Visual Astronomy of the Deep Sky

Roger N. Clark, Ph・D・

CAMBRIDGE UNIVERSITY PRESS

Cambridge

New York Port Chester Melbourne Sydney

&

SKY PUBLISHING CORPORATION

Cambridge, Massachusetts

Published by Sky Publishing Corporation

49 Bay State Road, Cambridge, Massachusetts, 02238 一 ! 290 and by the Press Syndicate of the University of Cambridge CB2 1RP

The Pitt Building, Trumpington Street, Cambridge CB2 1RP 40 West 20th Street, New York, NY 10011：USA

10 Stamford Road, Oakleigh, Melbourne 3166, Australia

© Roger N. Clark 1990

First publis 】ied 1990

Printed in the United States of America

British Library cataloguing in publication data

Clark, Roger N.

Visual Astronomy of the Deep Sky.

1 Astronomical bodies. Observation -Amateurs' manual

I. Title

522

Library of Congress cataloguing in publication data

Clark, Roger N. (Roger Nelson)

Visual Astronomy of the Deep Sky / Roger N. Clark

p. cm. Bibliography： p. Includes Index.

ISBN 〇 933 346 54 9 (Sky Publishing Corporation)

ISBN 〇 521 36155 9 (Cambridge University Press) 1.Astronomy 一 Amateurs⁷ manuals.  4. Nebulae - Atlases —

Amateurs' manuals. I. Title

QB64.C58 1990

523-dc 19 88 10290 CIP

ISBN 〇 933 346 54 9 (Sky Publishing Corporation) ISBN 〇 521 36155 9 (Cambridge University Press)

LU

For I can end as I began.

From our Jiomc on theearth wc look out into the distances and strive to imagine the sort of world into which we arc born., Today we have readied far out into space.

Ourimmediatencighborhood wc know ratlierin timatcly.

But with increasing distance our knowledge fades... until at the last dim horizon w e search among ghostly errors of observations for landmarks that arc scarcely more substantial. The search will continue.

The urge is older than history.

It is notsatitiedand it will not besuppr csscd.

Edwin P. Hubble

Contents

CONTENTS

in Orion

NGC 2261, Hubble's Variable Nebula in

Monoceros

M46 (NGC 2437), NGC 2438, Open

Cluster in Puppis with NGC 2438, a

Planetary Nebula

M67 (NGC 2682), Open Cluster in

Cancer

NGC 2903, Spiral Galaxy in Leo

M81(NGC 3031),Spiral Galaxy in Ursa

Major

M82 (NGC 3034), Peculiar Galaxy in Ursa

Major

M96 (NGC 3368), Galaxy in Leo

M105 (NGC 3379), NGC 3384, NGC 3389,

Galaxies in Leo

M 108 (NGC 3556), Galaxy in Ursa

Major

M97 (NGC 3587), the Owl Nebula: Planetary

Nebula in Ursa Major

M66 (NGC 3627), NGC 3628, M65 (NGC

3623), Galaxies in Leo

M109 (NGC 3992), Galaxy in Ursa

Major

NGC 403& NGC 4039, the Ringtail

Galaxv in Corvus

M99 (NGC 4254), Galaxy in Coma

Berenices

M106 (NGC 4258), Galaxy in Canes

Venatici

M 100 (NGC 4321),Galaxy in Coma Bere

nices

M84 (NGC 4373), M86 (NGC 4406), and

13 other galaxies in Virgo

NGC 4449, Galaxy in Canes Venatici

M87 (NGC 4486), NGC 4476, NGC 447&

Galaxies in Virgo

NGC 4565, Edge-on Galaxy in Coma

Berenices

M90 (NGC 4569), Galaxy in Virgo

M104 (NGC 4594), the Sombrero Galaxy

in Virgo

M94 (NGC 4736), Galaxy in Canes

Venatici

M64 (NGC 4826), the Black Eye Galaxy

in Coma Berenices

M63 (NGC 5055), Galaxy in Canes

Venatici

NGC 512 & Peculiar Galaxy in

Centaurus

NGC 5139 (Omega Centauri), the Great

Globular Cluster in Centaurus

M51(NGC 5194), the Whirlpool

Galaxy in Canes Venatici NGC 5195

M83 (NGC 5236), Galaxy In Hydra

M4 (NGC 6121),Globular Star Cluster in

Scorpius

cules

M20 (NGC 6514), the Trifid Nebula in

Sagittarius

M9 (NGC 6523), the Lagoon Nebula in

Sagittarius NGC 6530

M16 (NGC 66111,the Eagle Nebula in

Serpens

M 17 (NGC 6618), the Omega Nebula and

Open Cluster in Sagittarius

Mil (NGC 6705), Open Cluster in

Scutum

M57 (NGC 6720), the RingNebula in

Lyra

M27 (NGC 6853), the Dumbbell Nebula in

Vulpecula

NGC 6888, the Crescent Nebula in

Cygnus

NGC 6946, Galaxy i n Cepheus

NGC 6960 and NGC 6992-5, the Veil Nebula

in Cygus

NGC 7000, the North America Nebula in

Cygnus

M15 (NGC 7078), Globular Cluster in Pega

sus

M2 (NGC 7089), Globular StarClusterin

Aquarius

NGC 7293, the Helix Nebula in

Aquarius

NGC 7331,Spiral Galaxy in Pegasus

NGC 7662, Planetary Nebula in

A ndromeda

Conclusions

Appendices

A. Suggested reading

Recommended books

Beginning: learning the sky

Star atlases

Handbooks and observing guides

B・ StaT clusters for finding your limiting

mag n 辻 ude

C・ Air mass, atmospheric extinction, and

other calculations

Calculating zenith angle and air mass 268

Other useful computations

• D. Symbols a nd their definitions

• E. A catalog of deep-sky objects

• F. Optimum detection magnifications for

deep-sky objects

How the computation is done

What the ODM means

The role of contrast

Bibliography

Index

Photograph Credits

Chapter 3:

Figure 3.2b: Johnny Horne

Chapter 6:

Figui・亡6.hPalomar Observatory

Chapter 7:

M3】, M32, M]1〇: Palomar Observatory

NGC 246: Jack B. Marling

NGC 253: Palomar Observatory

M33： Naiional Optical Astronomy Observatories

M74: Naiional Optical Astronomy Observatories

M76: Laird A. Thompson, Canada-France-

Hawai felescope Corporation

NGC 89]: Mount Wilson and Las Campanas

Observatoires, Carnegie Institute of Washington

M77: Lick Observa(ories

NGC 1365: Wayne C. Annala, Copyright Uni

versity of Hawaii, Insiitute lor Astronomy

M45: Mount Wilson, and Las Campanas Observatories, Carnegie Institute of Washington

Ml: Eve red Kreimer, The Messier Album

M42, M43: Mount Wilson and Las Campanas

Observatories, Carnegie Institute of Washington

Trapezium: Lick Observatories

NGC 2023, NGC 2024, IC 434;James E. Gunn

M78, NGC 20? 】:Evered Kreimer, The Messier

Album

NGC 226]: National Optical Astronomy Observatories NGC 2903: Palomar Observatory

M81：National Optical Astronomy Observatories

M82: Laird A. Thompson, Canada -Francc-

Hawan relescope Corporation

M96: Evered Kreimer, The Messier Album

M97： Evered Kreimer, The Messier Album M66, NGC 3628, M65: Laird A. Thompson,

France-Canada-Hawan『e】escope Corporation

M109: Evered Kreimcr, The Messier Album

NGC 4038, NGC 4039: Palomar Observatory

M99: Evered Kreimer, The Messier Album

M106: Palomar Observatory

M100： Evered Kreimer, The Messier Album

M84, M86]13 other galaxies: Akita Fujii

NGC 4449: K.A. Brownlee, courtesy of Deep Sky Magazine

M87, NGC 4476, NGC 4478: Evered Kreimer, The

Messier Album

NGC 4565: Palomar Observatory

M90: Evered Kreimer, The Messier Album

M 104: National Optical Astronomy Observatories

M94: Palomar Observatory

M64: Martin Germano

M63: Evered Kreimer, The Afessier Album

NGC 5128: David Malin, Ang】o-Aus(ra】ian Telescope Board

NGC 5139: National Optical Astronomy Observatories

M5 ], NGC 5195: National Optical Astronomy Observatories

M83： Evered Kreimer, The Messier Album

M4: Martin Germano

M】3: Palomar Observatory

M20: National Optical Astronomy Observatories M8: National Optical Astronomy Observatories M16: National Optical Astronomy Observatories Ml1:Ben Meyer

M57: Laird A. Thompson, Canada-France-

Hawaii 'relescope Corporation

M27: National Optical Astronomy Observatories NGC 6888; Martin Germano

NGC 6946: National〇piical Astronomy Observatories

NGC 6960: Mount Wilson and Las Campanas

Observatories, Carnegie Institute 〇)'Washington NGC 6992-5: Mount Wilson and Las Campanas

NGC 7000: Ron Pearson

M15: National Optica] Asironomy Observatories

M2: Evered Kreimer, The Messier Album

NGC 7293: National Optical Asironomy Observatories

NGC 733】:Palomar Observatory

NGC 7662: Jack B. Marling

Preface

To stand beneath a dark, crystal-clear, moonless country sky is an awe-inspiring experience. Those thousands of stars, many larger than our own Sun, can make us feel small indeed. Il seems possible to see to infinity, though we cannot reach beyond arm's length. The beauty of the universe defies description.

Turn a telescope on a seemingly empty part of sky and swarms of new stars come into view — and possibly a faint glow of fuzzy nebulosity. Yet the heavens are subtle. Imagine that the fuzzy patch at the threshold of visibility is really a trillion suns 一 a galaxy larger than our own, in which our Sun is but a liny speck. Incomprehensible; yet somehow we try. Seeing that galaxy first-hand, even through a small telescope, is much more inspiring than the large, beautiful photograph in the astronomy book back indoors. Nothing can compare to viewing the universe directly.

The city dweller looks up at night and if lucky, sees a few stars and thinks "That's nice". But show that same person a very dark country sky and he or she will be awe-struck. Such a sky can be so spectacular that the Milky Way casts a shadow, and so many stars may be visible that even experienced observers have trouble finding constellations.

Even after many years as an amateur astronomer, I am still awe-struck on a dark moonless night. I was a very active amateur in the late 1960s and early 1970s, making both visual and photographic observations of everything within reach. By 1971,I had observed all the Messier and many NGC objects with an 8-inch telescope. I was especially interested in astrophotography, and taking pictures of the heavens consumed most of my spare time. Visual observing seemed just an eiijoyment, something to do while getting warmed up for those long hours ,guiding the telescope for that prize astrophoto.

In the late 1970s, my amateur career came to a standstill while I worked on a Ph.D. in planetary science. By 1982 the amateur bug was biting hard again, and all my old, pleasurable observing memories brought me back to active status.

Now that I had seen "everything⁵⁵, I was willing to spend lime making detailed drawings of what I saw. I began a literature search for material on how the eye performs in low-lighl-level conditions the handbooks for amateur astronomers available today, I found a surprising lack of information on observing deep-sky objects, or on what can actually be seen of them through telescopes.

The typical handbook devotes considerable space to observing the Moon and planets, but when the subject of galaxies and nebulae comes up, it has less to say. Usually it just recommends that because these objects are faint, low power should be used. Such works have inculcated the idea among amateur astronomers that one should have a "richesl-field telescope⁵⁵ and low-power eyepieces for deep-sky work.

This concept seemed wrong to me. My impression al star parties in the dark skies of the Cascade Mountains of Washington state was that an 8-inch f/10 telescope ,gave a more detailed view of most objects than richest field telescopes of the same or even slightly larger aperture! This was one reason I built an 8-inch fl1.5 し assegrain. Although I had started to build an 8-inch f/4.5, my opinions changed so much that I decided on the longer fbcal length partway through the prefect. I heard a few experienced amateur astronomers express the same thoughts—but none could give a good reason why high f/ralio telescopes seem to work belter on faint objects.

PREFACE

During research for this book I found the answer. The impression that higher "ratios by themselves give a better view is largely wrong. The true reason is partially what this book is about. The magnification used, not the f/ratio, determines what can be seen in a very faint object. A higher "ratio telescope simply yields higher power with a given eyepiece, and therefore it is more likely to be used that way.

In August,1982, I gave a talk to the Hawaiian Astronomical Society, an amateur club, in which I discussed how the human eye detects light at low levels and how various objects appear through telescopes. Many beginners see the beautiful pictures in astronomy books and expect the same views in their telescopes. This often results in disappointment. But I have found that if amateurs know more of the characteristics of the eye, and use the telescope a little dif1 ferently, quite a lot of detail in galaxies, nebulae, and star clusters can be seen.

The response to that talk convinced me that this material should be presented to all amateurs. It was then that I decided to write this book.

Roger N. Clark Denver, Colorado

Acknowledgements

As with any large work, there are many who helped in the creation of this book. I would like to thank Herman Dittmer, Ray Fabre, Ivan Gieslcr, Bob Gunnersoil, Ruthi Moore, Mike Morrow, Milt Sher, and Bill Tittcmore for their reviews of various drafts of the book. Their comments helped my ideas evolve to the present work.

I wish to extend a special thanks to Alan MacRobert of Sky Publishhig Corp, for his extensive review and editing. Alan is the only editor I have known who can take a rough manuscript and change it into a flowing work without changing any of the original ideas. His knowledge of astronomy and attention to detail assured that every thought was correct and understandable.

Finally I wish to thank my wife, Susan, for reviewing parts of the book and sons Matthew, Christopher, and Tyler for putting up with my many hours spent at the computer, and the many nights observing far from home.

About this book

Before the late nineteenth century all astronomy —amateur and profssi 0011—was visual. Everything depended on skilled use of the eye. Today, however, the professional astronomer rarely looks through the large telescopes at his or her disposal. As photographic film became more sensitive, both professionals and amateurs devoted more time to photography. In the last few decades, the advent of sensitive slsctronic light dstsc-tors diverted professional and even some amateur work further from visual astronomy. Direct viewing — at least of deep-sky objects — is now mainly for those intsrsstsd just in beauty and inspiration.

The typical amateur astronomer today has a telescope with a main mirror or lens 4 to 1() inches in diameter. A modern instrument of this size often provides a more detailed view of deep-sky objects than the larger telescopes used by the pioneering astronomers to make the great discoveries of the past. There are literally thousands of interesting objects, primarily ,galaxies but also star clusters and nebulae, within reach of the small telescope. Most are very faint, hundreds of times fainter than can be seen with the naked eye. However, a telescope with only 6 inches of aperture gathers about 400 times the light of the unaided eye, so these beautiful but dim objects can be brought into view.

The first-time user of a telescope is often disappointed that galaxies and nebulae do not look like the photographs in astronomy books. In some respects, the eye is no match for the camera. The beautiful photos are the result of very long exposures on sensitive film that builds up an image out of light too faint for the eye to see al all.

Nevertheless, the human eye is a very sophisticated light detector in its own way, and if used correctly, it can reveal considerable detail in galaxies, nebulae, and clusters.

Some beginning amateurs, inexperienced in the use of the eye and telescope, dismiss most deep-sky objects as faint smudges and merely search for a few easy ones before loosing interest. Others, thrilled by the ability to see galaxies millions of light years away, diligently search out the faintest ones possible. Soome amateurs turn to 0111er sights such as the planets, where even small telescopes can match the largest ones because of limitations by the Earth's atmosphere. Other amateurs turn to photography. The difliculty of obtaining a spectacular astrop^liol^o^, however, soon becomes evident. Compensation for the Earth's rotation requires that the telescope be moved precisely to track the stars. This requires somewhat soophisticated equipment, lots of patience, and long spells of staring at a guide star while correcting the rate of a never-perfecl telescope clock drive. Coount-less amateurs have gone through all of these stages and I am no exceptions.

With this book I hope to imparl new knowledge that will rekindle enthusiasm for visual studies of galaxies, nebulae, and star clusters. Visual observing is not only highly enjoyable; it can even be a useful tool for research, such as in the search for supernovae in other ,galaxies.

Many books are available on the basics of astronomy, so such material is not presented here. The appendices will direct the reader to excellent sources of basic information. This book is, however, designed for the beginner as well as the most advanced observer. Soome chapters contain complex diagrams and math, but the beginning astronomer with a limited math background will find a simple summary at the end of each chapter. Il might

VISUAL ASTRONOMY OF THE DEEP SKY

be beneficial for all readers, during their first encounter with the book, to read the chapter summaries before the technical portions. Chapters 3, 5 and 7 contain little math and need not be skipped by the beginner.

The capabilities of the eye are presented in Chapter 2. We will see that the eye is more sensitive to faint, extended light sources if the light is spread out over about 5 to 10 degrees. Because the ability to see low-contrast features in faint objects depends on their angular size, the visual observer will detect different amounts of detail at diff^i'ent magnifications. The implications of this fact for observing faint astronomical objects are discussed in Chapter 3, which also presents an introduction to telescopes, mountings, eyepieces, and how to Gnd celestial objects.

Chapter 4 analyzes the faintest star observable in a telescope and shows how this is a function of both the magnification and sky brightness, as well as the more commonly known factor of telescope aperture.

In Chapter 5 the techniques of making and keeping ,good records and drawings are discussed^. We analyze the visibility of the galaxy Messier 51 in Chapter 6 to illustrate the detection of low-contrast features in deep-sky objects. In this instance, the visibility of a galaxy's spiral arms is addressed, and we see that there is an optimum magnification for viewing faint detail.

Chapter 7 describes the visual appearance of more than 90 deep-sky objects. A catalog of photographs and drawings is presented to illustrate what can be seen with a typical amateur telescope. This chapter is laid out so that ■ a photograph and drawing are reproduced on facing pages at the same scale, to show exactly what in the photograph can be viewed in the telescope. Viewing distances from the drawing to the eye are given so the drawing can be seen at the same apparent size as at various, magnifications in a telescope.

The appendices include charts of star clusters for use in determining the faintest star visible in a telescope, a catalog of more than 600 deep-sky objects and the optimum magnifications to use on each, and equations for determining rise and set times of an object, as well as its altitude and azimuth.

This book presents many new concepts, some of which may be difficult for the experienced amateur to accept. The most revolutionary is the ''optimum magnified visual angle： This is the magniGcation at which an object should be viewed for best detection.

Research on how the eye detects faint objects eontradiets several basic pieces of conventional wisdom. One is the belief that low magnification should be used to concentrate a faint nebulosity on a small area of the eye's retina. This would be true if the retina worked passively, like photographic film. But it doesn't. The visual system has a great deal of active computing power and combines the signals f?om many receptors to detect a faint extended object. Increasing the magnification spreads the light over more receptors, and the brain's processing power can then bring into view fainter objects having lower contrast.

Another interesting concept is how the optimum magnification for a given object varies with the size of the telescope. Since the surface brightness is less in a telescope of smaller aperture, the optimum power is higher than in a bigger telescope! This applies only to deep-sky objects, and is illustrated nicely in Appendix F. Brighter objects, such as details on planets, fall on a different part of the eye's detection curves. In that case, about the same magnification should be used on all telescopes. Planetary observing is not discussed in this book, but this conclusion is very interesting nonetheless.

Another new concept is the highest magni-Gcation that may ever be usefully employed on a telescope. It is normally accepted that the highest power is about 50 to 60 times the objective in inches. This limit is correct only for bright objects such as the Moon and planets. For fainter objects the eye has less resolution and needs to see things larger, so higher powers are called for. At the limit of the eye's detection ability, the highest useful magnification is on the order of 330 per inch of objective! These extremely high magnifications are useful in specialized cases such as detection of detail in a small planetary nebula. For the drawing of NGC 7662 in Chapter 7, a magnification of nearly 600 was needed with an 8-inch telescope.

These results are based on an elaborate study of the eye's performance carried out during world war II and published in 1946 by Blackwell (see the bibliography). This information has been around for quite some time and is occasionally presented in some forms (e.g. Roach and Jamnick,1958). However, it has not been fully understood. It took considerable computer processing to convert BlackwelTs original data into a form useful in an astronomical context. Thus, it is not surprising that these concepts have not been previously discovered.

The human eye

INTRODUCTION

The eye is a remarkably adaptable light detector, able to funclion in bright sunlight and faint starlight, an intensity range of more than ten million. This is a considerably greater range of brightness than can be handled by man-made instruments such as photographic film or television cameras.

The eye consists of the cornea, which provides most of the lens action; the lens, which provides the focusing ability; the iris, which restricts the aperture and the amount of light entering the eye; and the retina, which detects the light and sends the signal to the brain. The slriicliire of the eye is shown in Figure 2.1.

In the retina, light is recorded by a photochemical reaction: when light impinges on a photochemical substance, a reaction sends an electrical signal to the brain. The sensitivity of the eye depends on the amount of chemicals present. It is primarily by varying the amount of photochemical material, not by opening the iris, that the eye can adapt to an amazing range of light levels.

RODS AND CONES

The light-detection devices in the retina are of two types: rod cells and cone cells. The cones are concenlrated in the Joveaゝ where visual acuity is greatest. This small area, appearing less than a degree in diameter, is the center of vision. We normally aim the fovea directly at an object to see maximum detail.

However, the cones are not as sensitive to light as the rods. Al the center of the fbvea (on the visual axis), there are no rods al all. The rods increase in density from zero al the center of the fbvea to a maximum al about 18 degrees ofl-axis. The density of rods and cones is shown in Figure 2.2. There are no rods or cones al all on the spot where the optic nerve leaves the eye. This is called the blind spot.

The rods and cones respond best to different colors of light, with the rods slightly more blue-sensitive than the average of the cones. This is shown in Figure 2.3. The cones are entirely responsible for color vision, and since they require bright illumination one secs no color in dim light, when only the rods are working.

The rods and cones are connected to nerves via ganglion cells. In the fbvea, a ganglion may serve a single cone, but in the periphery of the retina, where rods are prevalent, one ganglion may be connected to )()0 rods. Il is via the ganglion cells that the electrical signals from the rods and cones arc transferred to the optic nerve and the brain. The detection of faint light apparently depends on how many ganglion cells are involved. Those in the periphery each serve rods covering an area about 20 minutes of arc in diameter. The detection and conlrast-discrimination capabilities of the eye involve the summation eflecls of several ganglion cells. These are the capabilities that al low light levels are of interest to the visual astronomer.

UNITS OF BRIGHTNESS

Two factors govern the eye's detection of light. One is the total brightness of an object, and the other is its surface brightneso Surface brightness is the total amount of light divided by the area over which it spread.

A physicist might describe an object's total brightness by units of power (that is, energy flow), such as walls or number of photons per second. However, more specialized terms have been introduced for visual perceptions. The term illuminance describes the total light output of an object in the wavelengths seen by a typical human eye. One of the earliest units of illuminance measure was a candle. Requirements for precision led to the unit called the candela, the total visual light emitted in all directions by a standard candle made of a specific material and of a certain size, The lumen (Im) is another common unit of measure, and is equal to a candle divided by 4 兀(=12.5664).

Surface brightr^ess, intensity per unit area, is described by another term, luminance. Note the subtle, but important, difTcrEncc between luminanee and illuminance. Common units of luminanee are eandelas per-square meter, or lumens per square meter.

Astronomers use their own brightness unit: the stellar magnitude. And instead of linear msasursmsnts of distance on a surface, they use angular msasursmsnts of distance on the sky.

The magnitude was invented because the eye responds to light approximately logarithmically. One magnitude corresponds to a change in brightness by a factor of lUO”, which is about 2.51. Five magnitudes is a factor of 100 in brightness and 10 magnitudes is a factor of 100 times 100 or 1(0 000.

Figure 2.1.The human eye. (From Sky & Telescope       1984

Since astronomical objects cover an area of sky, their surface brightnesses arc described in magnitudes per square arc-second. The full Moon, for example, is a half degree (1800 arc-seconds) in diameter, so it covers 2.5 million square arc-seconds of sky. Dividing its brightness by its area gives it a surface brightness of 3.6 magnitudes per square arcsecond.

Astronomical objects difler vastly in both total brightness and surface brightness. The full Moon and the planet Mars have nearly the same surface brightness; their total amounts of light are so different only because the Moon covers a much larger area of sky. The Moon and the Sun, on the other hand, have nearly the same apparent size. Here if s a diflerence in surface brightness that causes such dissimilar amounts of light.

An object of a certain total brightness (such as the Moon) also illuminates the sur-fkee of the Earth with a certain number of lumens per square meter. Any astronomical object illuminates the Earth's surface in such a manner. Examples are in Table 2.1.

The illumination an object causes on the Earth's surface is directly relevant to astronomy. A telescope objective has a given area on which light from the object falls. The illumination per unit area times the area of the objective determines how many lumens are delivered to the eye.

Conversion between common units of surface brightness is shown in Table 2.2. The surface brightnesses of some familiar astronomical objects are shown in Table 2.3.

DARK ADAPTATION

When a person walks from daylight into a darkened room, the pupil of the eye opens to a maximum of about 7.5 millimeters (0.3 inch) in only a couple of seconds. However, the eye cannot yet see very well in the dim light. The effect of opening the iris changes the light entering the eye by no more than a factor of about 16. But as minutes go by, the eye's sensitivity increases by a factor of many thousands.

This dark adaptation is due to a chemical process. In darkness a chemical called rhodopsin, or visual purple, is manufactured and builds up in the rods and cones. The amount of visual purple governs the sensitivity of the eye. Dark adaptation is mostly complete after approximately 30 minutes, as seen in Figure 2.4a, though a slight buildup of visual purple continues for as long as two hours.

As can be seen f?om Figure 2.4b, the greater the angle an object subtends — that is, the

(temporal side)                       (nasal side)

ANGLE FROM FOVEA

Figure 2.2. Distribution of rods and cones along a horizontal line across the retina. Parallel dotted lines represent the blind spot. (From Sky & Telescope April,1984.)

Table 2.1. Common illumination levels

Total                Luminance caused at

stellar                      Earth's surface

Source                       magnitude             (lumens/square meter)

Table 2.2. Conversion ostellar magnitudes per square arc-second to candelas per square meter

larger its apparent size 一 the greater the sensitivity of the eye to it at all stages of dark adaptation, and especially after 30 minutes、 This is an important concept to understand, for not only the brightness but the size of an object in the telescope will affect its visibility.

CONTRAST DISCRIMINATION

Visual astronomical observations depend not just on detecting faint light but also on contrast discrimination. Both abilities are involved in seeing such things as spiral arms of galaxies and dark rifts i n nebulae, and in simply perceiving any object against the sky background. Contrast detection thresholds, as a function of background surface brightness for several object diameters, are plotted in Figure 2.5. This diagram shows that, for a given background (c.g. the night sky), less contrast is needed to sec a larger object.

The data in Figure 2.5 were used to plot minimum detectable contrast versus angular size at constant values of background luminance to make Figure 2.6. Here we notice that for objects with small angular sizes, the smallest detectable contrast times the surface area is a constant. As an object becomes larger, this product is no longer constant. The angle at which the change occurs is called the er■址ical visual angle. An object smaller than this angle is a point source as far as the eye is concerned. (A point can be considered the angular size smaller than which no detail can be seen.)

This critical angle is shown in Figure 2.7a plotted R)r various background luminances. Figure 2.7a shows that as the background becomes fainter, the size of a '"point source"

Figure 2.3. The relative response of the rods and cones as a function of wavelength (color) of light. Light of wavelength 5000 angstroms is green, 4000 angstroms is blue, and 6300 angstroms is red. Graph a shows the eye's response in linear units, while graph b shows the response in stellar magnitudes, a logarithmic scale. The data in both graphs a and b are scaled to the same value at their maximum to illustrate the color response of the rods and cones.

The cones are less sensitive than the rods, so the peak of the cone curve falls below that of the rod curve. How much below depends on the dark adaptation of the eye. When fully adapted, the rod peak is higher by slightly more than (bur magnitudes. ^FThis is shown in graph c. Here it is seen that the cone curve lies completely below the rod curve. Thus, the rods arc more sensitive at all colors when dark adapted than the cones. Derived from data in Table II of Kingslake (1965). The cone relative to the rod sensitiv 让 y in graph c was derived from data in Crossier and Holway (1939).

〇 2 4 6 8 (S①pコ七①suods①匕①Nle-①工

Dark Adaptati on at Differe nt Positi ons on the Reti na

10      20      30      40

Time in Dark (minutes)

Figure 2.4. 〇) Dark adaptation measured with a 2° diameter test object placed at various angular distances from the fovea. Derived from data in Middleton, 1958.

Dark Adaplati on：

Different -Sized Objects at

Cen ter of Vision

25

10        20        30

Time in Dark (minutes)

b) Dark adaptation as measured with test objects of different angular sizes located at the center of vision. The sizes are the diameter in degrees at which the objects could just be detected. Derived from data in Hecht et al.(1935) after Bartley (1951).

Table 2.3. Approximate surface brightnesses 或a^^mmonly observed objects

Note: Sky brightness values at the horizon can vary by a factor of ten and are adapted from Middleton (1958).

For the Sun, Moon, and planets, the total stellar magnitude and diameter (d) in seconds (") for which the computation was performed are listed in parentheses.

becomes larger for objects that are just detectable. In other words, the eye's resolution or ability to see detail is much coarser in the dark.

The eye's ability to see a point source, such as a star, increases as the background luminance ,gets dimmer. This is shown in Figure 8. Dark country skies arc better than city skies for seeing faint stars because the background is fainter — not because the country skies are significantly more transparent. A stm)nomeis often refer to sky darkness as "ltanspateelcy,l' but this is a misnomer.

A low-conir'ast object is more easily detected if it is larger. For an extended object such as a galaxy viewed in a telescope, mag-niHcation does not change the contrast with the background, because both the sky's and the object's surface brightnesses are affected equally. Some visual observers have stated that a dim object's contrast with the sky background increases with higher magnification, but this is clearly wrong. The contrast merely looks greater because of the increased detection capabilities of the eye.

This facet of human vision has not been described in print to my knowledge, so I will coin a name for the maximum magnification that will help detection: the "optimum magnified visual angle1'. This angle is shown in Figure 2.6 and also Figure 2.7b. For those readers familiar with calculus and the slope of a curve, the optimum magnified visual angle occurs when the first derivative (the slope) of each curve in Figure 2,6 is equal to -I.

If an object is at the threshold of detection and smaller than the optimum angle, more magnification will make it easier to see. When the object is magnified beyond the optimum angle, its surface brightness decreases faster than the eye's contrast detection threshold, and the object will become harder to detect. Remember that even for an

object somewhat above the detection threshold, higher magnification may bring out details within the object that are smaller than the optimum angle at a lower magnification. An example of what this means in practical terms k)r the observer is given in Chapter 6.

AVERTED VISION

We have seen that the rods are more sensitive to light than the cones, and that they are concenlrated in the periphery, not in the central part of the eye where visual acuity is best. Most amateur astronomers quickly learn about averted vision: a faint object may be invisible when you 100k straight at it, but it pops into view if you look slightly to one side. As was shown in Figure 2.2, the density of the rods increases away from the fbvca, reaching a maximum at about 18° to 20° off-axis. However, this generally docs not correspond to the maximum sensitivity region. The sensitivity of the eye varies somewhat with the person, with the direction oflPaxis, and with the diameter of the object.

10⁴

10³

1〇2

10

1

10¹

io²

Backgrou nd Surface Bright ness, B。 (mag nitudes / arc-sec )

Figure 2.5. The minimum contrast needed to detect an object of a ,given angular size shown as a function of background surface brightness, B 〇 、 The larger an object appears to the eye, the easier it is to detect. For small, bright objects on a bright background, a contrast less than 0.01 is enough for detection. But against the very dim night-sky background seen in a telescope (fainter than 25 magnitudes per square arc-sec), a large object must have a contrast of nearly 1.0, and a small object more than 10(), to be detected. Derived from data in Table VIII of Blackwell (1946).

10 100 1000

Appare nt Angular Size (arc-mi n)

Figure 26 The smallest contrast needed to detect objects of various sizes on various backgrounds. This diagram is the most important one in the book, so it's worth taking the time to figure out its complexities. This is the same data as in Figure 2.5, except that contrast detection ability is plotted against angular size for various background surface brightnesses (magnitude per square arc-second).

When an object is magnified in a telescope, the contrast between object and background does not change since both are magnified equally. However, the object becomes larger as viewed by the eye. Therefbre, moving horizontally across the chart corresponds to increasing the magnification.

As wc start with low magnification on the left side, the contours of background surface brightness are diagonal straight Tines. At a point called the critical visual angle, the lines begin to curve. Objects smaller than this value appear as point sources (the smallest detail that can be distinguished). As one moves to the right of the eritieal visual angle Tine, the faintest detectable surface brightness decreases faster than the background surface brightness. Thus, fainter objects 一 or detail within objects 一 can be seen as magnification is increased.

This is true only until the ''optimum magnified visual anglc" is reached. Thereafter, higher magnification decreases the detection threshold faster than surface brightness. A faint object is most visible when magnified to this angle. Chapter 6 is devoted to an example of the use and implications of this fact. Derived from data in Table VIII ofBlackwell (1946).

An exaniple of peripheral vision capabilities is shown in Figure 2.9. The periphery is about 40 tinics (4 miagnitudes) more sensitive than the fovea. As Figure 2.10 shows, peripheral vision is best when the object appears 8°to 16° ofT-axis in the direction toward the nose. The areas up, down, and toward the ear (temporal side of the head) are not quite as sensitive. The blind spot lies 13° to 18° toward the ear.

EXPOSURE TIME

Contrary to what nearly every astronomer believes, the eye seems to have an integration capability similar to photographic film, though much more limited. For the detection of the faintest objects, the light must accumulate on the retina for around 6 seconds. When searching for faint objects in the telescope field, scanning the area too fast will considerably raise (worsen) the detection threshold. Fixating on a point in the field of view while using averted vision should work best. Practice is required to concentrate on a point while using averted vision, because the eye tends to jerk around slightly. Fatigue seems to compound the problem, an inherent problem for the casual astronomer trying to stay awake all night.

100

5         10         15        20        25

Backgro und Surface Bright ness, B_o (mag nitudes / arc-sec?)

Figure 2,7a. The eye's resolving power, unlike a camera's depends on an object's surface brightness. The critical visual angle is the angle below which no detail can be seen and objects appear as point sources.

COLOR

The human eye is a remarkable detector of color under bright, daytime conditions. But the color receptors, the cones, do not function at all in the low light levels of night, so no color is seen. The threshold of color is about 21.5 magnitudes per square arc-second (0.00003 candelas per square meter). At this intensity both rods and cones are working. Because rods and cones are most sensitive to different colors (Figure 2.3), the perceived brightness of an object near the transition light level can depend on what color it is. This Purkirj effect is well known to variable star observers, who often have to compare two stars of difTi'cnt colors.

In amateur telescopes, color can only be seen in the brightest portions of a few deepsky objects, such as the Great Orion Nebula (M42) and bright planetary nebulae. Usually the only color seen is pale green, but if the contrast is high (as might be found in the mountains (ar       city lights), pastel reds

sometimes appear in bright emission nebulae such as M42. However, color sensitivity seems to be among the greatest variables of the human eye. It is the author’s experience that the sensitivity to seeing red in M42 varies considerably from person to person

5         10         15        20        25

Backgro und Surface Bright ness, B_o

⁽mag nitudes / a「c-sec²)

Figure 2.7b. The "optimum magnified visual angle” of an object depends on surface brightness. This angle is the size (or which a faint object, or detail within an object, should be magnified in order to maximize the possibility of detection.

when all Took through the same telescope. At the threshold of color vision, one should keep in mind, the perceived color may not be the true color of the object because of the Purkinje effect - and no deep ・sky object is much above the threshold.

Note that the threshold of 21.5 magnitudes per square arc second is fc)r the surface brightness at the eye, after the image is mag-nf ied by the telescope. Depending on the magnification, a telescope usually reduces the surface brightness by 0.6 to 7 magnitudes per square arc second. Appendices E and F give the mean surface brightnesses of various objects. These lists make it clear that few objects offer any possibility for detecting color. (Many objects, however, have bright spots several magnitudes above the mean; see the entry for M42 in Appendices E and F and in Chapter 7).

VISION AND HEALTH

Many other factors afTect the eye's sensitivity to faint light. For the eye to function properly there should be no vitamin deficiencies, and levels of blood sugar and oxygen should be adequate. At high altitudes, where the air is thin, some observers discover that heavy breathing (but not to the point of hyperventi-iating) enhances their night vision. Smoking will reduce the sensitivity of the eye. Contrast

o 5

(s ① pn±u 6eE)」s ①10 ① 4①〇一e 丄

5         10         15        20        25

Backgro und Surface Bright ness, B_o

(ma^g nitudes / arc-sec²)

Figure 2.8. The faintest star that can be detected by the trained human eye, shown as a function of the background surface brightness. This curve was derived from the data in Figures 2.6 and 2.7a. The faintest point source, magnitude 8.5, was determined by Curtis (1901).

sensitivity decreases as blood-alcohol levels increase, so one should not drink alcoholic beverages bcfbre observing. Eating candy or other food gives a blood-siigar boost. Vitamin A and zinc arc crucial for maintaining the eye's sensitivity, but taking supplements will not improve one's vision if the body already has enough.

Sunlight can be very dstrin1snta1 to dark adaptation. When the eyes arc exposed to bright sunlight for long periods, dark adaptation can take several days. Before an observing session, xiilaviolel-ffilc广ing dark glasses should he worn outdoors for several days.

As a person ages the eye does loo. The transmittance of the eye's lens declines and the pupil is not able to open as wide. Exposure to ultraviolet radiation reduces the lens transparency and also ages the retina, so unprotected exposure to bright sunlight (such as at the heach or in bright snow) should really be avoided al all times. Some sunglasses provide no protection al all against ultraviolet light. Ordinary eyeglasses, on the other hand, can be ordered with ultraviolet-flltining lenses. These are a fine idea for any astronomer who wears ,glasses.

The pupil will open to about 7 or 8 millimeters when a person is in his or her teens and early twenties, but by age 80, it may open to only about 3 to 5.5 millimeters. ^rThe age of the observer should be considered when choosing low power so that the telescope's exit pupil is not larger than what the eye can accept. The next chapter discusses exit pupils; see Bowen (1984) for further discussions.

SUMMARY

To delect bint objects use averted vision. The best direction to avert your view is so the object lies 8° to 16° toward the nose from the center of vision. Viewing 6° to 1 2° above or below the object is almost as good, but placing the object toward the ear should be avoided because of the blind spot.

A faint extended object (eg a galaxy, a bright spot within a galaxy or nebula) should

〇

Figure 29 Peripheral vision. The average sensitivity of the eye is shown at diflcrent angular distances from the fbvea (center of view). The eye can see about 4、magnitudes fainter when an object is placed a few degrees ofl-axis rather than stared at directly. 'Vhe data were derived by averaging the response curves from three individuals. Derived from data in Crossier and Holway (1937).

be viewed with enough magniHcation so it appears several degrees across to the eye. To be detected, it must be surrounded by a darker or lighter background so the eye can distinguish contrast. Various magnifications should be tried to bring details into the range of best detection. At each magniGcation, considerable time must be spent examining for detail. Higher magnifications should be tried until the object is totally lost fi~om view.

The eye should be dark adapted for at least 30 minutes so the photochemical visual purple is at fiill abundance. Bright stars and extraneous lights will tend to destroy dark adaptation.

-ods P u ーーm

-20° -10° 0° 10° 20°

Dista nee from Fovea

Figure 2.10. Peripheral vision further broken down. The detection threshold is shown for each field quadrant of the eye. The highest sensitivity occurs about 8° to 16° from the fovea where the image appears closer to the nose, although above and below the fovea are nearly as good. Placing the image further the nose (in the temporal quadrant) should be avoided because of the blind spot. Derived from data in De Groot et al.(1952)-

3

The eye and the telescope

TELESCOPE BASICS

In the last chapter, wc saw that five factors influence the visibility of a faint astronomical object: its brightness, the brightness of the background, the objects apparent size as viewed by the eye, the eye's dark adaptation, and where the image faTTs on the retina.

The first three factors depend in part on the telescope. ATT that really influences them is the telescope's aperture and magnification. Other details of what goes on between the eye and the teiescope objective arc immaterial, as long as the optics do not significantly absorb or scatter light. This rule has interesting im-pTieations for the long-standing question of the best type of telescope for deep-sky objects. The answer is, it doesn't really matter.

The design of the telescope, however, may restrict the achievable magnifications and field of view, and some designs do absorb or scatter light Tess than others. Redactors are generally better in this regard than reflectors if the aperture is Tess than about 8 inches.

Telescope design is a compromise of many factors. The most important is the aperture. This is the size of the objective, the main lens or mirror which collects the light. The larger the objective, the more light it gathers and the brighter the image. A bigger objective can also resolve finer details, but in practice the Earth's turbulent atmosphere often blurs the image so that a large telescope can sec no more detail than a small one.

A large-aperture telescope certainly costs more than a small instrument of similar design and is also bulkier and harder to store and transport. For the amateur astronomer, portability is often crucial because the telescope will be carried by car far from city lights. If the telescope is kept indoors, it should not be too heavy or complex to move outdoors and set up. A troublesome giant of an instrument wiTT be used only infrequently. There are many telescopes designed to optimize performance while minimizing the conveniences.

A telescope acts as both a light-gathering and magnifying device, whether used visually or for taking photographs. The objective gathers light and focuses it to form an image, which floats in the air Tike a tiny upside-down picture of the scene. The distance from the objective to the image is called the ^focal length (F). (Some complex telescope designs have effective jocal-lengths that differ from this distance.)

A key characteristic of any telescope is its focal ratio or f/ratio. This is simply its focal length (F) divided by its aperture diameter (D)：

"ratio = F/D.                (equation 3.1)

A telescope with a 4-inch diameter objective and a focal length of 40 inches, for instance, is called f/10.

In order to see through the telescope, an eyepiece must be used. The eyepiece magnifies the image the same way a hand Tens magnifies newspaper print. The telescope's power, or magnification [m) equals the effective focal length of the telescope divided by the focal length of the eyepiece (/):

m = F / f.                        (equation 3.))

In general, the higher the magnification, the Tess sky can be seen at once.

ABERRATIONS

There are many telescope designs, and every one suffers to some degree from optical defects known as aberrations. These result

when rays of light from a point on a distant object fail to reach the correct point on the image (also called the focal surface). Some such errors are inherent in the system design; others are added by imperfect construction. The six main ones arc: chromatic aberration, coma, astigmatism, spherical aberration, distortion, and Petzoal curvature.

Chromatic aberration arises because different colors of light arc bent by difllerent amounts when they pass through a lens. Unfortunately, no kind of glass known has the same index of refraction (light-bending strength) for all colors. Simple lenses suffer from chromatic aberration the most. Images formed by a simple lens are surrounded by obvious blue and red fringes. When two or more lenses with diflcrent index-of^i'ef'action properties arc combined, the chromatic aberration can be controlled to some extent because two or more difllerent colors can be made to focus at the same place.

Coma is named for the comet-like images of stars seen oflPaxis (away from the center of the field). The farther the star is from the objective's optical axis, the worse the coma will be. Coma also depends on the "ratio of the system, and for a given "「atio and angle ofPaxis, it increases with the aperture.

Astigmatism may be caused by errors in the manufacture of an optical component or may be inherent in the design. An example might be a "sphciicaf' lens surface that is not perfectly spherical but slightly cylindrical, for instance if the light from the left and right edges of the lens is focused at a different point than light from the top and bottom edges. Astigmatism can also be caused by improper alignment of an optical system.

Spherical aberration is a telescope's inability to focus all the light at the same distance from the objective. For example, a beam of parallel light that encounters a spherical mirror will not all be focused at the same distance from the mirror. The light that encounters the mirrorS edge will be focused closer than light that strikes the center. Parabolic mirrors do not suffer f?om spherical aberration when focused on objects very far away. Zonal Aberration is spherical aberration at a particular radius from the center of a lens or mirror.

Distortion is the squeezing or stretching of a part of an image toward or away from the center. A common lest of distortion uses a square grid; the lines may bend toward or away from the center, giving the appearance of a pincushion or a barrel. Hence the common names of pincushion distortion and barrel distortion.

Petzval curvature (or field curvature) results from the image being not ffat but slightly concave or convex. Typically, the best focus at the edge of the field lies closer to the objective than the best focus at the center. (Astigmatism also a fUects the surface of best focus.) Eyepieces too can have field curvature, so stars at the edge of the view are rarely as sharp as on-axis — unless the eyepiece curvature happens to compensate for that of the objective ゝ

TYPES OF TELESCOPES

Telescope design is a compromise between aberrations, cost, portability, fbcal length and field of view, among other things. The diflcrent types of telescopes described below arc illustrated in Figure 3」.

The refractor

A ref'actor is a telescope whose objective is a lens. To reduce chromatic aberration, the lens is almost always compound, or made oi two or more disks of glass (lens elements). Usually the objective is achromatic (made of two elements), though a few expensive ones arc apochvomatic (usually having three elements).

The main defect of refractors is chromatic aberration, but astigmatism and spherical aberration can also be problems depending on field size, focal ratio, and design. A high focal ratio is required in most achromatic lens designs in order to minimize the aberrationg. The commonly accepted f/ratio for refractors is f/15, though if they are small and not intended for high magnifications, f78 or even f/6 will give acceptable definition. Binoculars are refracting telescopes with an image-erecting

Figure 3.1. Basic telescope designs. In each case, starlight arrives from the left and is focused to a point on the image plane, which is seen in profile as a short line.

system and a low-power eyepiece for each

eye.

A dvantages of the refractor:

The enclosed tube helps keep the optics clean.

Redactors are relatively free of coma.

There is no central obstruction (see refecting telescopes below).

The construction is rugged and stays in optical alignment.

A lens scatters Tess light than a mirror.

Disadvantages:

The cost is often many times that of the same size reflecting lelescope、

The Tong ffocaT length makes it difficult to achieve a large field ofview.

The long focal length means a long tube, which requires a larger mounting, reduces portability and increases susceptibility to being shaken by wind.

Astigmatism is present ofPaxis.

The reflector

A reflector uses a mirror for its objective, so it is free from chromatic aberration. (ATT eyepieces, however, employ lenses, so the eyepiece can add noticeable false color to a reflector.) Only the surface of the mirror needs to be optically perfect, whereas in a ref'actor the entire volume of glass must be of high optical quality. An objective mirror has only one optical surface, making it Tess expensive to manufacture than an objective Tens, which has at least four.

Reflectors come in two basic designs: Newtonian and Cassegrain.

The Newtonian reflector

The Newtonian telescope, named for its inventor Isaac Newton, usually has a parabolic objective mirror. The focal point is between the mirror and the object to be viewed. In order to sec the image, a small flat mirror is usually placed just inside the focus to reflect the converging light to the side of the tube where the observer can place an eyepiece or other equipment such as a camera.

Advantages:

Low cost.

Freedom f?om chromatic aberration.

Focal ratio can be short (as low as about f/4), giving a large field of view and good portability.

Disadvantages:

Substantial coma at low f/ratios.

Alignment may often need some adjustments

The refective coatings are easily scratched and more diflicult to clean than a Tens.

The small diagonal mirror blocks some Tight, and also slightly reduces resolution and contrast.

The Cias^^t^i^irain reflector

The classical Cassegrain telescope has two curved mirrors: a paraboloid just like the Newtonian, and a small convex hyperboloid instead of the Newtonian's flat secondary. The hyperboloid secondary mirror magnifies the image formed by the primary. In the most common conffguration, the image is directed baek through a hole in the primary to a convenient location behind, where eyepieces and other equipment can be placed.

The coma in a Cassegrain is the same as in an equivalent focal length Newtonian. In cfleet, the Cassegrain is a compact high f/ratio Newtonian. In addition, a flat diagonal mirror may be tempora「iTy substituted for the hyperboloid secondary, creating a shorty-focus Newtonian telescope.

Advantages:

Freedom from chromatic aberration.

The f/ratio can be fairly Tow, givinga relatively large field ofview, though not as large as in a Newtonian.

A Cassegrain is more compact than an equivalent Newtonian.

Disadvantages:

Substantial coma.

Alignment is more difficult than for Newtonians.

The reflective coatings are easily scratched and more diflicult to clean than a Tens.

The hyperbolic secondary mirror is larger than the fat secondary ofa Newtonian, blocking more light and further degrading image sharpness and contrast.

Variations on the Cassegrain include the Gregorian, which has a parabolic primary and a concave ellipsoidal secondary, and the Ritchey-Chretien and Dall-Kirkham, which look like ordinary Cassegrains but have slightly'diflbreiit mirror figures.

Catadioptric telescopes

Catadioptric or compound telescopes use a combination of lenses and mirrors. The lenses do not refract light enough to cause chromatic aberration, but merely correct for aberrations in the mirrors.

The Schmidt・Cassegrain

The Schmidt-Casseg「ain has a spherical primary mirror and a convex, usually spherical secondary mirror. A thin lens or corrector plate in front corrects the spherical aberration. This lens has an unusual aspheric figure.

Advantages:

All the advantages of a Cassegrain.

A sealed tube keeps dust off the mirrors. Better freedom from aberrations than a classical Cassegrain.

Disadvantages:

Same as those for Cassegrains.

More expensive than Newtonians or classical Cassegrains.

Curved focal surface.

The Maksutov Ctas^^ieg'rain

A Maksutov telescope has a spherical primary and secondary much like the Schmidt-Cassegrain, but the corrector lens is a deeply curved meniscus.

Advantages:

Similar to those of the Schmidt-Cassegrain except the tube is even shorter.

No aspheric surfaces. (Minor spherical aberration is sometimes corrsctsd by aspherizing the meniscus.)

Disadvantages:

Similar to those of the Schmidt-Cassegrain.

Curved focal surface, but better than for similar Schmidl-Casscgrains.

TELESCOPE MOUNTINGS

A telescope's optics are only half the story. The best optical system will be useless if it is on a wobbly, quivcry mounting.

A telescope mounting must not only hold the instrument solidly, but allow it to be pointed anywhere in the sky smoothly and easily. Three basic types of mountings are in common nsc:】)the German cqiiaKoial,2) lhe fork, and 3) the Dobsonian. They are illustrated in Figure 3.2. There are many variations on these basic designs.

AH mountings can be divided into two groups: altazimuth and equatorial. If one of the mount's axes is made parallel to the earth's axis, then the telescope is called an equatorial. It swings in the direction of celestial north-south and east-wesl, and can be made to follow the stars as the earth rotates by turning only one axis. This tracking of the stars can be done with a motor known as a clock drive.

An altazimuth mount, on the other hand, swings up and down (in altitude) and side to side (in azimuth). The German and fork mountings arc equatorial, whereas lhe Dobsonian is altazimuth.

The German equatorial mounting (Figure 3.2a) is good for long telescopes such as refractors and long-fbciis Newtonians. But it requires a heavy counterweight, and when it Allows an object crossing lhe meridian, the telescope must be taken off the object, swung around to the other side of the mount, and re-aimed.

The fork mount (Figure 3.2b) is probably the best for telescopes with short tubes. The eyepiece does not move much as the telescope is pointed to diflerent parts of lhe sky, and there is no counterweight. A disadvantage is that the sky around the celestial pole sometimes cannot be viewed.

A variation of the fork mounting is lhe yoke, which is supported by two piers, one on either end of a framework that holds the telescope. The yoke mounting is very steady but not portable and not commonly ffound in commercial instruments fbr amateurs.

The Dobsonian mount (Figure 3.2c) is the most stable design commonly used by amateurs. It is sort of a fork mount, but because it is an altizmuth the design is simple while mechanical stresses are minimized. Commonly built of plywood with teflon pad bearings, the Dobsonian mount can be very compact, and it has no counterweights. Its main disadvantage is dificulty in tracking

Figure 3.2a, The author's homemade telescope, an 8-inch classical Cassegrain on a German equatorial mounting. The primary mirror is f/3.5, and the secondary mirror magnifies the focal length to 2336 mm (f/11.5). The smaller finder has a 60-mm objective with a 300-mm focal length and a 38-mm Erfle eyepiece, giving 7.9X and an 8° field of view. The larger finder is a 3.1-inch refractor normally used with a 20-mm Erfle eyepiece giving a 2° field of view and 31X. stars, especially near the zenith. It is often used to mount short-focus Newtonian telescopes of very large aperture. Such a mount costs very little to build — basically the price of the wood 一 yet it can be as stable as a massive equatorial.

EYEPIECES

Eyepieces aL short-focal-length lens systems that magnify the image formed by the telescope objective. Because the eye is most comfortable when focused on objects at infinity (10 meters or more in practice), the eyepiece is designed to convert the light from a telescope image into a system of parallel rays. The eye lens and cornea focus these rays onto the retina just as a camera lens focuses light from a distant object onto film.

Eyepiece designers try to minimize aberrations while maximizing the field of view, Most eyepieces have two or more lenses (Figure 3.3). The one closest to the eye is called

Figure 3,2b. Johnny Horne's 12.5・inch combination Ncwtoo(t：；Cassee^rraln telescope is shown on a fork mount.

the eye lens, and the one farthest is called the field lens. The focal plane of the eyepiece, commonly called the image plane, should be positioned to correspond to the image plane of the objective. This is what you do when you focus a telescope.

The light that leaves the eyepiece forms a cone ofparallel ray bundles. The angle of this cone is called the apparent field of view, because this is the angular size the eye sees as the round "window" of the telescope's view.

Each bundle of rays from a point on the image has the same diameter as it leaves the eyepiece. This diameter depends on the magnification of the telescope and the size of the objective. These ray bundles come together behind the eyepiece to form a disk called the ^uexit pupil." It can be seen as an actual disk of light floating just behind the eyepiece. The

Figure32c. A telescope in a typical Dobsonian mounting. This 6-inch f75 Newtonian with a square tube was built by the author. The Dobsonian mount is lightweight and very portable.

iris of the eye is placed here for optimum viewing. If the exit pupil is too close to the eyepiece, the observer will not be able to get his or her eye close enough to see the whole field of view. The distance from the eye lens to the exit pupil is called, appropriately enough, the "eye relief \

Eyepieces used for astronomy should have antireflection coatings on all air-t〇glass surfaces, in order to reduce loss of light, loss of contrast, and spurious images caused by reflections from glass surfaces. ^<cMulticoatings^>, are more eflective than single-layer coatings.

Many eyepiece designs have been in・ vented. Here are some of the more common ones.

Huygens. The Huygens eyepiece has two plano-convex lenses and overall low performance. The curved sides of the lenses face toward the objective, and the image plane lies between them. The apparent field ranges from 25° to 40°. This eyepiece works satisfac・ torily on telescopes with f?ratios of f/8 and higher. The eye relief is only about 25 percent of the focal length.

Ramsden. The Ramsden eyepiece also con・ sists of two simple plano-convex lenses, but the curved surfaces face each other. The image plane is just outside the field lens. The Ramsden is a very low-cost, low performance eyepiece, but slightly better than the Huygens. 'The apparent field ranges from 30° to 40°. It works satisfactorily on telescopes with "ratios of f/7 and higher. The eye relief is about 30 percent of the focal length.

Kellner・ The Kellner comes in three types, I, 11, and III. Types I and II consist of a simple and an achromatic lens. Type I has a single plano-convex field lens and type II has a single double-convex eye lens. Type III is also known as a Plossl and is discussed below. In all cases the image plane is just outside the field lens, and the apparent field is 35° to 50°. Types I and II have an eye relief of about 30 percent of the focal length, and all three work well with “ratios of f/6 and higher. The Kellner provides better color correction than either the Ramsden or Huygens.

Orthoscopic・ This is one of the finest tele・ scope eyepieces. It consists of a triplet field lens and a plano-convex eye lens. A well-designed orthoscopic can be fully corrected for distortion, and has very good color correction. It works well with telescopes having (/ratios as short as f/4.5. The apparent field is 30° to 50°, and the eye relief is as much as 80 percent of the focal length.

PlOss 1・ The Plossl eyepiece consists of two achromats in a design similar to the type I and II Kellners. It rates as one of the Gnest telescope eyepieces. The apparent field is 35° to 50°, and the eye relief is about 75 percent of the focal length. The Plossl works well with f/ratios as short as f/4.5.

Erfle. This eyepiece has a very wide apparent field, about 50° to 70°, with 65° common. It has three achromats and provides good definition in the center, but aberrations become obvious toward the edges. The eye relief is about 30 to 40 percent of the focal length. The Erfle works well with "ratios of f/4.5 and up.

Nagler. The Nagler is a proprietary design having seven elements and an astonishing 82° apparent field. It is designed for use with (74.5 Newtonian telescopes, though it will work well with higher f/ratios. The field of view can be breathtaking, though it is sometimes difficult to see the entire field at once because it appears so large. This is a very expensive eyepiece, often costing fbur to five times as much as a good quality Erfle, Plossl, or orthoscopic.

In recent years eyepiece sales have become a very competitive business, and new (and expensive) proprietary designs are coming to the market.

The Barlow lens. A Barlow is a negative lens that enlarges the image plane in a telescope. It is not an eyepiece but an eyepiece accessory. Barlow lenses are usually used to increase the telescope's effective focal length two to three times so an eyepiece will give a correspondingly higher magnification. A Barlow lens is a necessity for short-focus telescopes if high magnifications are desired.

For example, say your telescope has a focal length of 1200 mm (47 inches), and your shortest focal length eyepiece is 6mm. That eyepiece on that telescope gives a magnification of 200 X. Shorter focal length eyepieces are impractical because their eye relief is too small. But using a 2X Barlow lens would give 400X and a 3X Barlow 600X with the same 6mm eyepiece. With three eyepieces and a Barlow lens that is continuously variable, almost any reasonable magniGcation can be obtained. The Barlow docs add more optical surfaces in the light path, reducing the light transmission of the telescope. But the nega-

«■— Eye Relief―*

Image        Field              Lens

Plane        Lens

Figure 3.3a. How an eyepiece focuses light for the eye.

Kellner, Type 2

Erfle                                           Nagler

Figure 3.3b.

tive Barlow lens actually flattens the Petzval curvature of most visual telescopes and thus makes them perform better.

FIELD OF VIEW

When telescopes first came into common use in the 18lh and early 19th centuries, eyepieces were unsophisticated — at first a single simple lens, then later the Ramsden and Huygens. These eyepieces had small apparent fields of view. Looking through them was almost like looking through a peashooter. To obtain a large true field of view on the sky, long focal length eyepieces had to be used, resulting in low power. As better eyepieces were invented that offered wider views, visual discoveries of wide-field objects became possible.

For a ,given telescope focal length, it is the choice of eyepiece that determines both the magnification and the true field of view. The eyepiece magnifies the true field of view to a large circle projected in front of the eye. The size of this circle is the apparent field of view. The magnification (m) also determines the apparent diameter (oj of an object with a true angular diameter (o) in the sky:

作=2arctan[ m tan(0.5 oj], (equation 3.3) which can be simplified to

Qp —m(2(.                        (equation 3.4)

Equation 3.4 is accurate to within a couple percent when the apparent diameter, a_p,is less than about 30° and m is greater than about 10.

An eyepiece for a ,given magnification should be able to view as much as possible of the image formed by the telescope's objective. The linear size (s) of an image in the focal plane is given by

s = 2 F tan(0.5 aj,             (equation 3.5)

where F is again the telescope focal length. An eyepiece of focal length f will magnify the image so that, to the eye, it will subtend an apparent angle

(2p = 2 arctan(0.5 s/f).          (equation 3.6)

If the image's linear size, s, just fills the field of the eyepiece, we find the maximum apparent angle that can be viewed by the eyepieces. This is the apparent field of the eyepiece, a_c.

Different types of eyepieces have diflerent apparent fields. A simple lens may have an apparent field of only about 10°. Table 3.1 lists the characteristics of several syepiscs types.

At low powers and wide fields, we run into a practical restriction. The eyepiece should not allow you to see a larger image than the rest of the telescope provides. Otherwise, you'll just be viewing the inside walls of the eyepiece holder or other telescope parts, such as light baffles.

Modern telescopes commonly have eyepiece holders of sithsr 1.25 or 2.00 inches (31.7 or 50.8 millimeters) inside diameter. Since the eyepiece barrel must fit within this, the maximum achievable image size is about 1.14 and 1.90 inches (29 and 48 mm) for 1.25-and 2.00-inch holders, respectively. For a given eyepiece apparent field, the maximum usable focal length (B) can be computed by rearranging equation 3.6:

7^ = 5 / [2 tan(0.5 ce)].         (equation 3.7)

where s is 29 or 48mm, for the two sizes of eyepiece holder. The maximum useful focal lengths of eyepieces with various apparent fields are shown in Table 3.1.

Erfle and Nagler-type eyepieces have the largest apparent field. A focal length of about 23 millimeters is the maximum for an Erfle in a 1.25-inch eyepiece holder. Buying a 40-mm Erfle in a 1.25-inch tube defeats its purpose, since the apparent field would be restricted to 40°.

THE ROLE OF A TELESCOPE'S F/RATIO

The angle of the converging light cone from the objective to the image plane is determined by the "ratio. Light cones are illustrated in Figure 3.1. The refractor and reflector diagrams have "ratios of f/3.3 and the Cassegrains 76.5. Even shorter "ratios are illustrated by the primary mirrors of the Cassegrains: 〃1.3. For narrower lighi. cones, the f/ratio is larger, the telescoped aberrations are smaller, and eyepieces work better.

As the f/ratio decreases, the surface brightness in the image plane increases. Given two six-inch telescopes with focal lengths of 30 and 60 inches (f?5 and f/10, respectively), the "5 telescope will produce images with higher surface brightness than the f/10 telescope

Table 3.1. Eyepiece characteristics

Focal length (mm) that views an image

Apparent               of diameter:

when viewed with the same eyepiece. A 12-inch (75 telescope will produce images with the same surface brightness as the six-inch f/5. However, in the 12-inch the image would be twice as large.

The f/ratio determines the useful range of magnifications that may be obtained with standard equipment. With long focus (high f/ratio) instruments, low powers and wide fields are diflicult to obtain. For short focus (low f/ratio) telescopes, it is diflicult to obtain liigh power. You can always use a barlow lens to boost magnification, but another optical element is introduced and some low (/ratio systems may begin to show aberrations. Choosing the telescope's f/ratio is important because you want to be able to use low power to get wide lields of view to see large objects against the sky background, yet you also want high power to see fine detail.

The eye, however, is not aflected by the f/ratio of the telescope. Only the size of the object in view (the magnification) and its brightness (telescope aperture) are ultimately important. Consider two 8-inch (203 millimeter) teleskopes, one an f/4 Newtonian, the other an V\2 Cassegrain. Suppose two eyepieces with the same apparent fields of view are available, one of 21 millimeters focal length, and one of 7 mm. The 7-mm eyepiece on the f/4 telescope gives a magnification of 116 (116X), and the 21-mm eyepiece on the (712 Cassegrain also gives 116x. Each telescope-eyepiece combination provides the same true field of view on the sky. In this ease, all deep-sky objects viewed through the two telescopes will appear identical. Neither telescope has an advantage ゝ We can stale a general rule:

Telescopes of equal aperture and good, clean optics that do not scatter light will show identical views of deep-sky objects at the same magnifications, regardless of their f/ratios.

THE EXIT PUPIL

As mentioned earlier, a telescoped exit pupil is the little disk of light projected behind the eyepiece. The eye should be placed here to see the field of view best.

The exit pupil is actually a small image of the telescope objective. In a reflector, it will show a silhouette of the central obstruction and spider (the support vanes to the secon ・ dary mirror) and may become distractingly visible if the eye is kept too far back from the eyepiece.

The diameter of the exit pupii,e_p, is the objective diameter, D, divided by the magnification, m:

e_p = D / m.                     (equation 3.8)

Table 3.2. Minimum useful telescope magnifications

Aperture

2.54

5.08

7.62

10.1

• 12.7

• 15.2

• 17.8

• 20.3

• 25.4

• 30.5

• 35.6

• 40.6

• 45.7

• 50.8

55・9

61.0

7 6.2

91.4

The magnifications are for a 7.5 mm exit pupil.

If all the light collected by the telescope is to enter the eye, then the exit pupil must be smaller than the eye's pupil. In Chapter 2 we saw that the iris of a young adult wiTT open to about 0.3 inch (7.5 millimeters) when ffuTTy dark-adapted. This value sets a lower iimit to the magnification that can be used on a given telescope. By setting D to 25.4mm (1.0 inch) and e_p to 7.5mm, we fnd that the minimum useful magnification is 3.4X per inch of objective diameter.

Table 3.2 gives the minimum magnifea-tion for common telescopes based on this relation. However, it is usually wise to be conservative since individual observers' eyes may vary, and the maximum size of the pupil shrinks as a person ages. Thus a value of 4x or even 5x per inch of objective may be more appropriate. If lower magnifications arc used, the iris of the eye may block some light, reducing the effective aperture of the telescope.

SEEING AND RESOLUTION

The size of the smallest detail that a telescope can reveal (ignoring atmospheric turbulence) is called resooution. This depends on the size of the objective, since that is what determines the size of the telescoped diffraction pattern.

Tlie diffraction pattern of a point source, such as a star, is a circular disk of light surrounded by bright rings. At visual wavelengths, the angular dlame(e(i(2, of the disk is

a — 5.45 / D                    (equation 3.9)

where the aperture of tlie telescope, D, is in inches and a is in arc-seconds. Tiiis size represents the diameter of the difr「aclion pattern's first dark ring, just outside the central spot (Figure 3.4). (The diameter of the second dark ring is twice that of the first.) Tiie angular size a of the disk is called the Rayleigh resolution limit.

If a bright image in the telescope is magnified too much, it appears fuzzy due to the telescope's limited resolution. The generally accepted magnification limit for bright objects is about 60 times the diameter of the objective in inches. As ail example, considera 3-inch telescope; the central disk of the di匸 fraction pattern is nearly 2 arc-seconds wide. The eye is able to resolve about 1 arc-minute for bright objects, so only about 30 power is needed to enlarge the difTaction disc up to the resolving power of the eye. This is only 10X per inch of telescope objective diameter. At 90 power (30x per inch of oIMective), the eye can easily see the diffraction disk and rings if tiiere is little atmospheric turbulence, and at 180 power (60x per inch of objective) the disk is obvious. On extended sources such as planets or scenes on the ground, the image is made up of multiple overlapping difTrac-tion patterns, so it appears fuzzy.

With fainter subjects it's a different story. As we saw in Chapter 2, tiie eye has Tess resolution in dim light. Tiie magnification limit is found by dividing the eye resolution (1800 arc-seconds when the light is extremely dim) by the telescope resolution, using equation 3.9. The limit is 330X per inch of objective. And so, for faint stars and most deep-sky objects, there is essentially no limit to the useful magnification at all. For example, many planetary nebulae have small angular sizes but relatively high surface brightnesses. Very high magnifications may be employed to b ring detail within such a nebula into view.

High magnifications, however, cause more problems than limiting the field of view. Any vibration or looseness in the telescope mount becomes intolerable. Irregularities in the atmosphere 一 the constant quivering, churning and boiling ofan image, which astronomers call the seeing 一 usually make an image fuzzier than the telescope's theoretical resolution limit, and high power only magnifies this problem. Furthermore, the 330X per ineh of objective mentioned above is useful only at the very limit of detection. Any object brighter than the threshold of detectability will appear fuzzy at such powers. Because most fields will contain some brighter subjects, the overall impression will be that everything looks fuzzy (and it is, all except the one thing at the extreme limit of detectability).

Magnification also reduces the surface brightness of everything in view. It must not reduce an object's surface brightness below the eye's detection limit, of course, or the object will disappear. Therefore, the only case where 330X per inch might be justified is the detection of a star against a bright background, as discussed in Chapter 4.

FILTERS

Wouldn't it be wonderful if there were a device that darkened the sky background while leaving stars and nebulae bright? We saw in Chapter 2 that the detection capability of the eye depends on an object's contrast with its background as well as on its size and surface brightness. One way to reduce the sky background is to get far away from city lights. But even where there is no artificial pollution, the Earth's sky glows from scattered starlight and from airglow. Airglow is like a permanent, low-grade aurora. It is caused by charged particles from the Sun encountering our atmosphere at very high altitude.

Airglow occurs at only a few wavelengths (isolated colors), and if a filter could be made to remove these wavelengths while passing the rest of starlight and nebular light, contrast would be increased and the view would

Figure 3.4. A typical diffraction disk of a star image in a telescope. At top center is a photograph showing the diffraction rings, taken by Allyn J. Thompson. The central spot was intentionally overexposed to show the rings. The 3-dimsnsiona1 plots of diffraction patterns indicate brightness by height above a square base, viewed obliquely. Faint diffraction rings appear as concentric ripples around the center of a star's Airy disk. At left is the pattern of a star in a telescope having no central obstruction, while at right is the pattern for a telescope having a round central obstruction 30% of the objective's diameter. The central obstruction removes light from the central spot and rsdistributss it into the rings. From Stoltzmann (1983).

appear better. If city lights emitted only a few wavelengtlis, they too might be ffitered from the observer's view. Fortunately, this can be done — at least partially. To understand how, we'll need to examine the nature of light celeitial objects.

The liglit from a star consists of many wavelengths or eolors. Such light is called conlinuum radiation, since its spectrum appears nearly continuous.

Some nebulae also give off continuum radiation. But many of the best and briglitest emit light at only a few, specific wavelengths. These two types are rjlection and emission nebulae, respectively.

Reflection nebulae shine by reflected starlight, similar to the way a cloud in the Earth's sky reflects sunlight or tlie way the molecules in the Earth's atmospliere scatter it. Because the molecules and some dust particles in our atmosphere are smaller than the wavelengths of visible light, blue light is scattered more efliciently than red, and thus the sky appears blue. This type of scattering is called Rayleigh scattering after the scientist who first described the eflect. Reffection nebulae "shine" in part by Rayleigh scattering of starlight, so the spectrum is continuous, much like that of a star but usually bluer.

Emission nebulae, on the other hand, shine in only a few colors, They come in two types: planetary and diffuse nebulae. Both shine because starlight excites speciftc types of atoms； which rc-cniit this energy only al certain wavelengllis.

The briglitest spectral lines (colors) of emission nebulae are listed in Table 3.3. The atom that emits the radiation is listed, Allowed by a Roman numeral tlial indicates its ionization state. (I is un-ionized, II means the atom is missing one electron, III means two electrons arc missing, and so on.)

A forbidden line is not really forbidden to happen; it just cannot be observed in labor-

atories because the gas density must be lower than in the best artificial vacuums. Normally when an atom is excited, it emits a piioton within about 10*' second. But the excited states that result in forbidden lines can last minutes to hours. In the laboratory, atoms cannot remain undisturbed that long because they eollide with each other or the walls of the co^itain^er^.

Because Tess energy is needed to excite the forbidden Tines, they are much stronger than ordinary Tines when they can occur at all. Hydrogen is the most abundant clement in emission nebulae, but the f^orbidden lines of oxygen emit the most light.

Just as light from deep-sky objects comes in two types, continuum and discrete wavclcngtiis, so does ligiit pollution. For example, moonlight and ligiit from ineandes-cent bulbs is of the eontinuum type. Fortu-naiely, the ligiit from airglow and most streetlights is at discrete wavelengths in different parts of the spectrum than most of the light ^om nebulae. So the two can be separated with an appropriate filter.

Such filters are known as nebula filters or light-pollution fillers. The term "nebula fillcド' is more appropriate, since views of nebulae are improved most.

The usefulness of such a filter depends on the nature of the interfering light. Table 3.4 lists common sources.

In examining Tables 3.3 and 3.4： we see that most light pollution is at wavelengths diflerent from the light of nebulae. This is illustrated graphically al the Tops of Figures 3.5 to 3.8. Referring back to Figure 2.3, recall that the peak response of the human eye's night vision is near 5000 angstroms (at the color green), or the same wavelength as the strongest nebular line at 5007 angstroms. But there are strong light pollution lines close by, near 4 400 and 5400 angstroms. A filler must cut the spectrum pretty finely to reject these wavelengths and still have a high transmission at 5000 angstroms. This can only be achieved with modern interference-filter icchnology.

Interference filters have very thin layers of partially reflecting material separated by thin transparent layers. Depending on a layer's thickn^esses, different wavelengths of light will undergo constructive or destructive interference. The number of wavelengths that can be controlled depends on how many layers are deposited. Ordinary colored filters are added to block wavelengths far bom the primary transmittance region.

Interference filters can have very high transmittance at very narrow wavelengths. They are ideal for isolating the light of nebulae and have uses in many other areas of science. In the early 19705s the technology to make them was quite expensive, but now they can be mass produced and cost no more than a high quality eyepiece.

What about continuum light pollution? This is harder to deal with. It can be rejected only by making the filter absorb all light not at the wavelengths of the nebula. The wavelength bandpass of an interference filter can indeed be made extremely narrow. For example, filters used by amateurs for observing solar prominences have a bandpass of only 0.7 angstrom at the wavelength of hydrogen alpha, 6563 angstroms. At wavelengths as fhr away as 6500 angstroms, the filter may transmit less than one thousandth as much as

せート」'ゝ

(s ① DE 一 U6BE) 〇 suods ①エ ①--1-①匸

Figure 3.5. The spectral transmission of the Lumicon UHC filter is shown for light arriving face-oil (solid line) and at an angle of about 20° (dashed line). Note how the bandpass (the central peak) shifts to shorter wavelengths as the fflter is tilted, and how the blocking of the light outside the bandpass becomes less eflective. A relative response of -2.0 magnitudes is a transmittance of 16%, -5.0 magnitudes is 1 %, and -8.0 magnitudes is 0.06%. The positions of major nebular lines f?om Table 3.3 and the major light -pollution lines from Table 3.4 are shown above the graph. The dashed pollution lines are natural airglow lines of oxygen. The broad dashed line that peaks around 5800 to 6000 angstroms is the Lucalox streetlamp. Continuum pollution sources are not shown because they affect all wavelengths to some degree. at its peak. Similarly, a nebula filter could be constructed that trans mitted only the light al 5007 angstroms.

However, interferenee fillers have some undesirable charaeteristies. In partieular, they work at the correct wavelength only if the layers making up the filler are exactly the korreet - thickness. Two things disturb the thihkncss of these layers: temperature and tilting of the filter.

As the temperature changes, the layers will expand or contract, and the transmitted wavelength changes accordingly、The narrow-band solar filters are mounted in a small oven to precisely control their temperature. A solar filer's wavelength can actually be scanned slightly by changing the oven temperature.

When a filter is tilted, the transmitted wavelength becomes shorter, the bandpass broadens, and more unwanted light is transmitted. If you have aeecss to a nebula filter, try tilting it and you will see its color change from greenish to purple.

These effects limit the design of nebula filters for practical purposes. All nebula filters are made with bandpasses broad enough so temperature changes have no effect. Tilting the filter, however, does alter its characteristics.

Because stars emit continuum light, when any wavelengths at all are blocked, stars will appear fainter. The narrower the bandpass, the more starlight will be lost. Furthermore, many stars are bright enough to stimulate color perception, so if the filter transmitted only the nebular line at 5007 angstroms (green), all stars would appear ,green. Thus some manufacturers have made filters that transmit some blue and red light so the color balance is closer to normal. Light pollution at these wavelengths is not very great.

Light-Polluti on Lines

Nebular Lines

4000     5000     6000     7000

Wavele ngth (an gstroms)

Figure 3.6. Transmittance curves of the Edmund Deep-Sky Filter (solid line) and Lumicon Deep Sky Filter (dashed line).

Light &om nebulae is also reduced because no filter transmits 100% at any wavelength. At its peak response, it may transmit 80% to 90%, which would reduce the light from the nebula by 〇，1 to 0.2 magnitude even if all the light were emitted in the filter bandpass. Nebula filers work because they increase the contrast between the object and background more than they reduce the light from the object. A nebula filter improves visibility only when the improved contrast outweighs the loss of light. This depends on the eye's eharacteris-tics in the particular situation, the filter design, and the type of the light pollution.

Let's examine some filters and the implications for their use. The spectral response of one of the better nebula filters in common use, the Lumicon UHC Filter, is shown in Figure 3.5. Note that the main bandpass (the central peak) is centered at a wavelength _a . little longer than the nebular lines at 5007 and 4959 angstroms. This may seem like a； mistake, but is a very clever design. When the ■ filter is tilted 20° (dashed line), the bandpass shifts l()a shorter wavelength and the 5007 line still falls within the long-wavelength side of the bandpass.

Light from a telescope is always a converging or diverging cone, so the filter must be able to transmit the strong nebular line over a range of angles. For example, if the apparent field of an eyepiece is 40°, then the light at the edge of the field is tilted 20° from the 〇卩氏乩 axis. If the Lumicon UHC filter is placed between the eye and eyepiece, then all the light from the 5007 line reaches the eye. However, if an Erfle eyepiece is used, any nebula near the edge of the field of view (30°

〇

—6

一 8

4000     5000     6000     7000

Wavele ngth (an gstroms)

Figure 3.7. The Lumicon H-Beta Filter spectral response is shown for light on-axis (solid line), and of 匸 axis (dashed line). Note that a small tilt of the filter shifts the bandpass so the H-beta line at 4860 angstroms is no longer transmitted.

ofPaxis) may be completely invisible because the filter bandpass shifts enough to cut o(F the nebular line.

On the oihcr hand, the light from the edge of the objective even in a fast telescope of f74.5 is only 6.3° from the optical axis. So. if a wide-field eyepiece is used, the filter should be placed between the eyepiece and the objective, not between the eyepiece and the eye.

One observers trick takes advantage of a fiilter's off-axis problem. The filter can be used to help detect a faint object by tilting it back and forth between the eye and eyepiece. The nebula will blink in and out of view.

If we examine other ma1lufact:ursrs5 filters, we see some interesting results. 'The Edmund Scientific Deep-Sky Filter (Figure 3.6. solid line) has a slightly wider bandpass than the Lumicon UHC so that the 4861-angstrom H-beta line is also transmitted when the light arrives o ロ・ axis. However, when the Edmund filter is lilted only slightly, the 5007-angstrom line is lost. Some objects do have their strongest emission in H-beta. The nebula IC 434 surrounding the Horsehead (B33) is a good example. Thus the Edmund filter appears to be a good H-bcta filler. The Meade Instruments Deep-Sky Filter has a spectral response very similar to the Edmund filter, but does not transmit any blue or ultraviolet light.

The Lumicon Deep-Sky Filter (Figure 3.6, dashed line) follows a different strategy. It has a very wide bandpass that transmits light from both the 4861- and 5007-angstrom nebula lines even when the light is substantially 〇］匸 axis. Thus it is good for use with wide-field eyepieces and fast f/ratio tele:-scope:s. Such a wide-bandpass filler will transmit more light from galaxies and star clusters but also more light pollution.

Lighht-Pollution Lines ----1-------1------【・・・・，-•・い*-ゝ“”…’•十i I----1       ..........................

(s ① pn 七 ubEE)① suods ①工 ①-1-①工

Wavele ngth (an gstroms)

Figure 3.8. rhe Daystar Nebula Filter transmission on-axis (solid line) and about 20° ofFaxis (dashed line).

The Lumicon H-Beta Filter (Figure 3.7) has a narrow bandpass designed for use on objects that emit most of their light at 4861 angstroms. But when light passes through this filter only slightly ofF-axis, the bandpass wavelength becomies too short to transmit much of this light (dashed line). This fflter is only good for use between the eyepiece and the objective and should not be used even for casual observations between the eyepiece and eye.

The Daystar Nebular Filter (Figure 3.8) acts similarly to the Edmund fflier, but the bandpass is at a slightly longer wavelength, the tranimittanec is higher at the center of the bandpass, and the ofF-axis blocking is as good as on-axis. These factors are probably why some observers declare this fflter better than the Lumicon UHC fflter in actual observing practice, though the author believes them to be very elosc.

The manufacture of nebula Alters is so new that different ones may be available by the time this book is published. The factors discussed here, however, will help in evaluating any of them. Finally, observers about to make a purchase should remember that the difference between even the best nebula filter and none at all is usually rather subtle.

USING THE TELESCOPE TO FIND OBJECTS

So far this book has discussed observing techniques. But the best deep-sky techniques are useless unless the objects can first be found.

In order to find anything in the sky, you must become familiar with the bright stars and constellations - just as when you drive into a new city you get oriented by major landmarks. Once you're in the right celestial neighborhood, a detailed map is required for locating small, faint objects. Constellation guides and star atlases are listed in Appendix A.

Celestial coordinates

The stars appear fixed on an imaginary celestial sphere, which is always centered on the observer. The celestial sphere appears to rotate once every day because we observe it from the rotating Earth. Lines projected from the center of the earth through the north and south poles extend to the north and south; poles of the celestial sphere. The stars appear-to rotate around these poles, as was known in ancient times.

Astronomers have set up a coordinate system (or describing positions on the celestial sphere similar to latitude and longitude on Earth. The Earth's latitude lines: arc projected outward from our globe onto the sky to become the lines of declination. The celestial equator, straight overhead at the Earth's equator, defines 0° declination on ■ the celestial sphere. The north celestial pole is at + 90° declination, the south celestial pole at -90° declination.

"Longitude'' on the celestial sphere 也, called Right Ascension. Longitude lines are a little more difficult than latitude to prefect onto the sky because the stars are constantly rotating across the longitude lines on the Earth. For the 0° point astronomers have chosen the place at which the Sun crosses the celestial equator from south to north on its yearly journey around the sky. This point is called the First Point of Aries (though it is now in Pisecs because of a slow movement of the coordinate system over the known as precession).

Imagine aiming a telescope at thc First Point of Arics and keeping the telescope fixed with respect to the earth (on a motionless mount). As thc Earth turns, the right ascension of stars crossing thc view increases. Right ascension is commonly measured im hours, minutes, and seconds of time instead ■ of degrees, minutes and seconds of arc. There are 24 hours of right ascension just as there are 360 degrees of longitude. (Therefore, a minute or second of right ascension is not thc same as an angular minute or second of arc. Each is 360/24 or 15 times greater than the correiponding unit of longitude.)

Note that as declination increases (pos i・ tivc) or decreases (negative) ftom the celcs-tial equator, the right ascension lines become closer together. They converge at the poles.

Since the Earth's axis is tilted with respect to its path around the Sun, the apparent track of the Sun on the sky is north of thc celestial equator for half the year and south for half The sun's apparent track on the celestial sphere is called the ecliptic, and tilted 23.5° to thc celestial equator. The planets generally follow the ecliptic too because the orbits of all except Pluto are inclined only a few degrees from the plane of the Earth's orbit.

The line on the celestial sphere that passes through the pole and directly overhead, and extends to the north and south points on the horizon, is called the meridian. The point dircetly overhead is called the zenith.

The Sun is at 6 hours right ascension at the beginning of summer in the Northern Hemisphere, so right ascension 18 hours (the opposite side of the sky) will be on the meridian at midnight

At the beginning of autumn in the Northern Hemisphere, right ascension 〇 hours is on the meridian at midnight. At the beginning of winter, 6 hours R.A. is on the meridian, and at the beginning of spring,12 hours.

With these facts, and a little study of the approximate right ascensions of the cogstcllatlogs, you should be able to figure out what cogstcllatlogs are up at night any time of the year.

For example, say it is 9 p.m. on August 20; what constellations are on the meridian? Consider that one month later is the bcglgglgg of fall, when 〇 hours R.A. is on the

Figure 39 The celestial sphere. The observer is aligning the equatorial mount's polar axis by making it parallel to the Earth's axis. (Courtesy of Sky & Telescope.)

meridian at midnight. Since there are twelve months in a year and 24 hours of right ascension, the constellations move 2 hours per month. Counting 2 hours backwards from 〇 hours (the same as 24 hours), we find that on August 20, right ascension 22 hours should be approximately on the meridian at midnight. Three hours earlier the right ascension would be three hours less, so at 9 p.m., right ascension 19 hours should be on the meridian. A star map shows that constellations near RA 19 hours include Lyra, Aquila, Sagittarius, Corona Australis, Telescopium, and Pavo.

Precession

The Earth is spinning like an ofFbalance top in that the direction of its axis is very gradually changing. Thus the stars are gradually changing their declination and right ascension. The direction of the Earth's axis follows a slow cyclical motion, with a 26,000 year period, called precessiort. Thus, star charts need to be updated with a new coordinate grid about once every 50 years. Since precession is very predictable, the coordinates for a ,given year can be changed to those for another year by putting them through mathematical formulae.

Whenever a coordinate is given for an object, the year is specified when that coordinate is correct. This time is referred to as the epoch more precisely, the equinox. Most coordinates are currently given for equinox 2000 if the object is "flxedT like a star, or for the actual date (e.eg 1988.5) if the object is a short-term phenomenon like a comet.

The following simplified formulae for precession are reasonably accurate. If RA and Dec are an object's right ascension and declination (RA expressed in decimal hours and Dec in decimal degrees), the yearly changes ARA and ADec in these coordinates are given by:

△ RA = 0.051 +

0.022 sin(15RA)

(equation 3.10a)

ADec = 0.33 cos(15 R.A.)

(equation 3.10b) where ARA is expressed in minutes、t)i time and ADec in minutes of arc. Multiply by the， total number of years desired, negative if going back in time, and apply the corrections to RA and Dec. More complex formulas must be used for stars near the poles, but the ones given here work well in most of the sky.

Many modern observatories have very precise positioning capabilities, and dhangs coordinates to the epoch of the date and time the observation is made. By adding farther corrections such as for the flexure in the telescope mount and atmospheric refraction, the telescope can be aimed to within a couple of arc-seconds of an object.

Such modern technology has led to a generation of professional astronomers who do not need to know where something is in the sky. If they know its coordinators, a computer tells when it is observable and points the telescope right at it. Amateur astronomers are now beginning to use such technology too. But even a totally automatic telescope must first be accurately aligned on the celestial pole for the system to work, and that can only be done with a knowledge of where things are in the sky.

Furthermore, "flying on instruments" takes much of the fun out of skywatching, and leaves the viewer helpless in the face of problems. Most amateurs recommend against it ffor beginners, who need to learn the sky for themselves. Once the brighter stars and major constellations can be identified, the fainter objects that cannot be seen with the unaided eye can be found by using one of the following methods.

1 Star hopping

Star hopping should be learned by all amateur astronomers. It is the method of starling at a known naked-eye star and using a chart to move carefully from star to star until the desired object is reached.

The best way to plan such a search is to draw a circle on a piece of clear plastic representing the field of view of the finder telescope at the correct scale fbr the star chart. A wire ring the right size works just as well. The search route can be tested by sliding the circle across the chart. The route is then duplicated on the sky. Examples of star hopping are illustrated in Figures 3.10a through e.

THE EYE AND T1IE TELESCOPE

2. USng setting circles

Setting circles are graduated scales on a tele-icoped equatorial mount. They allow the telescope to be positioned on an astronomical object by "dialing in" the object's coordinates. This is not as simple as it sounds, and it should not be considered a substitute for learning to itar-hop,

Thc first step in using setting circles is to align the mount's right-aiccniion axis (alio called the polar axis) so it points to thc celei-tial pole, One method is to insert a special ffndcr telescope right into the mount's polar axis、Such ffnderi have a special reticle that shows the star pattern at thc celeitial pole. It is a simple matter to align the star pattern on the reticle with that on the sky. A few manufacturers offer such ffnderi inside their mountings.

A second method of alignment is to chosc two stars at the same declinati on about 6 hours apart, with one on the meridian. The telescope is aimed at one star and then swept to the other; the mount is adjusted until the telescope can bc made to point to both stars by only moving in right asceniion (that is, with the declination axis locked).

For example, consider one star on the meridian and one 6 hours further west. If you center on the first and try to move to thc second, any error will bc largely due to an alignment error in azimuth (the polar axis is pointed to one side of the pole). If the telescope ends up pointing south of the second star, then turn the entire mount until the star is centered. Now move back to the star near the meridian. This move is sensitive to an alignment error in altitude (the polar axis points too high or low). If the star is not centered, then move the mount in altitude to correct the position. Theoretically the mount should now be aligned, but repeat the procedure to check.

The accuracy of polar alignment you need depends on how accurate your setting circles are, what your field ofview is when searching for 0bjects, and how closely you want them centered. The accuracy requirement for visual observing is much less severe than for astrophotography. If your field of view is about 1°, and the setting circles are graduated

Figure 3.10a. How to find the open cluster M41 by star hopping from Sirius in the constellation Canii Major. The circles show a typical finder telescoped 6° field of view. First center Sirius in the ffnder, then sweep 2/3 of a (inder field south and a bit east, keeping track of star patterns along the way. (East is always the direction of increasing right ascension.) M41 should be visible as a faint, sparkling haze. Chart reproduced from Sky A lias 2000.0 by W l irion.

every 1° in 4x11131:101 and 4 minutes in right ascegsiog, then the polar allgimeit should be better than 1°. But it probably does not need to be better than about 1/2°. If, on the other hand, you wish to center small planetary nebulae at high power in a 10 arc-mliute field of view, you need big, observatoryquality setting eirelcs that can be read to about 2 arc-mliutes, aid the aligiment should be better than about 5 arc-minutes. Such allgimeit will take time aid better methods than described here, aid is probably got feasible with a portable iistrumeit.

Oice the mount is aligned, the setting circles must be set to read correctly. This is easily done by ceiterlgg the telescope on a

Figure 3.10b. Flidlig faint NGC 253 in the coistellatloi Sculptor is more dlfllcult than f idlig M41. The galaxy is in a star-poor region only about two degrees from the south galactic pole. The brightest star in the region is Beta Ceti,7° to the north aid slightly west of NGC 253.

Start by placlig Beta Ceti fg the center of the field of view. (You must know the coistellatlois well enough to locate Beta Ceti!) Just outside the southern edge of the 6° ffeld is a pair of 5th-magiltude stars. Find them aid center them. Now NGC 253 is just south of the field of view aid a little east.

Move south until the two 5th-maggiitude stars are just outside the north edge. NGC 253 should be just east of ceiter. Moving a fractloi of a degree east should put NGC253 in the ceiter of the feld of the main telescope as well as the fi ider. The correct spot can be estimated by comparing the positions of the faiit stars in the fider with those on the chart. Chart reproduced from Sky Atlas 2000.0 by W. Tlrloi. star whose coordinates are known and then moving the setting circles until they read the correct position.

If the polar alignment was not adequate, you might have difficulty ffnding objects. One way to circumvent this problem when observing in a localized region of the sky is to aim at a star in the region and turn the setting circles to read that star's coordinates. This way, any error in pointing nearby is minimized. When another area of the sky is to be observed, the setting circles would need to be reset using a star in that area. The time spent resetting the circles takes away from precious observing time.

Note: the right ascension circle must be carried along with the telescope by a clock drive, or it will give false readings just a minute or two after being adjusted to the coordinates of a known star.

In this author's opinion, setting circles should not be used except on permanent mountings by very experienced astronomers. The beginning amateur needs to learn the sky, and the learning will be impeded if setting circles are used. Many amateurs find that they remember object's positions among the stars (but not their coordinates) after only a year or two of active observing, and can position a telescope on one of those objects before most people would have time even to look up the coordinates. While examining a

Figure 3.10c. M5 1,the Whirlpool Galaxy, is a bvorite of amateur astronomers. Finding it is not too diHicult as it lies near the end star of the Big Dipper (Eta Ursa Maj oris). Start by centering the finder on that star. Move west nearly half a finder ffeld to center the 5rh-magnitude star 24 Canum Venaticorum. Now move south about a third of a field and west a little. M51 is just west of a 7th-magnitude star. Note the many other galaxies in this region, and how one would star hop to them. The brightest galaxies are the Messier objects: M51, M63, M94, and M101. Chart reproduced from Sky Atlas 2000.0 by W. Tirion.

star chart to star hop to an object, the amateur often sees other objects nearby worth searching out too. And who knows? By constantly searching for objects and comparing views to star charts, novae, supernovae, comets, or asteroids may be discovered.

FINDERS

The finder is a small low-power, wide-field telescope attached parallel to the main telescope. The main telescope has a very small true field of view, usually less than 2° and often less than 1/2°. A wider-field telescope is essential to point such a tiny field at something 一 otherwise you'd be searching for a needle in a haystack with a microscope.

Many telescopes come with finders too small and cheap for easily locating deep-sky objects. For star hopping, the finder must have a wide enough field of view to easily be pointed at objects (stars) seen with the unaided eye・ It must also have enough magni-

Figure 3J0d. Another way to locate objects is by ^ttoflsetting:ⁿ moving the telescope a known amount of right ascension and declination from a bright object to a faint one. Use the known size of your finder's view (in this case 6°) as your measuring device. M8. the Lagoon nebula in Sagittarius, is a favorite target of amateurs in the Northern-Hemisphere summertime. Offsetting to M8 and many other objects in the area can be done by first locating the "Teapot" of Sagil-tarius, outlined here. In dark skies, the Milky Way star clouds near the galactic center appear like steam rising fi*om the spout cf the Teapot. Start at the Teapot's top, Lambda (X.) Sagittarii, and move north 1°. (In oflset-ting it is usually best to do the shortest direction first because any distance errors will be minimized.) Next move west (the direction of decreasing right ascension) by 5.3°. M8 will now be centered. Chart reproduced from 旳 Allas 2000.0 by W. Tirion. ffcation to accurately aim at the object meant for viewing in the main telescope. A good finder is essential. If you can't find an object, what good is the main telescope?

A good ffnder has the characteriitici of good binoculars. The smallest binoculars really useful for astronomy are 7X 35 (7 power and 35 millimeters aperture), so a 7 X35 ffnder is adequate only on the smallest telescopes, such as 2.4-inch refractors. Most amateur telescopei should be considered undcrequipped if they have anything less than an 8x50 finder.

A finder also needs to be of high optical quality. Since ma ny of the brighter deep-sky objects appear as fuzzy stars at low power, real star images should be very sharp to help distinguish them.

The magnification of thc finder is important 100、If the main telescope is to bc positioned very accurately, thc magnification of thc ftnder should be at least one tenth that of the main telescope. If we call the magnification of the main telescope divided by that of the ffndcr the Finder Magnification Ratio (FMR). then the FMR should be less than 10. An FMR less than 5 is even better.

For example, suppose identical eyepieces were used on the main and finder telescopes. The apparent fields would bc the same, but the true field of view of the higher power main telescope would be much less. If the fnder has a power of 7x and a 7° field of view, and the main telescope has a power of 70X, the latter has a 0.7 ° field of view. An object must be placed within one twentieth of the ffnder's field of view from the center in order for it to appear in the field of the main telescope at all!

But the ffnder should be able to do better than place the object just anywhere within the main telescope field. It should really put it no farther than half the field of view (0.35° in this example) from the center. That is an

Figure 3.10e. In another example of offsetting, Ml 5, a globular star cluster, can be easily found northwest of the 2nd-magnitude star Epsilon Pegasi. Move north 2.3°. then west 3.4°. Note that Ml 5 is about 1/3° west of a 6th-magnitudc star. Chart reproduced from Sky A lias 2000.0 by VV.

Ti rion.

error of only one quarter the field diameter, or 10.5 arc-minutes for this ease.

A 7-power finder can do this if it is accurately aligned with the main telescope and has thin cross hairs. (An 8-inch focal length finder objective and a 7X eyepiece would require cross hairs 0.01 inch or less in thickness —a reasonable value.) But now consider that after an object has been acquired, the magnification may be increased to 150X or 200X. If the object is lost from the field of the main telescope, where the true field is now only 20 or 15 arc-minutes, it would be very diflicult to reacquire the object. A low-power eyepiece would have to be reinserted in the main telescope and you'd have to start all over again.

Because this book advocates observing at magnifications well above 1OOX on some deep-sky objects, a relatively high-power finder is also a ,good idea. Higher power, however, means a redueed field of view, and that can hinder comparisons with star charts. The field of view must not be less than about 5° or this becomes a real problem.

The solution to this dilemma is one of two: 1 a finder with exchangeable eyepieces, so that wide fields can be used for finding an object and higher magnification for precise aiming, or 2) two finders, one low power, one higher. Option 2 has the advantage that the higher-power finder could also be of ,greater aperture, and then very faint objects bceomc easier to find. Disadvantages are the added expense and weight.

The ease of locating objects with two finders, a small one of low power and a larger one of higher power, cannot be overstressed. For many years I had a single find ef on the 8-inch Cassegrain used for most of the drawings in this book. It is far better than most, because it has a 60 mm objective of 300 mm focal length (f?om an old telephoto lens) and a war-surplus Erfle eyepiece of 38 mm focal length and a 65° 'apparent field of view. The combination gives a breathtaking 8° field on the sky and 7.9X. The cross-hairs are graduated in 1° increments for easy star-hopping. However, after much of the research lor this book was completed, and I was using magnifications consistently higher than in my early years of observing, I added a 3.1・inch, "7.9 refractor. With a 20 mm Erfle, this telescope gives a 2° field of view at 31 X.

The ease of getting the general region of the object in the low-power finder, then moving to the 3-inch and almost always seeing the object and eentcring it, and then moving to the 8-inch, makes the added cost well worth it. Except for a very few objects, the 8-inch is used at magnifications of 117X and higher. The FMR ratios are then only 3.9 from the 7.9X finder to the 31 X finder, and only 3.8 from the 3 1 X finder to the 117x of the main telescope. The 8-inch would have to be used at a magnification greater than 310x for the FMR ratio to be greater than 10, but higher-power eyepieces can also be used jn the 3-inch refractor at such times. In addition, the 3-incli is a richest-field telescope, giving some marvelous views of the sky not possible with the 8-inch Cassegrain.

With these thoughts in mind, Table 3,5 was drawn up to recommend finder configurations.

MISCELLANEOUS TOPICS Caring for optics

If the optics are to perform to their limit, they must be clean and free from scratches. This is true of eyepieces, finders and the main telescope. All optics get dirty with use, and they must be cleaned with ,great care.

The worst problem is invisible: fine abrasive dust that will scratch suriaices during cleaning. Before anything else, very gently brush ofr the surfaces with a earners hair brush (which like other cleaning items is available at camera shops). Some companies sell cans of pressurized ,gas for blowing dust off surfaces such as lenses and mirrors, but beffore using one of these be sure it says it will not leave a residue and is safe for optics.

To clean small lenses such as eyepieces, the next step is to use lens-cleaning paper (obtainable at most camera stores) or sterile cotton swabs and a mild soap, alcohol, or lens cleaning liquid (also obtainable fom some camera stores). Use only ,genuine sterile surgical cotton, not synthetic "cotton" balls. The glass should only be patted and not rubbed because any rubbing action will tend to scratch dust across the surface.

Mirrors can be cleaned the same way, but the aluminum coating is extremely easy to scratch, so the utmost care is required. If the mirror is removed from its cell，it can be

immersed ii lukewarm water and mild, unscented pure soap for cleaiing. First run lukewarm tap water over the mirror for several minutes to dislodge dirt on the surfacc. Then soak it in a plastic tub or sink full of lukewarm soapy water. (Place a towel in a sink to cushion the mirror if' it is dropped.) Now swab the mirror very gently uinderwater with sterile cotton. Turn the wad of cotton in a backward-rolling motion so that as soon as part of it rubs the surface, it is carried away aid won't touch it again. When the wad has been completely turned, throw it out and use a new one. Rinse by ruiiing tap water over the mirror again for several miiutes. Finish with a riise of distilled or defoiized water (this does not leave stains), staid the mirror on its side on a towel, aid let it dry. If your tap water coitaiis many imp uritie.s, you should use distilled or deionized waler for the whole process.

Eyepieces should be stored in a safe case. Some companies sell eyepiece cases, or camera cases can be adapted, or you can build your own. Most cases are foam liied, and many anialeurs simply place the eye pieces in slots or holes in the foami. Such practice can damage the eyeplec e if ' aiy lens surface touches the fioam. Thc extreme case of damage would occur if the case was exposed to suilfght; the foam cai melt 0110 the lens! The eyepieces should have sonic form of lens cover, like a plastic cap, be (orc being placed in a foarn box. Or keep them in plastic sandwich bags.

Dew

Dew is extremely hazardous to optics. Repealed dewings can turn mirror coalings brown because when dew settles, dust settles with ft. The presence of waler cai also start a chemical etching process by impurities.

A telescope brought into a warm house from the cold will immediately dew up. The optics should be scaled before eitering the house so that little water will condense oi them, aid after reaching room temperature, the seals should be looseied so aiy trapped moisture cai escape ゝ

Dew cai also form on a telescope while it is being used outdoors, fogging the view. This can be preveited to some degree by having a long tube fi ftroi of the objective, about twice as long as the tube's diameter. Reflectors oily dew in the severest conditions, because the mirror is al the end oi ' a very long tube. Fiider telescopes should have a long shield ii front of the objective to preveit dew as well as to keep out stray light. Eyepieces are more dlfllcult to keep from dewing up, especially since they are gear the observer's breath. Eyepieces should got be left uncovered, but pul ii a protective box when not in use.

In really wet conditions, the box could be heated with two to three watts to drive off dew. The objective can also be kept dew free by placing one to three watts of heating elements around ft. Such heaters can be made with small resistors or resistive wire and a low voltage source such as a car battcry. (Do not use the car's own battcry because after heating all night it might not have enough power to start the car.)

SUMMARY

The purpose of a telescope is to gather light and focus it to form an image. An eyepiece magnifies the image and directs the light to the eye, whcrc it is refocused on the retina. Two basic parameters govern a telescope's ability to show an object: the amount of light gathered (determined by the size of the primary mirror or lcns) and the total magnification at the eye.

The magnification and the eyepiece's apparent fcld of view determine the true field of view on the sky. A short-focus tclcscopc (i.e. f/4.5) and a long focus instrument (such as f/15) of the same aperture will give essentially identical views of deep-sky objects when used at the same magnification with cycpicccs having the same field of view. The short-focus tclcscopc has an advantage in that it can achieve low powers and wide fields with standard eyepieces. The long-focus telescope has the advantage when working at high powers, as they can bc achieved with cycpicccs having reasonable focal lengths and comfonable viewing positions for the eye. Optical aberrations are also reduced.

The differences among telescope types are not as important for viewing deep-sky objects as for planetary or double-star observing, since most of the limitations for deep-sky work are in the human eye.

Even in perfect seeing conditions, an upper limit to thc useful magnification is set by the resolution of both the telescope and the cyc. The cyc can rcsolvc about one arc-minute when an object is bright, but resolution decreases as the object becomes fainter, and at the limit of detection, the cyc's resolution is only about 0.5°. In this situation very high powers are necessary to magnify the fnest details a telescope itself resolves so that they become visible to the eye.

For bright subjects, the magnification limit is oficn said to be about 60x per inch of thc telescoped objective diameter. (A SO-inch telescope is said to have a limit of6()0 power). However, for faint objects the limit is as much as 330X per inch. The lesson to remember is to use however high a magnification seems to work best.

Special flters can partially reject both natural and manmade light pollution. These nebula flters work by increasing contrast between certain objects and the sky back ・ ground. Even though the Alters decrease the light of all objects, if thc background is reduced cvcn further, thc improved contrast may more than compensate, and the object will actually appear brighter to thc eye. Nebula filters work best on emission and planetary nebulae, rather than on stars, galaxies, and reflection nebulae. The filters -generally should be used between the cyepicce and the telescope objective, because many such filters do not work well if thc light is. more than a few degrees o(Faxis. The effectiveness of filters from different manufacturers varies considerably, so try to use several in actual observing sessions, and choosc the best one to purchase.

For finding objects in the frst place, thc best method for thc amateur to learn is star hopping. First thc brightest stars must be learned with the aid of an all-sky map or planisphere. Then, using a detailed star atlas, thc telescope’s finder is pointed to a known naked-eye star. By using thc patterns on thc star chart, the finder telescope, along with the main telescope, is moved step by step to the object of interest.

The finder should have an aperture of at least 35 mm for main telescopes of 3 aperture or less, and at least 50 mm (2 inches) for teleskopes with apertures over 3 inches. The ffnder magnification should be no less than one-tenth the magnification used on the main telescope. Otherwise the main telescope cannot be pointed accurately. Amateur telescopes larger than about 8 inches Should have two finders, the second with an aperture and magnification midway between the small finder and thc main telescope.

4

The faintest star visible in a telescope

INTRODUCTION

The faintest 卩 oint of light detectable by the unaided eye was derived in Chapter 2. The eye's fundamental limit is around 5() to 15() photons of,green light arriving over a several-seeon(l卩 eriod, corresponding to a star as flint as magnitude 8.5. Seeing such a faint star requires perfect conditions and dark adaptation as well as exclusion of all extra nc()us light, including all other stars in the sky.

Thus it's not sur 卩 rising that no one sees 8.5-ma 卩 niludc stars with the naked eye in real life. A more typical limit is 7 or 7.5 for a skilled observer in excellent country skies.

Stars and other small objects form a special case for detection by the eye in the telescope. As we have seen, any object less than about 0.5° across as presented to the eye can be considered a point source if it is so dim it's at the threshold of detection. A star image is actually a ditrraction disk, but it is so small that, if faint, the disk is a point to the eye at any reasonable magnification at all.

Magnification does not change the brightness of a 卩 oint source in the telescope, but it does decrease the surface brightness of the background and reduces the field of view so other stars do not interfere. ThereGre： the fundamental limits of the eye can be reached when a telescope is used. Here it really is possible to see the equivalent of 8.5-magnitude stars naked-eye.

MAGNIFICATION

The fundamental magnitude limit B , of a telescope is given by

,叫=仏 + 2.5 log1o(が b / Q/)

(equation 4.1) where is the eye limiting magnitude (8.5 for the ideal case), D is the tc1cseope diameter. Dc is is the eye diameter, taken to be 7.5 millimeters, and t is the telescope transmission factor (which is usually about 0.7). This equation reriuces to

Mr = 3.7 + 2.5 logio (ひ') (equation 4.2) where 1 is expressed in millimeters. This formula was used to list the ideal limiting magnitudes for telescopes of various apertures in Table 4.1.

The surface brightness A/_tj of lhe sky or、an extended object (in magnitudes) is darkened by the telescope magnificalion and transmission (actor as follows:

Mb = -2.5 log］。(D勺 inD?) (equation 4.3) where m is the magnification. This is why magnification helps to detect faint stars when the sky is bright, or even under dark country skies compared to when low power is used. High magnification also increases the apparent angle between Geld stars.

Consider a dark country sky with a surface brightness of 24 magnitudes per square arcsecond. At the minimum usable magnification. computed from the equation

m_m = D / D®                 (equation 4.4)

which is 27 X for an 8-inch telescope, the sky brightness is redueed from its naked-eye level by

.Arn = -2.5 logt0(/)：             (equation 4.5)

or if / is ().7, 0.39 magnitudes / sq. arc-second、

Examining Figure 41.which shows the faintest star detectable by the eye, we find that at a sky background of，24.4 mag./sq. arc-sec., the naked-eye limit is 7.6. Using this value for B B , in equation 4.1 we find a limiting magnitude of 14.4 for an 8-inch telescope al its lowest usable power of 27 X.

Table 4.1 Ideal limiting magnitudes

Note: Large refraetors have brighter limiting magnitudes due to absorption in the thick objective glass.

Computed from equation 4.2

For higher magni^cations, equation 4.3 gives the reduction in sky brightness. For example, choosing a power of 90X and (—0.7 in equation 4.3, we see the sky background is 3 magnitudes fainter than that at 27X! Thus, the background is reduced to 27 magnitudes/ sq. arc-second, improving the cyc limit, A/,., to its best possible value, 8.5. The magnitude limit is then 15.2 in the 8-incli telescope, an increase of 0.8 magnitudes over that seen at 27 power.

A more dramatic example can be sccn byobserving stars in a suburban sky with a surface brightness of 2() magnitudes/iq. arc-see., where the naked cyc limit is 5.5. Al a magnification of 27 X on the 8-inch, stars of magnitude 12.5 would be just detectable. If the magnification were increased to 20(), the sky surface brightness in the telescope would bc reduced to 24.7 magnitudes / sq. arc-sec. and stars as faint as 14.6 would be within view. At 57() power, the sky suriace brightness in the telescope would bc reduced to 27.0 magnitudes / sq. arc-sec. and the 8-incli would reach its limit oi' magnitude 15.2.

This would bc true even in bad seeing, because a very faint star image blurred to a diameter of three arc-seconds is still a point as iar as the eyc is concerned, even at high magnification.

Al the very limit of detection, there must be no other stars within at least a couple of degrees (as sccn by the cyc) of the star being scrutinized. Greater distances are required if thc neighboring stars arc bright.

FINDING TONIGHT'S MAGNITUDE LIMIT

Because atmoipheric seeing docs not a flCct visibility of faint objects at the cyc's threshold, thc deep-sky observer s main mea・ sure of sky quality is merely the sky surface brightness. However, this is difficult to determine. Instead, observers traditionally take note oi' the faintest star visible to the naked cye.

'1-he atmosphere itself absorbs light and thus ailccts the faintest star observable. This atmospheric extinction inkrcases farther from thc zenith, as deicribed in Appendix C. 'rhus, when thc faintest star visible to thc naked cyc or in a telescope is determined, its altitude above the horizon should bc factored in 一 at least if the altitude is less than about 60°.

Making such a sky-quality estimate requires adequate star charts w让 h visual magnitudes recorded. For naked-eye determinations many charts arc available, some of which are listed in Appendix A.

For telescope work, open star clusters provide many stars in one view for easy limiting ・ magnitude determinations. Appendix B pnい senls a number of clusters around the sky for this purpose. They wcrc plotted f?()m data gathered by Hoag et aL (1961)and are shown in two charts per cluster: one with stars only, and on the facing page, the cluster with visual magnitudes given to the tenth with tlic decimal point omitted (129 is magnitude 12.9). Also given are thc platc scale and the viewing distance from the book to show how the cluster will appear at various magnificationi. Sparsely populated clusters were ehoscn so brighter field stars do not interfere. Clusters that have very bright members, such as thc Pleiades, arc not ink1uded since these interfere severely with viewing thc fainter members. Table 4.2 lists the star clusiecs in Appendix B and the magnitude range of thc stars plotted on the charts.

Unfortunately，open k1usters arc not evenly distributed around the sky, but are eomeen-tratcd near thc Milky Way. This caused -gaps in the Hoag study, su. ch as a lack of clusters between 7.5 hours and nearly | 8 hours right asceisioi. Clusters do exist ii this right ascegsioi range, but they are far south and were got measured ii the Hoag study. But at least oie of the clusters should be visible at some time of night each night of the year.

Ai example of using the star clusters to monitor site quality is shown in Figure 4.2. I waited to find the site closest to Denver, Colorado' that could be considered free of light pollution. The star cluster NGC 6910 was chosen for the test. Sites were selected at dlffereit distances from the metropolitai area that were uicoitamiiated by smaller, closer towns. 4'he best skies were found oily at a dlstaice of 70 to 80 miles away in the Rocky Mouitaiis. A site over 200 miles from Denver produced the same result as one at 80 miles, though there were always small towns at distances of about 5() miles. The theoretical limit was never reached, possibly because of these small cities, or because of atmospheric dust or airglow during the season of observations (summer), or more likely, the telescope traismissioi factor is closer to ().5.

〇

5         10         15        20        25

Backgr•〇 und Surface Bright ness, B_o (mag nitudes / arc-sec²)

Figure 4.1. The faintest star that can be seen by a trained observer depe nds oi the background surface brightiess of the sky. A good dark country sky has a surface brightness of about 24.4 magiitudes per square arc-secoid, giving a visual limit of magnitude 7.8. High magiiflcatioi on a telescope cai reduce the background below 27 magiitudes per square arc-second, allowing the eye to reach its fuidameital detectioi limit given in Table 4.1 Detection of faint stars in a nebula is more difTicult since the background (surrouidlig the star) is greater than that of the night sky.

Table 4.2. Star clusters illustrated in Appendix B for faintest-sita^r determination

Position (epoch 2000.0) ----------------------------- Magnitude Object                    R.A.                 Dec.                  range

All data from Hoag et a.(1961).

Dista nee From Den ver (miles)

Figure 42 "The faintest star visible at diflbrcnl distances from the Denver metropolitan area, population 1.8 million.

SUMMARY

The faintest star visible li a telescope depends on the aperture, power aid sky brightness. H igh power reduces the surface brightness of both the sky aid the star image (slice the star is a diffraction disk in the telescope). However, since the eye sees fkiit star disks as points, the surface brightiess of a fhiit star does not appear to change with increasing magnifeatioi. The background surface brightness does visibly darken, aid so faint stars become easier to sec.

The telescope, with its magnified aid restricted field of view, can be used to reach the eye's fuidameital detectioi limit, something a naked-eye observer can never achieve. This ideal condition is reached when the background is darkened suficleitly by magnification so that the faintest star possible can be seen. This cai be done whether observing li superbly dark country skies or ii city skies with Tight pollution. Similarly, faint stars li a nebula can be seen more easily at higher magnifications because the surface brightness of the nebula is reduced.

The faintest star observable gear the zenith with a specific telescope aid magnification is a fuidameital parameter documeitiig the conditions for observing deep-sky objects.

5

Making drawings and keeping records

INTRODUCTION

Anyone who observes the sky should keep a logbook. Even the most casual celestial sightseeing becomes more mearingf1i1 if a few notes are jotted down in a permanent record. For more serious observing, complete notes are essential. They document the viewing conditions, provide a record of what was actually seen, and create a baseline for comparing future observations and gaining perience. And, of course, they will form a rich personal diary of starry nights.

Good records sometimes serve purposes unanticipated at the time. Finding a supernova in a galaxy, or an unexpseted asteroid or comet, are examples that come to mind.

Amateur observers often just list the ob-)cets they see during a night. This has little value unless you just like to compile lists and figure out such tilings as how many galaxies you have seen. The next level of sophistication is to note the date, time, magnifieation and telescope used. This still has little value except to list-compilers.

At the next level, the observer might write a few comments such as:

Such a list has only a little more value. What is "easy?" Spiral structure means different things to diflerent people. <Barely seen" does not tell what it actually looked like. Was it elongated? Were stars visible through it? Which parts of it were seen?

I，he list above is typical of some of my early records. In preparing this book, I was very disappointed to discover their lack of usefulness. Back then I also made many drawings a half inch to two inches wide, but rarely included field stars. Thus there is no way to tell the scale of a drawing or the size of what was seen. Is that fuzzy patch tlie full extent of a galaxy including its spiral arms, or only the bright central region? A couple of field stars would have allowed comparison with photographs to ffnd out.

Even longer descriptions can seem adequate on future reading. In soome of my more exciting moments under superbly dark country skies, I wrote ，Just like a photograph", and "so spectacular and complex that a true drawing cannot be made.⁵' These comments recall fond memories but serve little other purpose.

To Coeumsnt the view fully, records must include the sizes of objects, the shapes and relative brightnesses of all their parts_T and their placement with respect to stars. This chapter outlines ways to do so - either in writing or by one of three methods for making drawings.

WRITTEN DESCRIPTIONS

Any record of a sighting should include certain basic data: the observer, date, start and ending times of the observation (be sure to state the kind of time and date, such as Universal ，Time or Mountain Standard Time), the observing site, the eyepieces and their magnifications, a sky quality note such as the faintest star visible, wliethcr a clock drive was used, and tlie approximate altitude or air mass of tlie object. (See Appendix C for a discussion of air mass.)

A written description should include estimates of the object's size, the direetlons aid distaices of its details from refcrcncc points, and the details' relative briglitnesses. Ai example:

NGC 4565 Edge-on galaxy in Coma Berenices 5/11 83 Start： 10:00 じT

8-inchJ/11.5 Cassegrain     Location： Waianae Ranch,

Hawaii

Eyepieces: 20mm Erfle (111X)124mm Erfle (188X)

Faintest star at zenith: about magnitude 143 at 188X. Obsener： R. Clark.

This object was haM to find, being in a star-poor areaゝ Once found, the view was amazing / I had seen the galaxy in poorer skies in the      1970s but did

not think the dork lane was visible with an 8-inch. Now the dark lane could be seen at 117X and 188X by several observer easily (no averted. vi^：^^ton).

The galaxy is about 11 arc-minutes long〉exte^^d-ing northwest to southeast. The boundaries of the arms are J airly sharp, except at the lips where they fade slowly into the sky background. 丁he central hub is oval (about 2 by 1.5 arc-minutes); the long axis is aligned with the arms. The edges oj the hub fade fast slowly, then quickly into the sky background. Best view is with the 12.4 mm Erjle at 188X. in which the galaxy extends over the whole field.

The dark lane is about 4 to 5 arc-minutes long and about 0.3 arc-minute wide. The inner nuclear region (the central 025 arc-minute) is much brighter and lies southwest of the dark lane.

On each side oj the nucleus in the long direction is a bright spot about 〇'25 arc-minute in diameter. Another bright spot was glimpsed occasionally about 4 arc-minutes from the nucleus along the galaxy's long axis.

A star lies 1.5 or 2 arc-minutes noorheast of the nucleus. It is janter than the nuclear region, probably 12th or 13th magnitude. A second star, a little fainter, is about 4 arc-minutes south of the nucleus and slightly west. A brighter on (mag.10 or 11?) is 8 arc-minutes west and 3 arc-minutes south of the nucleus.

End of oobeeuvUion: 10:42 UT.

DRAWINGS

It's often said that a picture is worth a thousand words. Composing a thousand words takes much time aid thought aid still may leave the reader with the wrong mental image. So when it comes to accurate recording of seicntilie data, there's often no substitute for a picture.

In past ceiturles a scientist was necessarily a draftsman. Nowadays scientists li almost all fields rely on photography to record images, aid the pencil aid sketchpad io longer rank as esseitial scleitific tools. Visual astronomy, however, remains ai exception.

Y ou'll need white paper, either plain or with light 'graph Tines forjudging distances, a soft peicll such as number 1 or 2, and a clean eraser. Some observers draw w让h chalk oi black paper, but others find chalk hard to coitrol. Drawings of deep-sky objects made at the telescope are generally done with scribbled pencil shading. Better-looklig fiilshed versions, made immediately afterward indoors, use what is called the figer-smudglng technique. Using a dull peicll, lightly scribble a somewhat smaller area than the desired shading. Then use a clean, dry finger to smear the marks until they blend together into a uniform texture. If the patch is too faint, scribble aid smear some more until the desired efl^ct is produced. If the smear becomes too dark, the eraser is used ii the same way as the pencil, followed by more smearing. But lighteniig a smudge is harder than darkeiiig it, so sometimes i「s necessary to start again.

Pencil drawings oi white paper are negatives: the brighter a star or nebulous patch, the darker the pencil mark or smudge. Some people fnd it hard to get used to a iegative image, but most quickly leari to meitally reverse dark areas on the paper to correspond to bright areas in the sky.

Doing aiythiig at the telescope other than looking through it requires light. Any extra light reduces the eye's sensitivity to faiit objects. But if the working light is too dim, drawings become Tess accurate. I recommend that a very dim flashlight be used. Red light is tradltloially preferred, but the rod cells are more sensitive to all colors than cone cells so there is io advantage to using red light. Figure 2.3c (page 9) shows this. In fact many amateurs tend to use too bright a red light. They have the false impression that if it is rcd, night vision will not bc impaired. In practice, however. I ffnd that very dim grccn or yellowish light works fine. Strong colors should bc avoided because they distort the eye's color perception at the telescope, if an objcct is bright enough to show color at all.

A good light lcvcl can be obtained from a two-battery ffashlight by replacing thc bulb with onc meant for three, four, or five batteries. This both dims and rcddcns thc light, while extending battcry lifC. Thc ffashlight should cast a smooth glow with no bright or dark spots. This can bc ensured by placing one or two pieces of paper in front of thc bulb to act as difliiscrs. Thc brightness can bc ffnc-tuncd by using two pieces of polarizing material between the bulb and paper to act as a variable filter. Buy a high-quality ffashlight 一 it is very annoying to have to shake or bang it to make it work.

A drawing can cover any amount of sky and bc any size on the paper you wish. Some authors recommend using just one cycpicec， drawing a eire1e to represent its field of view, and sketching what is seen in this field. This is a bad idea for several reasons. The most important is thc need to use many magnifica-tioni l()explore an object properly, one of the main thrusts of this book. Another is that you may want to draw a larger area than fits in the field, or . just a small detail.

One rationale for the clre1e method is to give others a fccl for what they might scc in their cycpiccc at the same magnification. But even al the same power, onc eyepiece can show a feld twice as big as another, 'l'hc right way to indicate an objectS apparent size is to give the distance f?omi which to view the paper so thc object looks as big as it did in thc lelescope. Angular distance on the sky corresponds to linear distance on a drawing or photograph. The relationship between them is called the scale (or frequently the ‘‘plate ska1c2'， a carryover from photographic plates). Often it is given in arc-icconds per millimeter. If an angle も in the sky corresponds to the distance lゝ on the drawing or photograph, then the scalepゝ is

qJ Is,.                        (equation 5」)

Oncc the scale is sct. the appropriate viewing distance v to the paper depends only on the magnification . of the telescope:

v = JI(2m tan(7t/2)).          (equation 5.2)

The drawings in Chapter 7 give both thc scale and viewing distances for various mag-nllleatio ns. By holdi n 呂 your cyc at these distances from the. page, you scc just how large an object appears at a .given mag^ificat.ion.

If an eyepiece's true ffcld in your tcleseope is known (scc Chapter 3), you can find thc linear size of the eyepiece fi.cld of view 仁 on thc drawing (or a photograph) from thc equation

仁=2mv tan(nt/2)；             (equation 5.3)

where も is now thc true field oi view d thc telescope with that eyepiece.

Even though many magnifications should be used to view an object, onc will often give thc best overall impression of the object. This magnification, along with a viewing distance of about 25cm (1() inches), will define a good scale for thc drawing.

Any drawing takes considerable limc, first at the telescope, then indoors doing a gcomet-rth correction and a finished version (discui-sed below). A very simple subject with only a couple of field stars, such as a faint. Icatu^re-lcii galaxy, may lake only 1()minutes al thc telescope. Most subjects take over 30 minutes, and complicated ones like thc Orion nebula (M42)： several hours. The more complex a subject, thc more you must go back and forth from eyepiece to paper, and each lime you return to the telescope it takes five minutes or so to regain full night vision even when using a very dim flaslilight.

After thc drawing is completed at thc telescope. a finished version is made indoors by the finger smudging method. For this finished drawing, a geometric correction should be made using known positions of stars to "straighten outu positional errors. This drawing usually takes longer than thc onc made at the telescope. In general, thc more lime spent, thc more aeeuratc the portrayal is. likely to be.

Good drawings do not require special artistic talent or experience, but they do demand close attention, much time at the tclcscopc, much time redrawing with ffnger smudging, and honesty in not recording details rcmem* bered from photographs but not positively sccn. All this can require more time than taking a long-exposure photograph. But the result is often as satisfying or more so.

Drawing even has certain advantages over photography. One is low cost. The simplest, bare-bones telescope is all you need 一 no clock drive, alignment devices, or electronic guider, not to mention camera, film, and a photographic darki■〇om. Secondly, the eye has a far greater 1at1tuCe，or dynamic range, than film. This means bright and faint details can be seen at the same time, which is something film cannot do (without very special techniques). For example, have you ever seen a photograph of M42 that shows the large, fehit outer wisps and the bright Trapezium? See Chapter 7 for many examples.

Drawing method 1： the initial-blind method

For this method you start "blind" with no information about the subject that might bias you; a photograph should not be at the tele-sdope.

First the held stars are drawn, then the ob)cet itself is sketched in, using the stars to position its parts correctly. Finger smudging is not used,、just gentle shading with the pen-di11 The working light should be so dim that you can't tell pencil shading from ffnger smudges anyway; the smallest detail visible in very dim light at a distance of 10 inches (25 ゝ 4 cm) is about a sixteenth of an inch (1.6 mm).

Because of this, a drawing made at the telescope will look terrible in normal room lighting. More to the point, a subject cannot be rendered very faithfully under dim illumination. But detail will be remembered (or a short while after leaving the telescope. This is the time to redraw the rough sketch indoors.

While doing so, a geometric eorreetion can be performed. This is done by finding a suitable photograph of the object and tracing only the field stars (and, if the object has very bright portions with sharp boundaries, these as well.) But remember that brightnesses may appear diflererit on the photo than per-de1ved by the eye. Draw the star brightnesses as remembered and rceorded： not as they appear on the photograph.

Lastly, the photograph should be put away and the hazy object finger-smudged in. Here one needs to be very hon est and not biased by the photo just seen. Only when thedrawing is complete may it be compared with the photo.

The final drawing should be checked for accuracy during another observing session： preferably under similar skies at the same time the next night. The object should first be studied in the telescope, then a bright enough light should be used to confirm the details in the drawing.

Drawing method 2: filling in details

I n this method you start with a pre-drawn, geometrically correct sketch and fill whatever details you can see in the telescope. Such a sketch could be traced &om a photograph but trouble may result because photos usually record stars that are too faint for visual use and the proper magnitude cutoff can't be told. A better source is a drawing already made by method 1 Such a drawing can be traced or copied, leaving out the detail at the limit of detection.

Using this preliminary sketch, you add any newly observed detail. Then a new final drawing is made indoors using finger smudging as in method 1.

Drawing method 3: the double-blind method

In this procedure no photographs are used before, during, or after the drawing at the telescope is made. This method, the Geest from bias, requires an accurately driven telescope and a grid reticle in the eyepiece.

The stars and subject are drawn on ,graph paper, where the squares match the reticle grid seen in the eyepiece. Everything can be plotted accurately this way; there is no need (or a geometrical correction in room light. However, nebulosity will still need to be redrawn by ffnger smudging. The grid may be rela1neC in the ffnal version, or removed.

This is the most difUcult drawing method, and retie1e eyepieces are not available in today's te1eseope market. They have to be custom made. A grid is not needed in all eyepieces, but in at least one so that some star positions can be accurately plotted.

SUMMARY

Good astronomical records provide a useful baseline for seeing the same and possibly more detail in thc future. They add meaning to a night's work and improve the observer's perceptiveness. Detailed records provide reminders of enjoyable and sometimes aweinspiring observing experiences. Careful drawings can also be displayed in thc same manner as photographs, showing others, eluding non-astronomy enthusiasts, thc beauty that can bc seen in the night sky-

6

A case study: the Whirlpool Galaxy Messier 51

INTRODUCTION

In this chapter wc will scc how thc principles diseuiicd up to now 一 including teleicopc size, sky quality, and magnification - aflbet thc features that can be seen in a partieu1ar galaxy.

Our case study is an object that nearly every amateur tries to observe not long after getting a tclcscopc: the Whirlpool Galaxy M51,with 让s spiral arms. This was the first galaxy in which spiral itructurc was discovered, in 1845 by the Earl of Rossc using a 72・inch speeu1um-meta1 rei1ector. It still has thc reputation as the galaxy with the most easily visible spiral arms. What telescope size, magni^c；ation, and sky conditions are required to scc them?

BRIGHTNESS PROFILE OF M51

Figure 6.1 shows a photograph of M51 with a line from thc nucleus to the edge. At 1 5 places along this line, the galaxy's light was measured photometrically by Francois Schweizer in 1976 as part of a study of the spiral structure of galaxies, providing us with an accurate brightness profile to analyze. Thc light levels along the line arc plotted in Figure 6.2 一 first thc actual levels that were measured, then with various amounts of light pollution added to illustrate the situati on un der in-ercasing1y poor skics. The same data arc listed in Table 6.1.

Note thc dip in luminance (surface brightness) 65 arc-seconds ^om the nucleus. This represents the background lcvcl between two spiral arms. What eonditions are needed to sce thc increase in brightness marking thc next spiral arm out, at 77 arc-sccondi?

Contrast is defined as

C = (B - Bo) / B_o               (equation 6.1) where B and B_o are the surface brightnesses (in linear units) of the object and background, respectively. In stellar magnitudes, this becomes

C =10^[-° -ml 1         (equation 6.2)

where M and AI_O are the surface brightnesses in magnitudes per unit area (such as pcr square arc-second). In the case we'rc considering in M51, Al = 20.4) and = 2).24 when the sky is very dark (24.25 magnitudes / sq. arc-scc; scc Table 6.1). This korresponds to a contrast C of 1.33. Note that the "background'' in this case is not the sky background, but the galaxy's glow between spiral arms.

If wc switch to higher powers, the surfkcc brightness dims because the light is spread out over a larger area. But the contrast remains the same. This fact is represented in Table 6.2 — which was drawn up for an 8-inch tclcicopc at various magnifications for thc six sky conditions in Table 6.1.

VISUAL DETECTION

Thc visibility of the spiral arm at a given contrast and surface brightness depends on its apparent size 一 that is, the tc1cseope's power — and the eye's sensitivity. Take a minute to examine Figure 6.3. At 27X, the spiral arm appears 13 arc-minutes wide to the cyc. Note where the vertical line for 27X meets the horizontal line for sky condition A (contrast =1.33). Here, thc sloping lines showing background surface brightness show that the eye can only detect such a small object ifit is as bright as 21.3 magnitudes per square arc-second. This point falls bclow thc heavy, roughly horizontal arc showing the detektion limit for an 8-inch telescope. So the spiral arm will not be visible.

Data in column A are from Schweizer (1976),Astrop9sicai/OwmaSupplement3iy pp. 313-32, from his Figure 5h at position angle 135° from the galaxy's nucleus. Data are B 3 magnitude e^s, which are reasonably close to visual magnitudes.

If we boost the power to 120X, the apparent width of the spiral arm swells to 60 arc ・ minutes. Its surface brightness is now fainter, 25.0 magnitudes per sq. arc-second. But the eye is much more sensitive both to faint objects and low-contrast objects if they are large. Under sky eondition A, the eye could see something 60 arc minutes wide even as faint as 25.7 magnitudes per sq. are-second1 So the arm is now quite visible in an B-inch scope. It could even be detected in a sky as poor as condition D.

Interestingly, the graph shows that boosting the power from 120X to 200 X would bring the spiral arm just into detectability in a 6-ineh scope under sky A. Even though the arm would be more apparent in the 8-ineh than the 6-inch at all powers, it is less apparent at 200X than it was at 120X.

Note that a faint object becomes harder to detect at both very high and very low powers. This is why the thick arcs for various telescopes curve up at both ends. Detcetion is easiest when the object is at the bottom of each arc - at the optimum magnified visual angle diseusss:C in Chapter 2. The optimum magnified visual angle is shown by the somewhat wobbly line climbing up the graph from lower left to upper right.

DISCUSSION AND SUMMARY

The sizes and contrasts of deep-sky objects vary greatly. So docs the size and contrast of detail within a single objs:el1 So a large range of magnifications is needed to see all possible detail. For any feature, there is an optimum magnified visual angle at which it can best be detected. Fortunately, the decrease of de tec ・ lability on each side of the optimum is small, so fine gradations of magnification are not needed.

The eye's response to light, like the r— sponses of our other senses, is logarithmic. So the sequence of magnifications used should follow a logarithmic trend. For example, you

Figure 6.1. Messier 51 The line shows where the photometric brightness measuremeits analyzed in this chapter were made. (Courtesy Palomar Observatory.)

Contrast (C)                                     M_o

Apparent size of

U 18

19

20

21

22

23

24

〇     50    100    150    200

Dista nee from Nucleus (arc-sec ・)

Figure 62 The data from Table 6.1 are plotted to show the coitrast of M5Ts spiral arms in diflereit amounts of light pollution. Sky A is for a dark country site, while F is typical of a Targe city. The spiral arm analyzed ii the next figure is the one 77 arc-secoids from the iucleus.

might want a series of eyepieces that each give about 1.6 times higher power than the last. Ifyour telescope's minimum useful power is 30X, then a reasonable series of magni ・ ifcatioms might bc 30X, 50X, 75X, I25X? 200X, 315x, 510X, and so on.

An aitoniihing fact emerges from analyzing thc cyc's semsitivity under astromomica1 conditions. A small telescope requires higher magnification to detect a faint object than a large telescope! This cflect is discussed further in Appendix F.

10⁴

1〇3

102

10

1

10^-1

io"²

M 5 1 spiral arm a t position angle 135°

Detection Range in

Various Telescopes

6-inch telescope

B-inch

一1 6-inch

1 2-inch

10 100 1000

Appare nt An gular Size (arc-mi n)

Figure 6.3. Thc visibility of the main spiral arm in M5) is analyzed for various tc1cseopcs, powers, and sky con ditions, using the data in Tables 6.1 and 6.2. For diiferent amounts of light pollution (horizontal lines labeled ''A" through "F''), the contrast between the arm and its background was computed, using thc values of M and M_o in Tables 6.1 and 62 Next, for each magnification on a telescope of a given aperture, the apparent background surface brightness (A/〇) was computed. Each contrast (converted to a logarithmic scale) and each magnii^cation (which determines thc spiral arm width) was used to plot a point. If the background surface brightness M_o is brighter than the detection limit B_o (the lines sloping downward to the right), the spiral arm can bc sccn. Heavy black arcs show the detection limits for difEerent sized telescopes. Thc spiral arm can bc seen at brightnesses and contrasti greater than each telescoped limit; that is, above the arc.

The arcs illustrate several points. First, large telescopes do better than small ones for detecting low-contrast features 一 or the same feature in a more light-polluted sky. The minimum useful magnification on a tclcscopc will not show all the detail. Instead, the optimum magnified visual angle should bc used. If the magnification is raised too high, the detail is again lost from view.

7

A visual atlas of deep-sky objects

This chapter illustrates many of the best galaxies, star clusters and nebulae as they appear in a modest amateur telescope. Most of the observations were made by the author with a homemade 8-inch f711.5 Cassegrain reflector from observing sites no better than those near any large American city. The drawings have good, uniform quality control, so an observer should be able to tell, after a few trials, whether he or she can expect to see less or more in any object illustrated, given his or her particular telescope and sky quality.

Facing each drawing is a photograph at the same scale and orientation. Thus the viewer can readily determine which features in photographs can and cannot be seen. Also given are the distances from which to view each drawing so the object looks the same size as at various magnifications in a tele-seopC1

Full data are given for each observation, including how long it took to make the preliminary drawing at the telescope. Much additional time was spent preparing each final drawing, as discussed in Chapter 5. All drawings were made by either method 1 or 2 described in that chapter.

THE PERSONAL EQUATION

Astronomers have long spoken of the "personal equation'' to account for differing results by visual observers. The personal equation is a correction to be applied to an indi-viduars data to bring it to some impersonal standard. The differences in what people see probably depend more on their experience than on actual difkrences in their eyes. The fundamental capacities of the eye are about the same for most people. For example, in controlled tests of the faintest visible star, the difference from one person to the next is probably less than one magnitude, and even this may be due largely to how well someone has learned to use averted vision.

Focusing a telescope corrects for nearsightedness and farsightedness. Other eye problems aside, visual acuity depends on the density of rods and cones in the retina; sensitivity depends on their photochemical action and links to the brain. This neural architecture is probably much the same in most people. On the other hand, years of practice can make a great difference in fine tuning the techniques of visual observing.

Hopefully the information in this book will greatly shorten that time. My own growth in observing ability is interesting in this regard. Starting as a very active amateur in 1968, I had observed all the Messier objects and many NGC objects by 197 1 I located many supposedly diflicult ones that turned out to be not very hard. Even the notoriously elusive Horsehead Nebula was easy in a dark country sky. By early 1982 I was making detailed drawings of everything observed, and that summer I decided to write this book. That autumn I did most of the research and analysis for the previous chapters.

While doing so, I realized I had not been reaching the fundamental limits of the eye. I had not known the concept of the optimum magnified visual angle: how to match the telescope power to the eye's detection characteristics. The result of this increased unCsr-standing can be seen in the drawings of the Orion Nebula (M42) made in January 1982 and January 1983, on pages 101 and 103. The second drawing shows much more dc-tail. Although sky conditions were slightly better, most of the improvement resulted from using the eye and telescope together more efUectively. A greater range of magni-fcations, aid speiding more time studying the object, resulted fi such features as faint arcs of gebulosity coming into view. Now, when examlniig M42, these details are quickly seen.

Previously each observer had to discover such techiiques by hit or miss, which often took many years or a lifetime. With an uider-standiig of the material presented ii this book, the time should be shorteied to perhaps a year or so. It does take considerable practice to develop good techiiques. After all, one can read a book on how to drive a cag but learning to drive happens behind the wheel. With these thoughts in mind we will now explore the variety and beauty that can be seen through small amateur telescopes.

AVERTED VS. DIRECT VISION

The appearance of deep-sky objects depends strongly oi whether direct oraverted vision is used. Direct vision has sharper resolutioi but Tacks sensitivity. Thus, looking straight at ai object will show its brighter parts in detail, while the fainter parts may be totally Tost. Chaigiig from direct to averted vision aid back cag produce some iiterestiig blliking eficcts. For example, when looking directly at a globular star cluster, the bright ceitral mass of stars may be partially resolved into individual piipoiits, but the faiiter outer regions are invisible. Averted vision will show a fuzzy, unresolved (or less resolved) central region, but the outer parts come into view.

Some open clusters show similar eflects. While using averted vision, faiiter stars may be seen ii the ceiter of the cluster but they will not be clearly resolved. Thus they give the impression of a faint nebulosity ii the cluster. This eflect is shown ii some of the drawings, such as of M11 aid M67, aid ft crops up in many old descriptiois of clusters by early visual observers.

In the case of difluse iebulae aid galaxies, if you cag see the object with direct vision at all, you cai probably iicrease the power to see more detail. Low powers should be used if the gebula is very large aid already at the limit of averted visiog. If all you want is to detect the object's existence, then a power should be used that magiifies its apparent size to about 3° or 4°.

But usually the nebula is brighter than the detectioi limit aid powers many times more than that required for simple detectioi can be used in an effort to see detail - perhaps swirls aid dark spots in a nebula, or Rlit mottllngs, spiral arms, or dark lanes in a galaxy. The range of visual experiences is actually quite large with modest amateur telescopes.

In some cases, stars coincide with the nebula or galaxy in view. A good example is the planetary nebula M27, which is ii a rich region of the Milky Way with many foreground aid background stars plus a 13th-magiitudc ceitral star. These stars cai be hard to detect against the n ebula's light. The ceitral star, for instaice, appears fainter than I3th-gfagiitude field stars; it is didlcult in an 8-lnch telescope at low powers or with mediocre sky coidltlons. But high powers spread out the n ebula's light while stars remain point sources so far as the eye is concerned. So the central star becomes much easier.

Oie of the hardest aspects of averted vision to master is holding the eye motionless on oie point for six seconds or more while trying to grasp detail in the periphery of your visions. The eye tends to jerk, especially if fatigued. Oi the other hand, in some conditions moving the eye (or gently jlggliig the telescope) helps bring ai object into view, because peripheral vision is highly sei sitive to aiy-thiig moving.

THE OBSERVATIONS AND DRAWINGS

The drawings in this chapter are the product of great effort to detect all the detail that could possibly be seen. This detail is i eces-sarily portrayed more promineitly than it actually appeared. If a true represeitation were drawn, it would take the reader similar time aid effort to discern it on the printed page. This would amount to several minutes (or simple objects, aid hours for something complex like the Orioi Nebula.

It's worth remembering, however, that the true detail and coitrast ii deep-sky objects is not subtle at al! 一 as the camera proves. Only the detectioi limits of the eye make them seem so. After all, our eyes were not designed for astronomy but (or the very dlffereit job of day-to-day survival on Earth.

Since you arc probably reading this book in adequate room light, the detail in each drawing will bc obvious and quickly sccn. Experiment by viewing the book in very dim light — so dim that some details need averted vision. One thing you'll notice right away is that the photographs become much more like the drawings.

I find that the drawings of M51 are particularly interesting for practising averted vision techniques this way. For examiple, try to scc how far you can dim the lights and still sce the spiral arms from various distances, using direct and averted vision. Thc viewing distances represent miagnifications — though the surface brightness of a pieture docs not diminish as you move closer, unlike the case with magnification in a tc1cseopc. Such practice is .good training before .going out into the field. '                                           ・

Included with each illusiration is the ob-jcktS Messier, NGC, IC, or other catalogue number, its right ascension and declination for equinox 2000.0, its constellation, and its type. Also .given are the Universal Times and dales when the observation for each drawing began and cndcd, along with thc observing 1oeation, telescope, cycpicccs, magnifications, air mass (scc Appendix C), and the faintest visible star (usually at the zenith at a given magnificaiion). Thc scale of the drawing is indicated, as is the orientation (direction of north) and in some cases thc drawing method used.

Thcrc is a brief description of each object's physical nature as currently known, but that is not the primary purpose of this book. Excellent companion guides that cover such material arc Burnham^Js Celesta! Handbook and The Messier Album. A more up-to-date listing of data for thousarids of deep-sky objects is Sky Catalogue 2000.0, Volume 2. See Appendix A「

The objects are presented in order of increasing right ascension. Distances from which to view the page are given to represent a range of telescopic magnifications. At any other magnification m, the viewing distance v in centimeters can easily bc comiputed from this simplified version of equation 5.1:

v =l/(2w lan(ps/l 2()))         (equation 7.1)

where p3 is the scale i n arc-minutes per centimeter and the argument to the tangent is in degrees.

My observing sites and conditions varied &om essentially perfect to quite poor, She best were at Manastash Ridge in the Cascade Mountains of 'Washington, site of the University of Washingtoifi 30-inch tclcscopc, and in thc Colorado Roekics. Observations at thc Washington site were made before 29721 Fhc dark sites there wcrc truly spectacular. The naked cyc could scc to magnitude '7.5, and thc summer Milky Way cast a diffuse shadow! The faintest star I documented was magnitude 14.8, in thc star cluster NGC 6910. This star was seen very easily by (bur observers in three 8-inch telescopei. Moreover it was close to other, brighter stars near the center of the cluster (scc the NGC 6910 magnitude chart in Appendix B), so thc true 8-inch magnitude limit must have been 15.0 and maybe slightly fainter.

Thc rest of the observing sites wcrc near sca lcvcl and close to major eitics. Onc is Patteri's Observatory, the old location of the Seattle Aslloiiomical Society's 12.5-tneh Newtonian telescope, whcrc observations were carried out in the laic 1960s and early 1 970s. The rest of the sites arc on the isl;and of Oahu in the state of Hawaii. Thc best location there to which amateurs had intermittent aeeess was a private ranch in tlie Waianae area, on thc west coast of the island. This spot was slightly better than that at Patten's, having an 8-inch limiting magnitude of about 14.5 on a .good nighl. (Hazc from ocean spray is a problem at all elevation Hawaiian sites.) The next best 1oea-tion was on an abandoned runway al Barbers Point Naval Air Station. Here the 8-imeh limiting magnitude was around 1 4.0, Somewhat poorer was my home in Hawaii Kai, at the east cnd of thc island, with 8-inch 1tmtts of 13.6 to 13.8. Finally, some observations wcrc made al Evva Beach, whcrc the 8-ineh limit was about 13.4. These limits are for good nights; they could bc considerably worse, owing to hazc.

Haze and volcanic smoke are two seemingly constant problems in Hawaii. The occan kicks up a lol of spray, which is carried all over the islands by strong trade winds. In thc spring of 1982, thc Mexican volcano El Chichon injected much dust high into the atmosphere, which had a devastating cffcct on observations by both professionals and amateurs. In April1982, while at Mauna Kea Observatory, I found the nakcd~eyc limit at new uiuuu iu uc aiuuna 争.u; normally it i_s 7 or better. By late June, on Oahu the faintest star observable was about 0.6 magni-wde worse than normal. In January and February 1983, Kilauea volcano was erupting, covering the Hawaiian islands with what is 'called "vog" (volcanic smoke plus fog), which worsened the limiting magnitude

Most observers don5t have to contend with such problems, but there are always variations from night to night. So the sky quality should be doeumented at the time of each observation.

Definitions

Several words and phrases used in the de-seriptions of the objects need to be defined:

Small telescope or small amateur telescope: one less than 6 inches (15.2 centimeters) in aperture.

Medium size telescope or medium amateur telescope: one 6 inches (15.2 eent1-meters) to 12 inches (30.5 centimeters) in aperture.

Large telescope or large amateur telescopes: one more than 12 inches (30.5 centimeters) in aperture.

Very poor skies: those typical of the middle of a large city with a population of a million or more. Such a sky is brighter than a country sky at full Moon, with a surfaee brightness overhead of about 19 magnitudes per square arc-sceond or brighter*.

Poor skies: those typical of the edge of a large city with a population of a million or moi■巳 The sky surface brightness overhead is about 20 magnitudes per square are-seeond or slightly brighter. The naked-eye limit is near magnitude 5.

Moderate skies: those typically found a few miles from large cities. THe sky surface brightness is 20 to 21 magnitudes per square are-sceond, and the naked-eye limit is magnitude 5.5 to 6.〇・

Good skies: those typical of the country a few tens of miles from a large eity： with few lights in the vicinity. The sky surface brightness is 21 to 23 magnitudes per square arcsecond, and the naked-eye limit is magnitude 6.0 to 7.0.

Excellent skies: those typically found only 90 miles (150 km) or more from a large dity： with no bright lights within a few miles. There is no haze in the sky. The sky surface brightness overhead is fainter than 23 magnitudes per square arc-second. These skies are common in the mountains of the western United States, but are rare along the east coast of the U.S. in the summer because of haze.

North, south, east and west are used in the astronomical sense: north means the direction to the north celestial pole, east means toward increasing right ascension. If an observer is in the Northern Hemisphere and facing south, astronomical east is to his or her left (which is also east on land).

Very low power: magnifieations less than about 30x.

Low power: magnifications from about 30X to 70X.

Medium power: from about 70x to 200X.

High power: from about 200X to 400X.

Very high power: over 400X .

M31, THE ANDROMEDA GALAXY (NGC 224), M32 (NGC 221),M110 (NGC 205), GALAXIES IN ANDROMEDA

M31: R.A. 0(炉 427 Dec. 41° 16’ (20000) M32： R.A. 00^fl 427 Dec. 40° 52, MHO: R.A. 00 40.3ⁿ Dec. 41。4V

Technical. Messier 31,commonly called (he Great Galaxy in Andromeda, is a large spiral tilted 13° from edge-on. It is the brightest galaxy we can view li the sky apart from our own Milky \\'ay. Arab astroiomers recorded it in the 10th ceitury as a small, dlffiise patch： but under the murky skies of Europe it eluded notice with the uiaided eye. No European recorded it until 1612, when Slmoi Y Tarlus saw it with the help of a telescope.

The Andromeda Galaxy is part of the Local Group of galaxies, which is about 5 mfllioi light-years li size; the four main members arc M31, M33, Mafiei1,and our owi galaxy. About 30 galaxies are currently recognized as members of the Local Group, most of them dwarf ellipticals or irregulars associated with our galaxy or Y 131.

Y131 has approximate^ly twice the mass of our galaxy, or about 300 (Amer.) bl11101 times the mass of the Sun. and puts out about 11 billioi times the Sun's light. Its distance is usually given as 2.3 million light-years, and if is approachiig us at about 80 kilometers per second.

Oi deep-sky photographs ヽ［31 covers fully 1.25° by 4.1°, which correspoids to a diameter of about 100,000 light-yearsゝ In his book Galaxies, Timothy Ferris gives an ii-teresting observatioi on space and time. Since Y131 is 2.3 million light-years away： we are seeing 2.3 million years into the past. However, since one side of the galaxy is about 100,000 light-years more d1sta1t.： a thousand centuries are spanned from one side to the other. Thus we are not only looking back in lime, we are seeing many limes at once!

Visual.M31 appears somewhat less than 4° long through most instruments, though some observers have reported seeing a 5° extent with fine binoculars, under excellent skies. These outer parts are extremely faint.

Unlike many bright objects, M31 often leaves a disappointing impression at first sight. The central region is very bright but fades quickly and smoothly toward the edges. The beautiful details seen in photographs are difficult visually in any telescope. In (act, to see the outer edge corresponding to about 2° length requires a very dark sky. Because of its size, very low power is needed to see all of the galaxy in one view. But at such magnifications, almost no detail appears.

With good skies and a moderate aperture, the dark lane to the northeast is easily seen at 30x and up. An 8-inch telescope will show this lane under moderate skies, as well as the bright star cloud to the southwest (upper right in the photograph) at higher powers. This star cloud is known as NGC 206. There are many dark patches in the galaxy, espe-dla11y near the bright central portion, but none have been reported by amateur observers.

The brightness of M31 varies tremendously from the nucleus to the outer edges. Assuming a size of 150 by 50 arc-minutes and a total magnitude of 4.0, the mean surface brightness is 22.3 magnitudes per square arc-scdond1 However, the nucleus is many magnitudes brighter than this, while the edges are fainter.

Near M31 are two of its companion galaxies: M32 and Ml! 〇・ These are dwarf eliiptic-al systems that have a relationship with M31 similar to that of the Magellanic Clouds with our own galaxy.

M32 is south of M31's nucleus. It appears on the edge of a spiral arm whose boundary is evident in good skies with a 6— or 8-ineh telescope. M32 was first seen by Le Gentil in 1749; it can be found by careful observers with a 2-inch telescope. Its angular size is 3.6 by 3.1 arc-minutes, and its total magnitude of 9.5 yields an average surface brightness of 20.7 magnitudes per square arc-second.

Ml10 (NGC 205), northwest of the nucleus of M31, is eonsidcrab1y more difficult than M32 beeause it is spread over a larger area and has a fainter total magnituCe：10181 It is 8 by 3 arc-mi nutes in size, and has an average surface brightness of 22.9 magnitudes per square arc-second. This galaxy has been seen with te1eseopes as small as 2.4 inches. In fact, a good friend of mine, Jon Seamans. has observed all the Messier objects with a 2.4-inch refractor.

M3〕has two more companion galaxies about 7° to the north: NGC 147 and NGC 185. Both are about the same difliculty visually as NGC 205. They are described in Appendix E and in Burnham's CMelial Handbook.

Fhotograph of M31,M32 and M110. The Great Galaxy in Andromeda, M31, is the main object at center. M32 is at upper cenier, and M110 (NGC 205) is the small object about 6cm to the lower left of the nucleus of M31. South is up. (Courtesy Palomar Observatory.)

2〇^/-----------

Drawing of M3L

Scale: 6.0 arc-mii/cm Viewing Distance (cm)

8'iich f门，25 Newtonian ----------------------

28mm Kellner (52X)      25x：23 200X：3

50X：ll 300X:2 100X： 6   400X：T

air mass:1 faintest star: 13.5 at zeilth, 52X; tracking

8/17/69 UT (time not recorded) at Pattens Observatory, Wash.; R. Clark

Visual. With a total magnitude of8.5 and an angular size of 2.5 by 4 arc-minutes, the nebula's mean surfaee brightness is 19.6 magnitudes per square arc-second. It has a 12tti-magnitude central star. In 7 X 50 binoculars the nebula appears very small but not quite stellar. In small telescopes it is a featureless disk.

The 8-ineh under good skies showed the central star as well as two Held stars superimposed on the nebula, but no other features were made out. The west side faded gradually into the sky background, while the other edges appeared sharp. Large amateur telescopes under excellent skies may be able to bring out some details.

Drawing of NGC 246.

Scale：1.5 arc-min/cm Vicwi ng Dista ncc (cm) 8-lneh 〃1215 Cassegrain

28mm Kcllncr (82 X)      25x：92  200x：ll

20mm Erflc (117X)        50x：46 310 X： 8

12.4 Erfle (188x)          100x：79 40Ox： 6

air mass:1.20, faintest star:14.2 at zenith, 188X; no tracking.

10/8/83 9:50-10：10 UT at Waianac ranch, Hawaii; R. Clark

NGC 253, GALAXY IN SCULPTOR

R^.A. 00^h 47.6^, Dec. -25° 18 (2000.0)

Technical. NGC 253 is an uiusual galaxy in that its iucleus appears to be ejectlig material, much in the mainer of the iucleus of M82 but less vloleitly aid in a way not obvious on most photographs. The core of the galaxy radiates about half the system's total eiergy. Most of this emlssloi is at infrared waveleigths, amouitlig to about 100 billion times the energy of our Sui. Gas is being spewed from the iucleus at a velocity of 120 kilometers per second.

NGC 253 is part of the Sculptor cluster of Galaxies: a group over 20° in diameter aid centered gear the south galactic pole. The dlstaice to NGC 253 is somewhat uicertali, but it's probably oily a little beyond the farthest members of the Local Group. The galaxy is thought to be around 40 000 light-years in diameter.

Photograph of NGC253. South is up. (Courtesy Palomar Observatory.)

Visual.NGC 253 is large and bright but often overlooked by Northern-Hemisphere amateurs because it is rather far south in a nondescript constellation. It contains dust lanes of great complexity in all areas. The galaxy is fully 22 by 6 arc-minutes in size and is inclined 17° from edge-on. At a total visual magnitude of 7.〇, its average surface brightness is 20.9 magnitudes per square arc-sceonC .

When high in a good sky, NGC 253 shows considerable detail in medium size telescopes. The 8-inch under moderate skies shows distinct brighter portions that cones-pond to spiral arms. The galaxy is bright enough to be seen in a 2-inch. In the 3-inch finder at 31 X,it is a smooth oval. But even large telescopes show no nucleus. This galaxy must rival M31 in beauty for Southern Hemisphere observers, although it is somewhat smaller.

Drawing oj'NGC 253.

Viewing Distance (cm)

Scdl：1.2 arc-min/cm

8-inch ffl1.5 Cassegrain

20mm Erfle (117x)

12.14mm Erile (188X, best view)

7 mm Erfle (334x)

M33 (NGC 598), GALAXY IN TRIANGULUM

R.A. 01^h 3ヨ・9ゝ Dec. 30° 39 (2000.0)

TeehhicaL M33 is onc of' the four spiral galaxies in the Local Group, but it is a small onc, estimated to hold fewer than 20 billion stars comipared to the 300 billion or so in the Andromeda Galaxy (M31).M33 is about 40 000 light-years across and tilted 55° from cdgc-on. It is probably only about 700 000 light-years from M31, which would appear thrcc times bigger from a planet in M33 than from Earth. M33 is about the same distance from our galaxy as M31 or possibly a little e1oicr. It was discovered by Messier on August 25,1764, and today is often called the Triangulum Galaxy or the Pinwheel.

Vissul. M33 is about 1° across, including the faint extensions, but it has a low surface brightness. Amateurs often miss it because they are looking R)r a smaller object. With a total magnitude of 5.3, its average surface brightdess is a dim 22.8 magnitudes pcr square arc-second.

M33 is a challenge to the noviec. Experienced observers seem to have little trouble Ending it, and it can even bc sccn with the naked cyc under excellent skics. The brighter portions cover an area 20 by 30 arc-minutes. Within 1° of the nucleus are many NGC and IC objects, the brightest of which is NGC 604. This is an H II region (a cloud ofionized hydrogen), similar to the Great Nebula in Orion but much larger, with an embedded star cluster. NGC 604 can bc seen as a round spot at the end of the spiral arm to the northeast of the nue1cus; it is at lower right in the drawing. I have sccn it in the 3-inch ff.nder at 89 x.

Most observing guides recommend very low powers for M33. Although low powers are ffnc f)r locating it, they will not show any detail. This probably aecoumts for the galaxyS often-reported lack of' features. Medium powers ))00X to 200X on an 8-inch tc1cseopc) will show the two main spiral arms and NGC 604 cvcn under only moderately good skics. Excellent dark country skies should show a wealth of detail with earciU1 observation using only a 6-incli iclcscopc.

-------------5'-------------

Drawing of M33.

ScIi：1.9 arc-min/cm     Viewing Distance (cm)

8-indh 111.5 Cassse^ain---

20mm Erfle (117X)        25x： 72 200X：9

12.4mm Erfle (188x) 50x：36   300X：6

100X：18   400x：5 airmiass:1.03, faintest star:13.8 ntzsnilli，丨 88X ： no tracking

9/5/83 12:00-12:50 UT at Hawaii Kai, Hawaii; R. Clark

M74 (NGC 628), GALAXY IN PISCES

R・A. 01^h 36.7^rn,     15。4T (2000.0)

TedmicaL M74 was first seen in 1780 by Pierre F. A. Mechnin. It is a faee-oon spiral about 30 million 1ightiyenrs distant and 80 000 light-years wide. Its mass is cslimatcd at about 40 billion suns. On deep photographs M74 shows beautiful spiral arms about 3000 light-years thick, with dust lanes tending to outline their inner edges.

VissuL M474 is often regarded as the most difficult Messier object to locate. With a diameter of 9 arc - minutes and a total magnitude of 9.0, M74- ought not to be hard. The difli-culty is probably due to improper use of mag-nif1cnlion. Again, guides oflen suggest using loow powers, since the average surface brightness is low at 22.4 magnitudes per square nrcisecond. But most of the light is eoneen-trated in the small nuclear region about 40 arc-seconds in diameter. So at low power the galaxy can loook like a field star. Higher powers show the nuclear region as a disk.

In the 8-inch under only moderate skies, 188X gave the best view. The bright central region was surrounded by a (111X1, uniform glow・卜174 cnn be seen in telescopes as small as a 214-inch refractor. Under excellent skies, I have seen it in the 3-inch finder at 31 X and even the 2.4-inch finder nl 7.9x.

Photograph of M74. South is up. (Courtesy National Optical Ast^rono-^iy Observatories.)

--------------------5'--------------------

Drawing 〇/M74.

Scale 1.2 arc-m in/cm

8-i nch ff 11.5 Casscgrai n 12.4mm Er fie (188x,

best view)

20mm Erfle )117 X )

Viewing Distance (cm)

25x：115 2O0X:14

50X： 57 300x：10

lOOx： 29 400x： 7

airmass:121,faintest star:14.0 at zenith, 188X； no tracking

1/16/83 6:45—6:55 UT at Barbers Point, Hawaii;

R. Clark

M76 (NGC 650-651), PLANETARY NEBULA IN PERSEUS

R. A. 01 4L9^m, De 15。34 (2000.0)

TeにhuiEcl. N176 is a small planetary nebula similar in appearaice to the Dumbbell Nebula, M27. Like all planetarles, its distagce is poorly known; published values range from 1 700 to 8000 light-years. The central star, magnitude 16.5, is extremely hot, about 60 000 Kelvin. Its light output is less than the Sun's, while the nebula itself emits two or three times the Sui's light. The nebula is 1 to 4 light-years across, depending on the accepted dlstaiice.

Visual. MI76 is called the "Little Dumbbell'' aid is probably the faintest Messier object. Visually, it looks like the bright portion of M427 but much smaller. With a total magnitude of 10, the average surface bright ncss is 18.7 magnitudes per square arc-secoid, high compared with most deep-sky objects. This surface brightness, due to the small size of 1.5 by 0.7 arc-miiutes, allows high powers to be used to search for detail.

M76 is b righter at its ends than in the middle, which caused 19th-century observers to catalogue it as two objects, NGC 650 aid NGC 651.The 8-lnch showed coisideraible detail at powers around 20OX aid up under only moderate skies. The small size and bigh surface brightness mean that good, steady atmospheric seelig is needed for viewing fine detail within the nebula — an unusual situation for a deep-sky object. The 8-lnch showed a distinct difference between the two com po-nents; the southern portion had a marked point, aimed south. There are many field stars near MI76 even in a hlgh-power view.

Photograph of M76. South is up. (Courtesy Laird

A. Thompson. Canada-France Hawaii Telescope

CoTpoTation.)

Draining of M76.

Scali:1.2 arc-m in/cm

8-lnch 1711.5 Cassserain 12.4mm Erfle (188x, best view)

9mm Kellner (26(3 X)

air mass: 1.44, faintest star: 14.0 at zenith,188 X; no tracking

1/16/83 7:20-7:42 UT at Barbers Point, Hawaii;

R. Clark

NGC 891,GALAXY IN ANDROMEDA

R.A. 02^h 22.4U Oec. 42° 2 T (2000.0)

Technical. NGC 891is a beautiful edge-on galaxy with a dark lane extending from one end to the other. This is one of a small group of galaxies that includes NGC 1023 and NGC 925. The distance to NGC 891is estimated to be bet ween 20 and 40 million light-years. The galaxy's total light is roughly 1 or 2 billion times the light of our Sun. The dark lane consists of dust clouds mainly along the gnlaetic equator, much like those in our Galaxy. In fket, all-sky photographs of the Milky Way appear remarkably similar to photographs of NGC 891.

Visual. NGC 891 has a total magnitude of 12.2 and an angular size of 12 by 1 arcminutes. Its mean surface brightness is somewhat low at 23.5 magnitudes per square are-second. Although NGC 891 may be glimpsed through a 3-inch telescope under excellent skies, a medium size telescope is needed to begin to show detail.

Through the 8-inch under good skies, the overall size and shape were seen well at 11 7X, but the dark lane could be detected only nt 188X. The dark lane was visible only near the nucleus. Larger telescopes under similar skies show the lane extending farther into the edge~on spiral arms. It has a surface brightness about 0.6 to 0.9 magnitudes fainter than the bordering bright zones. This magnitude difference implies a contrast of 0.57 to 0.44 (log contrast of -024 to -0.36). Such a contrast and surface brightness indicate that at least an 8-inch teleseope and good skies are required to detect the dark lane.

Photograph o/NGiC 891 SouLh isup. (Courtesy Mount Wilson and Las Campanas Observatories, Carnegie Institution 〇/ Washington.)

Scale: 0.75 are-m1n/em Viewing Distance (cm)

8-lnch f/ 11.5 Cassegrain -----------------------

20mm Erfle (117x)        25x：183 200x：23

12.4mm Erfle (18!3x)      50x： 92 300x：15

lOOx： 46 400x：11

air mass:1.1 7, faintest star: 14.2 a tzeiith, 188X; io tracking

10/8/83 13:33-13:50 UT at Waianae ranch, Hawaii; R. Clark

M77 (NGC 1068), SEYFERT GALAXY IN CETUS

R.A. 02h 42T, Dec. 一00。01(2000.0)

Technical. M477 is one of a class of galaxies with bright, very aetive nuelei. These arc the Seyfert galaxies, named after Carl Seyfert who studied them in the early 1940s. Thc nucleus of M477 is ejecting elouds of gas at a veloeity of 600 kilometers per second, and each of these elouds is estimated to contain as much mass as 10 million suns. The nuclei of sonic Seyfert galaxies vary in brightness, but that of MI77 does not.

Thc distance to 卜177 is about 75 million light-years, and thc diameter of its outermost region is around 100 000 light-years. Thc total mass is estimated to bc about 100 billion suns; the light output, 40 billion suns. Like most Seyfert galaxies,1477 is also a strong radio source. Radio astronomers know it as 3C 71.

Vis^uk MI77 is magnitude 10.0 and about 2.5 by 1.7 arc-minutcs in size. Its mean surface brightness is 20.2 magnitudes pcr square arc-sccond. Faint spiral arms extend to a diameter of 6 arc-minutcs but have not bccn reported visually. Thc galaxy has a bright inner spiral pattern 40 by 20 arciseconds in size, which is not seen in small amateur tele-seopes. Some observers have reported a mottled cffeet in large telescopes. A second, larger spiral pattern extends to 2.5 by 1.7 arc-minutes、

Thc visual impression is that of a bright central region surrounded by a diffuse oval, which in turn is surrounded by a larger and fainter oval. In the 8-ineh the bright inner region did show a brighter spot southwest of the nucleus. This spot corresponds to a bright part of a spiral arm, which might be recogniz-ablc as such in a large telcseope under very dark skies at 200X or more.

Photograph of M77. South is up. (Courtesy Lick

Obse^,rvato7^y.)

Scale: 0.6 arc-min/cm     Viewing Distance (cm)

8-inch f 11.5 Casssegain----

12.4mm Erile (188x)      25x：229 2OOX：99

9mm Kcllncr (260X)      50X：115 300X!】g

6mm Orthosc()pii:(369x)DOX: 57 400X：14

air mass: 1.64, faintest star:13.5 at zenith, 188x； no track ing

】/13/83 8:45—9:18 UT at Hawaii Kai, Hawaii； R. Clark                  ?

NGC 1365, BARRED SPIRAL GALAXY IN FORNAX

R. A. 03h 33.77, Dec, 一36。0& (2000.0)

Technical. NGC 1365 is a beautiful barred spiral galaxy, probably the finest example of its e1ass in the southern sky. It is the third brightest member of the Fornax Cluster of Galaxies, which is some 50 million light-years distant. Thc bar is about 45 000 light-years long; thc arms stretch the bar's ends for at least twice that distance.

NGC 1365 is also onc of thc most luminous of all known barred spirals, and in the 1970s was found to emit X-rays. Thc X-ray emis・ sion comcs mainly from the region of thc nucleus, as is the case with Scyfcrt galaxies. But NGC 1365 sccms to bc a ncw type. The X-rays from Scyfcrts are associated with fastmoving gas, while none is observed in this galaxy.

Visual. NGC 1365 has a total magnitude of 1)12 and an angular size of 8 by 3.5 arcminutes. The mean surface brightness is 23.4 magnitudes pcr square arc-second, whieh is, unfortunately, somewhat low. Thc galaxy should bc visible in small tc1cseopcs as a faint patch if it can be observed high enough in a good sky.

Through the 8-inch under good skies, the galaxy had a bright, fbzzy nucleus surrounded by the soft glow of the unresolved spiral arms. Even from my Hawaii observing sites, NGC 1365 is never high in the sky, and for more northerly observers, it is too low to oiler any chance of detecting detail cvcn in large tc1cseopcs. Southern Hemisphere observers with large tc1cseopcs and good skies may bc able to scc the spiral arms.

Photograph 〇/NGC 1365. South is up. (Courtesy ne C, Annala, Coopyight University of Hawaii, institute for Ast^ronomy.)

-----------------5’----------------

Dratving of NGC 1365.

Scale:1.2 arc-min/cim     Viewing Dlstankc (cm)

8-inkh f71115 Cassegrain -

20mm Erfle (11 7xj        25x：］15 200x：14

12.4mm Erfle (188X)      50x： 57 3(0)乂:1〇

100X： 29 400x： 7 airmass:1.9 7, faintcit star: 14.2 al zeniih,1 88 X no tracking

1 0/8/83 1 4:0(〕一14：1 〇 UT at Waianac ranch, Hawaii; R. Clark

M45, THE PLEIADES OPEN CLUSTER IN TAURUS

R.A. 03h 46.9^rn, Dec. 24 〇？^, (2000.0)

Technical. The Pleiades have been familiar the world over since ancient times. The cluster is very elose, rs1ativs1y speaking, at a distance of only 400 light-years. It is also unusually large, 3((light-ycars in diamstsr： so it is easily resolved by the unaided eye.

This unique grouping has had special significance in all cultures f)r which records can be found. Thc earliest recorded observation is found in Chinese annals from 2357 BC. The cluster had particular meaning in ngricu1tu-ral societies, since its rising and setting near sunrise and sunset marked important times in the growing season. Even Halloween is tied to the Pleiades. In thc Middle Ages the cluster cu1minatsC around midnight on the "Wiich's Sabbath,', which had its origin in ancient Druids' rites. (Since then the midnight culmination of thc Pleiades has shifted to November 21st owing to precession.)

References to thc Pleiades are found throughout literature, music, and religion. Thc great pyramid at Teotihuacan, 28 miles northwest of Mexico しity, has 让 s wesl faee dirceted to the setting of thc Pleiades (14° north of west there), and all the east-west streets of thc ancient city are oriented in the same direction. Other cultures that gave the Pleiades special significance include American Indian, Maya, Aztce, Australian Aborigine, Egyptian, Greek, Roman and Persian. Robert Burnham devotes many pages to Pleiades lore in his Bumham も Celestial Handbook.

The Pleiades are often called the Seven Sisters, though nine of the stars now have names. Seven of the names date back at least as far as the Greek poet Aratus in the 3rd century BC. (Aratus took his consts11ntion lore f?om writings by Eudoxus that were n1rendy about a century old, but are now lost.) Aratus spoke of a lost Pleiad, and indeed, many people can see only six stars here. Perhaps, it has been specu1nted： a star did fade sometime within the historical memory of the aneient Greeks — possibly Pleione, since 让 is variable today. References to the lost Pleiad are also found in Japanese lileraturc and legends from several other cultures.

M45 contains about 300 to 500 stars. Since all are at essentially the same distance, thcir apparent brightnesses reflect their intrinsic luminosities.「Fhe brightest Pleiades are all blue—white; fainter ones are yellow and reddish. When the stars' brightnesses arc graphed as a function of their color (the so-called color—mag nt ude diagram), they all fall on the curve known as the main sequence. This is where a star resides fb r most of its active lifetime while it converts hydrogen to helium. None of the Pleiades visible today has yet evolved off the main sequenee to the red rgiant stage.

All the brightest Pleiades except Maia arc spinning very fast, with rotation periods of about two days. They are also quite young, about 88() million years old, and are rapidly consuming thcir hydrogen fuel. There is one known white dwarf in the cluster. Astronomer Alan Sandage has theorized that in the distant pas し the Pleiades probably conlnincd two stars even brighter than those we see now, which have since become white dwarfs.

On photographs the brighter Pleiades are surrounded by a delicate web of neibulosity. This is a fine example of a reflection nebula, one visible because of starlight reflected and scattered by small dust grains. Color photography shows the Pleiades nebulosity to be quite blue. The reason is not just that the stars i11uminnting the dust are bluish. Thc grains tend to be smaller than the wavelength of light and thsrsf0re scatter blue light preferentially —a process similar to the Rayleigh scattering that makes our sky blue. Because the light is reflected starighl,让s spectrum is continuous, rather than concentrated in emission lines as in emission nebulae like M42 (the Great Nebula in Orion) or M8 (the Lagoon Nebula).

Throughout the Pleiades, the nebulosity appears streaked in long filaments almost like eirrus clouds. Magnetic fields may be responsible f)r n1igning this dust. Some of the filaments appear only a few nrc-seconds in width and many arc-minutcs long.

Thc brightest patch of nebulosity, NGC 1435, appears to surround and extend south of the star Merope, and hence is known as the Mcrope Nebula. It has also been called the Thumbprint from its nppearnnce on an early drawing. In 1965 the astronomer F. 〇^,Dell showed that the nebula is actually behind Merope, not enveloping it or in front of it.

Visual. M45 is a beautiful sight to the unaided cyc, in binoculars, or in a te1eseope. Its stars have a total magn 让 udc of 1.4 and are spread across 100 arc-minutes. Bceause the cluster is easily resolved by thc normal unaided cyc, its mean surface brightness (20.0 magnitudes per square arc-second) has little meamlmg-excepl for comparison to other objects in this book.

Most people can scc six or eight Pleiades with thc naked cyc, though people with good visual aeutty can often sec 10 to 1 2, and as many as 18 have been claimed. The number visible to the naked eye is not only a test of how faint one can scc, but also a test of visual acuity and thc sky's freedom from haze. The table of bright Pleiades on page 94 shows that 10 stars are brighter than magnitude 5.651 Only because they are so e1ose together are the stars of this magnitude and fainlcr hard to scc. I have never been able to scc more than six Pleiades while wearing glasses, even under exee11ent skics, and without ,glasses thc cluster appears as a blob. However, even though my nearsightedness is severe, the senstttv 让 y of my retina to the faintest of light appears to be normal.

Dcteetlon of bint te1eseoplc members of thc Pleiades is also eonfused by the glare of thc brighter serrs. Even under excellent skics, stars of only about magnitude 14.4 can just be seen with an 8-inch telescope in thc Pleiades. Elsewhere the limit is about 15.2. That is a loss of nearly a magnitude.

The Pleiades are often eonfUsed with the Little Dipper (Little Bear) by beginning amateurs and laypeople. Thc cluster docs have the shape of a small dipper about twice the size of the Moon, and it is easier to scc than thc Little Dipper of Ursa Minor because thc main stars are brighter.

M45 is certainly a favor 让 c object among amateur astronomers for its beauty in virtually any optical instrument with an adequate field of view. Binoculars provide a pretty sight, but a 3* to 8-inch telescope at about 30 power probably shows the most spectacular view. A wide-field eyepleee at 60X will still show all the bright stars at oncc, including the dipper. At higher powers only smaller portions of the e1uster are seen, though these higher powers can bc useful and arc dlseus-scd bclow.

Thc Pleiades nebulosity is a prized trophy for visual observers. Dust- and dcw-frec optics are essential for thc pursuit, since light scattered from the bright stars will hide thc nebulosity. The easiest portion is NGC 1435 south of Merope. Other nebulosity appears to surround Alcyone, Maia, and Electra. There is little or none around Celaeno, Taygcta, and Asterope, and defi n 让 cly none around Atlas and Plcionc.

Thc Merope Nebula was discovered by \V. Tempel in 1859 with a 4-inch refractor. In 1874 Lewis Swift found 让 deteetab1c w让 h a

nch refractor at 25X. Modern observers seem to bc having more diniculty, and it is often stated that a 6- to 8-inch telescope is required. This dlserepaney is no doubt due to lnereased light pollution, and probably also to lnereased atmospheric dust particles, which scatter starlight. The mean suriaee brightness of NGC 1435 is 21.6 magnitudes per square arc-seeond and the total magnitude is 6.8, only slightly fainter than the Helix Nebula, NGC 7293.

Modern observers have reported viewing the nebulae around other stars. John Mallas in The Messier Album reported nebulosity, in-e1udlng fine streaks, around Maia, Taygeta, Alcyone and Celaeno with his 4-inch refractor. At least some of this must bc spurious, because there is little or no nebulosity around Celacno and Taygeta. Walter Scott Houston saw an unusual sight through an 8-inch telescope on an exceptional night in the southern Arizona mountains: "When I looked into the cycpiccc, expeetlng to scc a few faint wisps, the field was laced      edge to edge w让h

bright wreaths of de1leate1y structured nebulosity...,'.

The brightest of the linear streaks are near Electra. One about 20 arc-sceonds wide extends from ,just south of that star about a third of thc way to Alcyone. A second streak about half as long runs parallel to it onc are-minutc brther south. They are easily confused w 让h artiitcia1 streaks caused by di 匚 iraetlon or rei1cetlons in the tc1cseopc.

Thc drawing shows what was seen through the 8-ineh under ,good skics. Thc lowest magnification with the 1.25-inch eyepleee holder was 82x, giving a field of view 40 arcminutes in diameter. This field eneompassed the bowl of the dipper qiHtc niccly. I had sccn the Merope Nebula many times before, at low power under cxec11cnt skics. 「This

VISUAI. ASTRONOMY OF THE DEEP SKY

SMh is to the hf t. Photogmph of M45. (Courtesy Mount Wilson and Las Campanas Observatories, Carnegie Institution oj Washington.)

Drawing oj M45

Scale： 4.4 arc-mii/cm     Viewing Distance (cm)

8-lich fl1.5 Cassegrain -------------------------

air mass:1.05, faintest star: 14.2 at zen1th： 18J8X no tracking

10/08/83 11:40-12:15 UT at Waianae ranch, Hawaii; R. Clark

Brightest stars ofthc Pleiades

observation proved diircrent, however, because the nebula could be seen well at as high as 82 X. And when the magnification was inereased to 117x, the nebula beeame easier! U ndcr excellent conditions, the Merope Nebula is easily seen in the 3-inch finder at 31x.

The Pleiades' many and varied stars and delieate nebulosity provide a beautiful view to the amateur. It is no wonder the cluster is called the most studied and photographed of astronomieal objects.

Figure 7.1.Thc brightest Pleiades, with their visual magnitudes. To the naked 总 ye Asteropc appears single; the magnitude is |he eombined light of both stars. The 6.2-magnitude star at upper left is also a naked-eye blend. From Sky & Teles&pe、 November,19851

Ml (NGC 1952), THE CRAB NEBULA: SUPERNOVA REMNANT IN TAURUS

RA. 05^h34.5^m, Dec. 22° 〇!^^, (2000.0)

Technical. Messier 1 has bccn known as thc Crab Nebula ever slnee the third Earl of Ros-sc observed it w让 h his 36-inch telescope in 28441 In the drawing hc made, the nebula's filaments suggest the legs of a crab. Photographs show a beautiful network of red filia-mcnts throughout a diffuse grcen oval.

Ml is onc of the youngest objects in the sky and certainly the youngest Messier object. It is the expanding remnant of a brilliant nakcd-cyc supernova that was sccn in July, 20541 Now 5 by 3 arc-minutes in size, Ml is growing by nearly a half arc-minutc per century. Changes have been photographed in only a couplc of decades.

M1 is about 6000 light-years distant and 6 light-ycars across. It is onc of thc strongest radio sources in the sky and also emits strong X-rays. Near the ecnter of the nebula is a lGth-magnitude star that (lashes 30 times a second in visible light, radio, and X-rays. This is a pulsar, the superdense neutron-star core of an old supernova. It is thought to bc only 20 kilometers in diameter, and so dense that a teaspoonful of its matter would have a mass of several million tons! The (lash rate c^irrcsponds to its period of rotation. Its very strong magnetle field spins with the star 30 times a second; energctle c1cetrons trapped in the field produce the radiation. Thc pulsar is actually pumping energy into the nebula, so thc expansion rate is lnercaslng.

Visual.MI can bc sccn in binoeu1ars and small te1eseopes as a small, faint patch. With a visual magnitude of 9 and a size of about 5 by 3 are-mlnutes, it has a mean surfaec brightness of20.6 magnitudes per square arc-second1 Large amateur tc1cseopcs under dark skics will show some of the filaments, though they are often diflicult.

At low powers (60x or less) fcw details are visible. At higher powers thc nebula's outline begins to depart &om a smooth oval. In thc 8-inch at 18«x, the "bay" to the east is visible, and the whole thing takes on the appcar-anee of two oblong nebulae side by side. No stars could bc seen inside the nebula with thc 8-inch, but many were around it.

Photograph of Ml. South is up. (Courtesy Evered Kreimer, The Messier Album,J,H. Mallas & E.

Kreimer.)

Viewing Distance (cm)

air mass:1.01,faintest star:1 3.7 at zenith,188x, n 〇 [racking

11/14/82 1 2:48—13:10 UT at Barbers Point, Hawaii; R. Clark

VISU AL ASTRONOMY OF THE DEEP SKY

M42 (NGC 1976), M43 (NGC 1982), THE GREAT NEBULA IN ORION

M42:RA. 05^k35.^^rnD^(^.-05²23^, (^000.0) M43; RA. 05^h 35J6ⁿ Dec. —05。16^f

Technical. Mcssicr 42 and 43 arc probably t1te brightest and most spectacular nebulae in the sky, rivaled only by the Eta Carinae Nebula. Often referred to as lhe Great Nebula in Orii01ヽ142 and M43 are a beautiful example ofan H 11 region: an emission nebula containing mostly hydrogen, ffuoreselng in the ultraviolet light of very hot, newborn stars in its midst. The light of the Orion Nebula eonslsts primarily of grecn emission lines of oxygen, with other colors from blue to rcd coming from emission of hydrogen： helium, nitrogen, and ncon.

Thc dlstanee of the Orion Nebula is usually given as around 1300 light-years and its size as about 3()light-years. The bright region is surrounded by vastly larger, dark clouds of gas and dust, which in fact fill much of Orion itself. Thc dusl is thought to bc primarily silicate (rock) particles only a micron in diameter. T'hc composition of the glowing gas has been given as: hydrogen 90.8 %, helium 9.08%, carbon 0.05%. oxygen 0.02%, nitrogen 0.02%, sulfur 0.003%, ncon 0.0009%, chlorine 0.0002%, argon 0.0001%, and ffuorine 0.000D%.

Dccp within the cloud many stars are forming. One sign lhal the dark cloud is much bigger than the portion wc sec is that the region is full of bright infrared sources -stars whose visible light is blocked by dusl. Ihc bright nebula appears to bc a hole blown in the dark e1oud's wall, allowing us to scc part way in.

In lhc brightest part o ftlic hole are lhe four bright Trapezium stars, which arc responsible for illuminating lhe ,gas. Thcsc very young, hot stars are estimated lo be only 100 000 years old. T'hcy arc among the youngest stars lhal amateurs can scc.

Visual・ Tlic Orion Nebula is easily sccn in the middle of the "Sword of Orion’ ' as a I'uzzy star. Curiously, however, its haziness is not mentioned in ancienl records. The first known dlseovery of the nebula was by Nicholas Pieresc in 16)1.

11 is very pretty in any instrumenl from binoeu1ars to the largest telescopes. N142 is 65 are-minutcs in diamclcr (twice the size of thc full Moon) with a total visual magnitude of 4. M43 is about 7 by 5 arc-minutcs and has a total magnitude of 8. Thc average suriace brightness of thc Orion Nebula is 21.7 magnitudes per square are-second - bul the faintest parts are much dimmer and thc ' Trapezium region very much brighter, in thc range of 17 magnitudes pcr square arc-sccond. This large range in brightness is diflicult to photograph, and most pictures overexpose the Trapezium region. Thc cyc has a larger dynamic range than Him and, given good skies, can scc thc faint and bright portions of thc nebula at thc same time.

Thc detail visible in the Orion Nebula is truly spectacular. Hcrc is onc of the few sights in the sky that, when seen through modest amalcur te1eseopes， impresses cvcn those not excited by astronomy. The imtraca-is beyond description, and even after hundreds of hours of viewing this nebula, it still remains a beautiful and wondrous sight.

Its very complcxity makes the nebula di 匚 flcult to draw. Thc drawing made with the 8-inch on page 101 took two nights (January 17 and 18：1983) and nearly six hours of work (2 hours of observations and 4 hours for thc ffnal drawing). This short a time was only possible beeause many hours had bccn spent viewing and sketching thc nebula on other nights. In fact a drawing done a year bclorc, shown 〇 n page 103, shows eonslderab1y less detail. At lhal time I had less experience making detailed observations, used lower magnlf)eatlons, and spcnl less time at the tclcscopc.

Thc "I'rapczium region conlains tlic brightest patch of thc nebula, about 4 by 3 arc-minutcs in size. The surface brightness here is so high lhal the amount of detail is often limited by almospheric turbu1cnkc (seeing) ralhcr than thc lim 让 ations of thc cyc - a silualion almost unknown in visual decp-sky work. With so much light, very high powers can bc used.

Many stars dot this region. The four brightest are the famous multiple star system 01 〇rioifs, They form a trapezoidal figure that inspired the group's name. The four stars are easily resolved in small telescopes； as their separations range from 8.7 to 19.2 arc-seconds and their mag ntu des from 5 to 8. Two additloial components, shining at 11th magnitude, can be made out with a 6-lnch telescope in steady air. The rest of the nebila is full of fainter stars: over 300 brighter than magnitude 17 are within just 5 arc-miiutes of the Trapezium. The nebula contains more than 50 variable stars with maxima ,greater than magnitude 14.

The fainter regions of the nebula coitaln many loops of nebulosity familiar from photographs. They are visible in moderate sized amateur telescopes. 1'he brightest arc, extendfgg south oi the east side, is visible in telescopes as small as about 2 inches. Since M42 is over one degree in diamcter： low powers are needed to view ft all at once. However, only at moderate to high powers do most of theelegant arcs become visible. In the 8-1 nch, they were best seen at powers near 200 x.

The faintest portions of the nebula form a loop at the southern end, opposite the Trapezium, giving the nebula a circular, almost bubble-like shape. At low power this outer loop is seen only on very dark nights. Surprisingly, though, at niedium to high powers it can be seen on moderate nights such as those on which the drawing of Jaiu-ary 1983 was made. The outer loop appears continuous at low powers, but at 11 7X in the 8-lnch, the individual sections seen on photographs could be resolved.

Powers of lOOx to 200x show a dark patch just south of the Trapezium. This patch is next to a star 2 arc-miiutes south aid slightly west of' the southern tip of the bright zone containing the Trapezium stars. This bright comer of the nebula is a nearly perfect right angle, aid its edges appear remarkably straight. It looks almost uniatural.

To most observers the Orion Nebula appears pale green. The visibility of other colors is somewhat conitoveesiaa A few observers report a faint reddish color, cspe ・ cially oi the southern edge of the Trapezium region - which is indeed qute red in photo ・ graphs. On extremely dark nights, I have seen the Trapezium and faint outer regions to appear green while most of the nebula appeared a vivid pastel pink, aid the bright arc extendlig south oi the east side appeared pastel blue. Blocking the bright area around the Trapezium makes the detection of red in the outer regions easier.

VIS UAL ASTRONOMY OF THE DEEP SKY

air mass:1.12, fainicst star: 13.8 ar zenith, 188x, no tracking

1/18/83 8:40-9:48 UT at Hawaii Kai, Hawaii;

R. Clark

visual astronomy of the deep sky

Photograph o/the inner region o[M42, the Trapezium area. South is up. (Courtesy Lick Observatory.)

2'--------------

Drawing of M42 Trapeziunn area.

Scale: 0.5 ase-m11/cm      Viewing Disaance (cm)

8-lich f/11.5 Casscgraii -------------------------

12.4mm Erfle (18 殳)     100x：69   300x2イ

200 x： 34   400x：1 7 air mass:1.17, faintest star: 13.5 at zeniih,188 X ; no tracklig

1/13/83 9:30—9:55 UT at Hawaii Kai, Hawaii;

R. Clark

NGC 2023

NGC 2024, IC 434 (THE HORSEHEAD NEBULA) NEBULAE IN ORION

IC 434:    R.A. 05^ft41.1^rnDec. -0T^>24^,

NGC 2023;ん4 05^t4L7ⁿ,Dec. -02。13' NGC 2024: R.A. 05¹ 41.9笃 Dec. -01° 5〇^, (2000,0)

Technical. The 2nd-magnitude star Zeta Orionis, the easternmost one of Orion's belt, is surrounded by bright and dark nebulae. Thc region is famous R)r the photographically speciacular Horsehead Nebula, B33. This is a dark dust clooud remarkably like thc head of a ghostly horse rearing up in silhouette against the bright background of thc emission nebula IC 434. Thc energy source illuminating IC 434 is the star Sigma Orionis, about 1 ° southwest of Zcta.

IC 434 appears to be colliding with, or perhaps burning its way into, a very large, dark eloud. The sharp, nearly straight boundary between them extends about 1° south of Zeta Orronis. The Horsehead, about halRvay down thc boundary's length, is a protuberancs of thc dark cloud. The boundary extends about 18 light-years from north to south, and thc Horsehead is about 1light-year across. Thc dark cloud covers the entire eastern part of the drawing and photograph, as can be seen by the relative absenec of stars on the right, and in fact it extends many degrees still farther cast. Thc whole complex has been placed at a distance of about 1200 light-years.

NGC 2024 is a complex nebulous patch about 15 arc-minutes east-northcast of Zeta Orionrs. It is an emission nebula, probably excited by Zeta, crossed by a large, dark lane running north to south.

NGC 2023 is a bluish reflection nebula surrounding an 8th-magnitude star a little less than half a degree farther south, castnorthcast of the Horsehead.

Visual. NGC 2024 is the brightest and easiest nebula in this fascinating region. It is very elose to 118^-magnitude Zeia Oi^Io^Is, so placing the bright star out of thc feld of view reduces glare. NGC 2024 shows considerable detail in medium size telescopes, and under good skies it is visible through the 3-inch finder at 3 1 X. Through the 8-ineli thc main dark lane could be seen, along with several smaller ones, as shown in the drawing. In a 13-incb telescope, many more dark lanes appeared.

NGC 2023, a diffuse patch around an 8tli-magnitude star, is quite easy to see ii the optics are e1enn and free of dew. The best way to check whether the nebula is real, rather than starlight scattsrcC in the atmosphere, telescope, or eye, is to examine other stars of similar brightness to make sure they arc fiee of ttnebulosity''.

^{IC 434} is very faint and considewd ^{one of} the most diHcult deep-sky objects to observe visually. It is often said that cxeellent skies and an 8- to 12-.nch telcseope at low power arc required. The Horsehead, B33, is even more diflleult, being only 5 arc-ii)fnutes across. Many amateurs find the dark lane m NGC 2024 and believe they've found the Horsehead.

Glare from Zcta Orionis is one reason for failing to detect IC 434, especially with dirty or dewed optics. Low power is another. Medium powers not only make IC 434 and B33 larger but also increase their apparent distance from Zcta Orionis. As can be seen in the drawing with the 8-inch telescope, IC 434 and B33 were detected imde.r only moderate to 〇good skies. Thc drawing by Ray Fabrc using a 1：3.1-inch Dobsonian telescope at nearly the same powers shows a much better view of both. This view of the Horsehead is similar to that in an 8-inch under excellent skies. If good to excellent skies can be (bund, the Horsehead Nebula should be fairly easy in a 6-Inch teleseope. I have also seen it through the 3-indh at 5()X under the excellent skies of the Colorado Rockies.

Unfbrtlmats1y，thsrs are no magnitude or surface brightness measures of either NGC 2024, NGC 2023, or IC 434. However, IC 434 is slightly harder to detect than the Merope Nebula in thc Pleiades. Thc average surface brightness of that nebula is 21.6 magnitudes per square arCiSe(：ond，so IC 434 is probably 22 or slightly fainter.

Drawing ofNGC 2023, NGC 2024_y and iC 434 by

Ray Fabre with a 13-inch Dobsonian reflector at 6"X and 90X . N°e the increased detail seen with the larger aperture coompared with the 8-inch draiw-ingf made al th e s ame time and site. The scale and viewing distances are the same as that for the 8-inch drawing.

air mass:1.1；faintest star:15.2 at zenith, 9°X: no lracking

2/12/83 7:15-8:00 UT at Waianae Ranch, Hawaii; R. Fabre

Photograph ooNGC2^^^23₃ NGC2024, andIC434. South is up. (CourtesyJames E. Gum.)

Drawing " NGC 2023, NGC 2024, and IC 434 with an 8-inch telescope. Th£ Horsee)£ad was barely seen with the 8-inch telescope. A power o/ 117'x was

blitterfor detecting it than 82 x ； the higher power may have helped reduce the glare fom Zeta Orionis.

Scale: 4.6 arc-mi】i/cm     Viewing Dlitamce (cm)

8-inch fl1.5 Cassegrain ----------------------

28mm Kellner (82 X)      10x： 75   lOOx：?

20mm Erfle (117X)       25X：3O 200x：4

50x：15 3OOX：2

air mass: ).)2, faintest star: 14.2 atzeniih, 117X； no tracking

2/12/83 7:0(0-8:00 UT at Waianae Ranch, Hawaii;

R. Clark

M78 (NGC 2068), NGC 2071, DIFFUSE NEBULAE IN ORION

M78:      R.A. 05^h 46.8^m,Dec. 00° 03'

NGC 2071: R.A. 05^h 47.2ヽ Dec. 00。18, (2000.0)

Technical. M78 is a small refection nebula a little more than 2° northeast of Zcta Orionis. It was discovered by Pierre Mechain in 1 78〇. The nebula is bclicvcd to be about 1600 light-years distant, roughly thc same as Zeta Orionis. It is 2 or 3 light-years across. Thc light it scatters is from the 10th-magnitude star HD 38563.

Just northcast of M78 is NGC 2071, another reflection nebula. The entire region is enveloped in a dark nebula, so stars are few. The total magnitude of M78 is 8, and with a size of 8 by 6 arc-minutes, its average surface brightness is 20.8 magnitudes per square arc-second. NGC 207 1 is 4 by 3 arc・ minutes, and its average surface brightness is somewhat dimmer than M7 8's.

Photograph of M78 and NG C 2071.South is up. (Courtesy EveredKreimer, The Messier Album.)

Viisuuli M78 often looks like a small comct. With a visual magiitude of 8 aid a size of 8 by 6 arc-mi1utes： it has a mean susface brightness of 20.8. Its 1osthes1 border is sharp, while the southcri part dims gradually, stasting near the iorthcri edge. Thc icbuia contaigs two lOth-magiitude stars, one near the iorthcri edge, thc other 53 arc-secoids to the south. A third aid faiitcr star is icar the southern limit of thc icbula, slightly east of the other two.

NGC 207] appears smaller aid fainter than M78. It has a size of 4 by 3 asc-ci1utcs： but there arc io magiitude estimates.

Drawing of M78 and NGC 2071.

Scale： 2.0 ase-m11/em      Viewing Di^siai^c^c (cm)

8-11eh £711.5 Cassegrain -------------------------

12.4mm Erfle (188X)      25x：69   200X：9

5OX：34   300x：6

100x：17   400x：4 air mass:1.09, faiitcst star 13.7 at zciith,1 88x, no trackiig

11/14/82 11:51-12:15 UT at Barbers Point, Hawaii; R. Clark

NGC 2261, HUBBLE ' S VARIABLE NEBULA IN MONOCEROS

R^.A. 06^h 39•广,Dec. 08° 43' (^2000.0)

Technical. NGC 2261 was discovered by Sir William Herschel in 1783, bul il has been known as Hubble's Variable Nebula ever since Edwin Hubble found, on photographs taken between 1900 and 1916, that it had changed shape. NGC 2261 is a small rcfleellon nebula that appears rather like a comct. Thc variable star R Mlonocerotis is at its apcx, and appears as if it were the "comet's" nucleus. R Mon varies by up to 4 magnitudes in an irregular way. Its variations arc responsible for the changing appearance of thc nebula, which it illuminates. However, thc nebula and star do not vary together. It seems that dusl clouds orbit close to thc star, blocking its light to a greater or lesser degree and casling thcir moving shadows on thc nebula.

Some changes have been seen lo occur in as '让 tic as one day, and shadows have moved by up to 1 arc-second in four days. Changes that might bc seen in large amateur telc-icopei can occur on timcscalci as short as a month. On nights of very good seeing, with large teleicopei R Mon docs not appear quite stellar. Apparently dust close to thc star forms a very small, brillianl reflection nebula.

Photograph 〇/NGC 2261.South is up. (Courtesy National Optical Astronomy Observatories.)

Visual.NGC 2261 is quite easy tn medium size telescopes even under oily moderate skies. I first found ft while randomly sweeping the winter skies aid immediately thought it was a comet. A check of several star charts and Bumham も Celedal Handbook set me straight. At magnitude 10 with a size of2 by 2 asc-minutes： it has a mean surface brightness of '20.1 magnitudes per square asc-seco1d. This is quite bright compared to most icbu-lae. Although no detail was seen ii the 8-lnch under moderate skies, ft should be possible to see detail under excellent skies if the shadows at the time arc high in coitrast. Since the surface brightness is high, and the icbula is a scflectio1 from dust gsai1s： it may appear bluish in large telescopes under dark skies.

Drawing oj NGC 2261.

Scale: 0.8 asc-min/em

8-iich (711.5 Cassegrain ]24mm Erfle (188x) 9mm Kcllncr (260x)

Viewiig Distance (cm)

air mass:1.07, fai1lesi star:13.5 at zenith, 18J3X； io tracklig

1 / 13/83 10:35-11:05 UT at Hawaii Kai, Hawaii；

R. Clark

VISUAL A STRONOMY OF THE DEEP SKY

M46 (NGC 2437), OPEN CLUSTER IN PUPPIS WITH NGC 2438, A PLANETARY NEBULA

M46:     R_tA. 07h4L9^m,Dec・ 一 14° 49

NGC2438: R.A. 07れ 41.9^m,Dec、一14° 43' (2000.0)

Teehnicca M46 was diseovered in 1 771 by Charles Messier. The cluster is about 5400 light-years distant and 30 light-years in diameter. It contains between 200 and 500 stars, with about 150 between magnitudes 10 to 13.

M46 is unusual in that it appears to contain a planetary nebula. NGC 2438. But thc nebula is not a member of the star cluster, since it is nioving through space at a different velocity. The nebula appears 65 arc-seconds in diameter. It is probably in thc foreground about 3000 light years from us, and has a diameter of about 1light-year. Its central star is 16th magnitude.

Visual. M46 is a pretty cluster at any mag-nificati on, where it all fits within the field of view. The cluster is round, with a Ciamster of 25 arc-minutes. Thc total visual magnitude is 8, and in small instruments and low power, where the cluster is not resolved, thc mean surface brightness is 23.6 magnitudes per square arc-sccond.

The planetary nebula is best seen at medium to high powers. At 150x or 200x with a wide-field eyepiece, the whole cluster and the planetary can be seen simultaneously. Thc nebula is 11 th magnitude, with a mean surface brightness of 19.81 In mediumsized telescopes, a star slightly northwest of center is often mistaken for the csntrn1 star, which is fainter. In thc 8-inch under moderate skies, the nebula appeared as a doughnut noticeably fainter on the west side.

Photograph 〇/M46 andNGC2438. South is up. (^o^^tesy EueredKreiimier, The Messier Album.)

Drawing of M46 and NGC 2438.

Scale: 3.0 arc』】】い川     Viewing Distaicc (cm)

8-lich "11.5 Cassegraii -

1 2.4mm Erfle (18Bx)       10x：115 100x：ll

9mm Kcllicr (260x)      25x: 46 20(° x: 6

50X： 23 300x： 4

air mass: 1.39, falitcst s:ar:13.5 at zciith, 188X, no tracking

1 /13/83 8:45—9:1 8 UT at Hawaii Kai, Hawaii;

R. Clark

M67 (NGC 2682), OPEN CLUSTER IN CANCER

R.A. 08^h 51,0^rn, De 11 49 (2000.0)

Tehhnihal. This open star cluster is unusual in that it is one of thc oldest known (estimates range from 3 to 10 billion years) and is very far from the plane of our Galaxy. Most open clusters, also known as galactic clusters,lic close to the galactic disk, whereas MI67 lies approximately 1500 light-years above it. The cluster is about 2500 light-years distant and 12 light-years in diameter. It includes some 500 stars brighter than magnituCs 16; the brightest is magnitude 10.

The stellar population is unlike that of most open clusters, but rather similar to the very old globular clusters. The only other open cluster known to have a similar pattern is NGC 18& in Cepheus. The most evolved stars in M67 have only about half the luminosity of similar stars in globulars, however. This difference is nttributcC to chemical composition: the typical globular is made almost entirely of hydrogen and helium, whereas M彳67 has a hcavyic1emcnt ratio similar to onr Sun.

Photograph of M67. South is up. (Courtesy Optical Astronomy Observatories.)

Visual. M67 has a total visual magnitude of 7, and it is 15 arc-minutes in diamcler. When unresolved in very small instruments such as binoculars, its mean surface brightness is 21.5 magnitudes per square arc-sccond. The drawing shows an intcrcsling visual cflcct of closely spaced stars. The cluster contains many faint stars not resolved in an 8-incli telescope. With averted vision, the eye can detecl their light without resolving them. This gives an appearance of a faint background nehulosity. The drawing is a rcpre-seKtation of this eflcct. With direct vision the "nebulosity" vanishes.

Drawing 〇/M67.

Scale：1.2 arc-min/cm

8-lmh(711.5 Cassegrain 20mm Erfle (11 7X j 12.4mm Erfle (188x)

Viewing Distance (cm)

rⁱrmrss: 1.05, faintest sar:14.0 al zenith, 188X, n o tracking

1/16/83 10:30-10:45 UT at Barbers Point, Hawaii;

R. Clark

25x：115 200x：14

50x： 67 300x：10

】00X： 29 40Ox： 7

NGC 2903, SPIRAL GALAXY IN LEO

R.A. 〇卯 32尸,Dec. 21^ 3T (2000.0)

Technical. NGC 2903 is a large, many-armcd type-Sb spiral galaxy, one of the brightest galaxies in Leo. It shows complex detail og deep-sky photographs. Thc only other galaxy of comparable brightiess in Leo is M66.

Visual.Ii angular size NGC 2903 is 11.0 by 4.7 arc-miiutcs, and its total visual magnitude is 9.7. The mean surface brightiess is 22.6 magnitudes per square arc-sccond. In the 8-fnch under moderate skies, NGC 2903 was seen as a diffuse glow surrouidfig a brighter nucleus. Under excellent skies ft should reveal some detail in ai 8-fnch or largcs telescope.

Drawing 〇 /N GC2903.

Scalle 1.1 nrdimin/dm      ■Viewing Distn1dce (cm)

8-inch f/1115 Cnsscgrnin -----------------------

12.4 mm Erfle (188x)     25x：125 200x：16

50X： 63 300x：10

100X： 31 400x： 8

air mass： LOO, faintest stta:1 3.5 at zenith, 188X ； no tracking

1/13/83 ll:4(0-12:40 UT at Hawaii Kai, Hawaii;

R. Clark

Notes: Intermittent clouds increased the time neeCeC to make the observation.

VISUAL ASTRONOMY OF THE DEEP SKY

M81(NGC 3031),SPIRAL GALAXY IN URSA MAJOR

RA. OS卯 55.60 Dec, 69° 04 (2000,0)

Technical.M81 is sometimes known as Bodc's Nebula, after its discovery by J. E. Bodc ii1774. It is a spiral galaxy tilted 32° ^om cdgc-on. Relatively close to us at about 8 or 10 million light-years, ft has a diameter of 40 000 to 50 000 light-years. MI81 is the brightest member of a group of about a dozen galaxies; its nearest icighbor aid closest rival in brightness is M82 (NGC 3034). Radio observations show hot gas extending twice the length of the spiral arms seen on photographs. This hot gas has been traced all the way across a ^bridge'' to M82, Presumably thc bridge is the result of a close encounter between the two galaxies about 200 million years ago.

Visual.M81 appears as a diffuse oval with a bright hcntes. It has a size of 18 by 10 arcminutes, aid its total visual magnitude is 8.O). The mean surface brightness is 22.3 magnitudes per square arc-second. IThrough the 8-fncli under good skies, only the slightest hint of the spiral arms was seen. Thc outer spiral arms are of very low coitrast. On short-exposure photographs through large telescopes, spiral arms with thin dust lanes can be traced to within half an ash-mi1utc of the nucleus, but none could be seen through the 8-fncli.

Thc easiest part of thc outer arms to detect is west aid slightly north of the nucleus. Thc arm here consists of several bright knots, which in large telescopes look like a string of falit stars,

Photograph ojM81. South is up. (Courtesy National Optical Astronomy Observatories.)

Drawing of M81.

Scale：1.3 arc-min/cm     Viewing Distance (cm)

8-inch (711.5 Cassegrain --------------------------

12 一4mm Erfle (188X,      25x：106 200X：13

best view)                    50x： 53 300x ： 9

7mm Erfle (334x)        WOx： 26 400x： 7 airmass: t.2t faintest star：14.3 at zeniih,188 X； no tracking

2/12/83 9:55-10:05 UT at Wninnnc ranch, Hawaii: R. Clark

M82 (NGC 3034), PECULIAR GALAXY IN URSA MAJOR

尺.4. O卯 56.1厂,Dec. 69° 42, (^^000.0)

Technical. M82 's an irregular galaxy discovered, along with MI81 about a degree south, by J. E. Bode in 1774. Fills galaxy is unusual because its ec1tral region is very dusty aid seems to be violently disrupted. It was long thought that the core of the galaxy had uidcrgoic ai explosion a few million years ago. Now it is thought that the inner region is merely in vigorous turmoil, probably owing to great bursts of star formation in the dusty areas. Many of the abuidait gas clouds arc moving outward at speeds of up to 50() km/ sec. The violent gas motions have produced many shock waves, which have compressed the rcmaiiiig gas aid dust to trigger the formation of yet more stars.

This thick interstellar matter provides spectaculas detail on photographs. Much of the gas is heated by the newly formed stars to a temperature of 100 kclvfn (—17()°C). This is very hot compared with typical litcrstcllar dust, wliic h is only a fcw degrees above absolute (zero kelvin). Thc stars that do this heating arc in many large clusters, each with as many as 1 O 000 members. The heated dust makes M82 the brightest ：galaxy in thc sky at thermal infrared wavelengths, aside from thc Milky Way. M82 is also a strong radio source.

Recent reeonstruetions of thc history of XI82 may explain what happened to ft. About 200 million years ago X181 passed close to X182, aid the gravitational tidal effects pcr-turbed thc orbits of billions of stars and great masses of gas. Thc result was collisions between ：gas clouds, causing many of them to collapse and produce new stars, while other clouds were thrown out of the disk plane of XI88.The ejected material eventually slowed aid fell back into the plane, causing more cloud collisions aid thc burst of star formation wc sce today.

M82 is about 8 or 10 million light-years distant aid 16 0()() light-years across. Its mass is estimated at only oic-tcith that of M81.

Visual. M82 shows coisidcrablc detail in amateur telescopes. At a total magnitude of 9.2 aid with a size of 8 by 3 aro-mir^ut^cs, the mean surlacc brighticss rs 21.3 magnitudes per square ase-seeo1d. This is a full magnitude more surface brightness than the mean of neighbor X181 (though ヽ18Ts central region is admittedly much brightes). M82s high surface brighticss allows high mag】】】-ficatlons to be used even with small w】— scopes.

In the 8-inch, the main 1rscgular dark laic in the ee1ter of the ,galaxy was very easy to see at medium powers under good sklcs. Thc dark lane, the galaxy's most promiicnt feature, is diHicult in 4-incli or smaller tcle-scopes. Other structure was visible in thc B-'icTi, such as the sharp drop in intensity about 1 are-m11utc cast of the iiucics.「hc dark lane gave the appearance of an arrow pointing west aid slightly south. A bright region was seen to the west of the nucleus. Powers near 200X seemed to give the best overall view. More detail might have been visible if the seeing was not so poor (star images were 2 to 3 arc-seconds in size).

Photograph f M82. South is up. (Courtesy Cana-da_France-Hawaii Telescope Coゆoration.)

Viewing Dislancc (cm)

Scaale 1.2 rrk-min^m

8-inch f711.5 Cassegrain 20mm Erfle (117X)

12.4 mm Erfle (188x, best view) 7mm Erfle (334X)

25x：) 25 200X：14

50x： 57 300X：)O

100X： 29 400X: 7

air mass： 1.55, faintest star: 14.3 al zcnilh, 188X; no tracking

2/12/83 9:15-^9:55 UT at Waianae ranch, Hawaii; R. Clark

M96 (NGC 3368), GALAXY IN LEO

R,A 10h 46少,Dec.11 49 (2000.0)

Technical. M96 is an 5a-typc spiral galaxy in thc Leo Cluster of Galaxies , which includes M95： M65, M66. and M )02. The group is around 30 million light-years distant. As with all type -Sa galaxies, a large central bulge Com1natss tightly wound spiral arms. M96 has a laint outer ring about 6 arc-minutes in diameter.

VisuuL M96 has a total magnitude of 10.2 and a diameter of 6 by 4 arc-minutes, for a mean surface brightness of 22.3 magnitudes per square are-sseond1 Most of the light is concentrated in thc central bulge, however, and the surface brightness thsrs is much higher. Through the 8-ineh in good skies, a dark lane was easily seen at medium powers. I did not know it existed before making the observation. The lanc is not visible in telescopes smaller than 4 inches.

Dawiing of M6

Scalle 1.2 arc-min/cm     Viewing Distance (cm)

8-inch f711-5 Caiiegrain ------------------------

12.4mm Erfle (188x)      25x：115 200x：14

5OX：57   30x：10

ld)x：29   400x：7

air mass:1.09, faintest sair 14.3 at zeniih, 188X; no tracking

2/12/83 11:10-10:31 UT at Waianac ranch, Hawaii; R. Clark

M105 (NGC 3379) NGC 3384, NGC 3389, GALAXIES IN LEO

M105:      RA10h 47,8^m, Dec.1.。35'

NGC 3384: R1A.10h 48.3^m, Dee.1.。38' NGC 3389: R.A.10^h 48.4和 Dee.1.。32' (2000.0)

丁“11川51・ M 105 and NGC 3384 arc elliptical galaxies and NGC 3389 is a typc-Sc spiral. All arc part of the Leo Cluster of Galaxies, which includes M95, M96. M65 and M66 at a distance of about 30 million light-years. NGC 3384 is only 8 arc-minutss from M105, and NGC 3389 is 10 arc-minutes from it.

Visual. Ml 05 is thc brightest member of the trio, with a total magnitude of 10.61 Being 2 arc-minutes in diamstsr： its mean surface brightness is 20.8 magnitudes per square arc-sscond — but, like most galaxies, it is far brighter in the middle than at the edges.

NGC 3384 is only slightly fainter at magnitude 1110. It is larger, 4 by 2 arc-minutes, yis1ding a mean surface brightness of 21.9 magnitudes per square arc-second. Most of the light is concentrated in the c^ 0^X1111 arc-minutS1

NGC 3389 is ths most diHicult: it has a total magnitude of 12.2 and appears 2 by 1 arc-minutss. This size corresponds to a mean

Photograph of M105 (aL 帝)NGC3384 (at lower Tigh), and NGC 3389 (at upper right). South is up.

((Courtesy Evered Kreimer, The Messier Alburn.) surface brightness of 21.7 magiitudes pcr square ase-seco1d. The light is dishsibuhcd fairly uniformly, so thc peak surface brightness is not much gseatcs： unlike with N1105 and NGC 3384.

All three appeared featureless through the 8-lnch under moderate skies. The elliptical galaxies showed brighter ceiters, and NGC 3384 appeared slightly eloigated, while Ml05 was round. NGC 3389 appeared as a very faint oval of u1ifosm esight1css. At medium powers with wide-field eyepieces, all three galaxies fit into thc ffcld of view aid provide a nice view.

--------------5’--------------

Drawing 〇^M105, NGC3384, QmiNGC3389.

Scaae:1.3 arc-mi n/cm

8-lnch "11.5 Cassegsa11

20mm Erfle (117x)

12.4mm Erfle (188X, best view)

9mm Kellner )26(0 X )

25x：106 2OOX:13

50X： 53 300X： 9

100X: 26 400X： 7

M108 (NGC 3556), GALAXY IN URSA MAJOR

Il lL6^m, Dec. 55° 4 (2000.0)

Technical.M 108 is a nearly edge-on spiral galaxy of type Sc. First discovered by Pierre Mechain in )78), it was not commonly included in Messier's catalog until the early 11970s. Ml08 is about 25 million light-years away and has many obscuring dust lanes and no pronounced central bulge.

Visual. Ml 08 appears 7.8 by 1.4 arcminutes in angular size, with a total brightness of magnitude 11118 and a mean surfkcc brightness of 22.0 magnitudes per square ar^c-second. It is 48 arc-minutes northwcsl of the Owl Nebula, M97 (NGC 3587). Through the 8-inch under moderale skics, no delail corresponding to dark lanes was seen. The nucleus looked like a star, and an aclual star is superimposed on thc galaxy's western side. The galaxy appeared as a uniform, elongated glow, fading at lhc edges into the sky background.

Photograph 〇/M108. South is up. (Courtesy Evered Kreimer, The Messier Album,)

Drawing oj M108.

Scale:1.2 arc-min/cm

8-inch fll.5 Cassegrain

20mm Erflc (117X) 12.4mm Erflc (188X, best view)

7mm Erfle (334X)

Viewing Distance (cm)

air mass: 1.41, fainlest stair 13.8 at zenilh, 188X; no lracking

5/15/83 8:41 - 8:52 UT at Barbers Point, Hawaii;

R. Clark

M97 (NGC 3587) THE OWL NEBULA: PLANETARY NEBULA IN URSA MAJOR RA.ll 14.9m Dec. 55° 02^f (2000.0)

Technical.M97 was discovered by Charles Mlesskr's compatriot Picrre Mechain in 1171-When William Parsois, Lord Rosse, observed it with his 7 2-iich speculum-metal sef€Ctos in 1848, he made a drawing semark-ably like ai owl's face, aid M97 has been the Owl Nebula ever since. It is thought to be about 1600 light-years distant aid about 1.5 light-years in diametes. Its gas is only 1/10 to 1/100 as dense as that of a typical plaietary nebula, suggesting that M97 is relatively old aid has been expandiig and thiiiiig for a Tong time. The 14th-magiitudc ce1tsal star has ai elective tempesature of 85 000 kelvin.

Visual.Thc two dark "owl's eyes" are hard to dctcct in small to medium telescopes. With a diameter of 2.5 are-mi1ulcs and a total magnitude of 11.0, the nebula has a mean surface brightness of 22.6 magnitudes per square arc-sccoid — very low for a plaietary nebula. By comparisoi, thc famous Ring Nebula, M57 ii Lyra, has a mean surface esightness of 1 7.9 magnitudes per square arc-secoid. With the 8-iich uider moderate to good skies, the eyes were barely seen. If I had got known of their existence I probably would not have detected them at all. When making thc drawiig I did not know the eyes' true osie1tatio1, so the fact that they arc placed cosrectly iidicatcs that they really were seen 一 unless blind luck was at work. This u1eertai1ty shows the extreme difllculty in observing the owTs eyes. Better skics or a larger teleseope are needed.

air mas: 1.36, faintest staa'： 13.8 at zciith, 188X； no tracking

5/15/83 8:25-8:40 UT at Barbers Point, Hawaii;

R. Clark

VISUAL ASTRONOMY OF THE DEEP SKY

M66 (NGC 3627) NGC 3628, M65 (NGC 3623), GALAXIES IN LEO

M65：           1&9ゝ De 13° 07

M66:     Rl^.li^h 20.2^, Dec.13 OU

NGC 3628： R.A.1N 20.3楓,Dec. 13°37‘ (2000.0)

Technical. This trio of galaxies fits into a low-powc： field of view. They arc members of the Leo Cluster of Galaxies, about 30 million light-years away, which also includes M95, M96, and M105.

All three galaxies arc typc-5b spirals, with NGC 3628 being nearly edge-on. M66 has thick spiral arms and dust lanes.  M65

appears like a miniature Andromeda Galaxy (N31);it has a dark dust lane on thc east side, and the spiral arms are wound much like M31 s. M66 has a diamster of 50 000 light-years; M65 60 000 11ght-ysars; and NGC 3628, about 90 000 light-ycars.

Visua !・  M65 and M66 can be detected in

binoculars, while NGC 3628 requires a small telescope under good skies. M66 has a total magnitude of 9.7 and an angular size of 8.0 by 2.5 arc-m1nutss，yielding a mean surface brightness of21.6 magnitudes per square arcsecond. M65 has a total magnitude of 10.3, a size of */.8 by 1.6 arc-minutcs； and a mean surface brightness essentially the same, 21.7. NGC 3628 is more diflicult, with a total magnitude of 10.3 and a size of 12 by 2 arcminutes, yielding a fninter mean surface brightness, 22.4 magnitudes per square arcsecond.

In the 8-inch, M66 shows an oval central region surrounded by the oval glow of the fainter spiral arms. The arms appear dimmer than the impression given by most photographs, no doubt because the central region is usually ovcrexposeel. The oval corresponding to the arms is oriented generally north -south, while that of the bright central region is more northwest — southeast. The southern arm looked pointed, an appearance familiar from photographs.

M65 showed a bright central region surrounded by the fainter spiral arms. Thc dark lane eould not be seen in the 8-inch. NGC 3628 appeared considerably Winter than either M65 or M66 and was only slightly brighter in the center. The spiral arms fhded gradually into the sky background, and no dark lanes eould bc seen during this observation. However, under exeellent skies the dark lane so evident in the photograph is visible in an 8-ineh telescope.

Photograph of M66 (at upper right), NGC 3628 (at bottom) and M65 (at upper left). South is up(Cour-んリ Canad—Franc-Hawaii Telescope Corporation.)

Drawing 〇/M66 (at up阿!ight, NGC 3628 (at bottom), and M65 (at upper lejf).

Scale: 2,3 nrc-min/cm Viewing Distnncc (cm)

8iindh "11.5 Cnsscgrnin -----------------------

20mm Erfle (11 7x)         1Ox：115 100x：l I

12.4mm Erfle                25X ： 46 200 x： 6

50x： 23 300X： 4 ai mass:1 J 0, faintest sias:14.3 nt zenith, lS^X ； no tracking

2/12/83 t):3D-10:26 UT a： Wninnnc ranch, Hawaii; R. Clark

M109 (NGC 3992), GALAXY IN URSA MAJOR

RA.ll^h 57.6^m, Dec. 53° 22^f (2000.0)

Technical. NGC 3992 was added to Messier's catalog as ^<tM109^,5 as recently as 1953, by Owen Gingerich. It is a barred spiral galaxy of type SBb. A short bar extends through the central hub. At its ends are long, thin spnal aTms that can be traced for about three-quarters of a turn.

Visual.N4109 is magnitude 10.9 and 6.4 by 3,5 arc-minutcs in size; the mean surface brightness comes to 22.9 magnitudes per square arc-second. Through the 8-inch under moderate skies, M 109 appeared as a difH-ise round glow. The central region appeared stellar. There was no hint of spiral structure.

Photograph of M109. South is up. (Courtesy Evered Krehner, The Messier Album.)

Draiving of M109.

Scali:1.2 ase-mi1/em

8-iich 〃11.5 Cassegrain

20mm Er fc (11 7X)

12.4mm Erflc (1B8X, best view)

Viewing Distaicc (cm)

25x：115 200x：］4

50x：5 7   300x：10

100x：29   400x： 7

air mass: 1.32, faintest siar：13.8 at zenith,188 X; no tsack11g

5/15/83 9:05-9:16 UT at Barbers Point, Hawaii;

R. Clark

NGC 4038, NGC 4039, THE RINGTAIL GALAXY IN CORVUS

NGC4038:RA.12^h     Dec. -1^° 52^f

NGC 4039: RA.12¹01.9^m, Dec, 一］8 5V

(2000.0)

Technical. NGC 4038 and 4039 are interacting galaxies. The pair is known as the Ringtail Galaxy or the Antennae because of two very long, thin "tails" that extend roughly north - south from the small central part. The distance to the pair is about 50 million light-years. Recent computer models indicate the two galaxies were quite normal before their encounter, and that the tails are stars and gas flung from the outlying regions of the original galaxies. The gas, enough to form about 1.5 billion suns, has been detected at radio wavelengths, being primarily hydrogen. The two galaxies have probably taken several hundred million years to reach their present state.

The galaxies5 main portions show intricate detail. Both lack a nuclear region, and measurements of velocities show complex motions that at first seemed diflrcult to explain. However, each galaxy is rotating about its own axis, and the two are orbiting each other. Computer models can account for the motions when this combination of orbits is considered.

Photograph 〇/ NGC 4038 (the bottom portion of the irreguarly shaped U) and NGC 4039 (the top portion). South is up. (Courtesy Palomar Obseⁱrva-tory.)

VsuuL NGC 4038-39 look unusual, quite unlike normal galaxies. They appear as a "U" shaped object with the open end to the west. The northern leg of the U appears lnr-ger. This is NGC 4038, magnituCs 11.0, about 2.5 by 2.5 nrc-minutcs in size not including thc tails. NGC 4039 is magnituCs 12 and 2.5 by 2.Q nrc-minutes not including tails. The mean surface brightness is 21.6 magnitudes per square are-second fir NGC 4038 and 22.4 for NGC 4039.

The parts seen in the 8-ineh were sssme-what sniallcr than the sizes above. Two ■ ’stars" were seen, one in each galaxy. They may be two of the many bright knots that appear on photographs. If so, better skies should reveal much of the intricate detail with an 8-inch. Thc tails are extremely faint, and it is questionable if they can be seen at all in medium size telescopes.

Drawing of NGC 4)38 (the larger object,) and NGC 4039 (the smaller, thin extension above it).

Scale：1.2 arc-min/cm

8-inch (711.5 Cassegrain

20mm Erfle (117X) 12.4mm Erfle (188X,

best view)

25X：115 200X：14

50X: 57 300X:10

100X： 29 400X: 7

M99 (NGC 4254), GALAXY IN COMA BERENICES

RA.12^h 188： Dec、14° 25' (2000.0)

Technical.M99 is a type -Sc spiral in the great Virgo Cluster of Galaxies. It is probably 50 million light-years distant and has one of the greatest redshifts in the Virgo Cluster: 2300 kilometers per second. m99 was discovered in 1781 by Pierre M^echaln. It is nearly face-on aid has a well-defned spiral pattern. The bright southeri arm extends westward uiusually far from thc iucleus. This was the second galaxy to be recognized as spiral, by Lord Rosse in 1848.

Visual.  M99 appears at magiitudc 10.4,

with an aigular size of 4.5 by 4.0 arcminutes, yielding a low mean susface brightness of 22.2 magiitudcs per square arcsecond. Ii the 8-inch, thc ccitral portfon appeared as a small, bright diffuse area, while the spiral arms formed a soft, uniform glow around it. No hint of spiral structure was seen under moderate to good skies. Detection of the bright southcri spiral arm has been rcponcd by oesesvers under good to excellent skies with 8-iich telescopes.

Scaai:1.4 are-mi1/em

8-ln ch f/11.5 Cassegraii 20mm Erilc (117X ) 12.4mm Erfle (188X, best view)

7mm Erflc )334 X)

Vicwfig Distance (cm)

air mass:1.3 & faiitcst srar: 13.8 at zciith, 188X; io trackiig

5/15/83 10:13-10:28 U’f at Barbers Point, Hawaii;

R. Clark

M106 (NGC 4258), GALAXY IN CANES VENATICI

R-A.12^h 19J)m, Dec. 44° 18 (2000.0)

Technical. NGC 4258 was added to Mes-sifr's catalog as "Ml 06" by Helen Sawyer Hogg in 1947. The galaxy appears to have erupted about 20 million years ago. Its nucleus is unusually bright at both visible and radio wavelengths. The spiral arms terminate 〇 knots of bright young stars. "Ghost" radio arms, unseen optically, trail behind the visible arms. Two clouds of material, amounting to several tens of millions of solar masses, appear to have been ejected from the nucleus in the galaxy's plane.

Visual. M106, magnitude 9.0, is 20 by 6.5 src-minutes in size, though the brighter part b only 8 by 3 arc-minutes. The overall mean surface brightness is 22.9 magnitudes per square arc-second.

Through the 8-inch under moderate to good skies, this galaxy was quite a surprise. I was quite familiar with photographs of it and had seen it many times before, sometimes under good to excellent skies. But these observations were done at low powers b efore the research for this book was completed. The surprise was a distinct dark lane easily detected next to the nucleus on the west side. It is rarely seen on photographs because the central region is usually overexposed. The lane was best seen at powers near 200x. The spiral arm to the north was also seen, though its contrast was low. The arm ended in a point about 3 arc-minutes from the nucleus. Excellent skies or larger telescopes would surely show beautiful detail in this galaxy.

Drawing ” M106.

Scale: 2.3 arc-min/cm

8-inch “11.5 Cassegrain

20mm Erlle (117X)

12.4mm Erfle (188x, best view)

25X：6O 2OOx：7

50x：30   300x：5

100x：15   400 x：4

air mass: 1.24, faintest star:1 3.8 at zenith, 188X; no tracking

5/15/83 9:18-9:31 UT at Barbers Point, Hawaii;

R. Clark

M100 (NGC 4321),GALAXY IN COMA BERENICES

R. A.12^h 22,9^im,      15° 49 (2000.0)

Technip     M 100 is a spiral galaxy of class

Sc seen nearly face-on. Its distance is estimated to be about 40 million light-years and 让 s linear Ciametsr slightly more than 100 000 light-years. Thc galaxy's total luminosity is about 20 billion times that of our Sun, its mass about 160 billion suns. Thcrc arc two main spiral arms and complex dust lanes. The arms can be followed all thc way in to the nucleus.

Photograph of M100. South is up. (Courtesy Evered Krehrner, The Messier Album.)

Visual. In small to medium telescopes, M100 appears as a bright ceitral spot sur-roundcd by a uiiform, diffuse glow. Thc total magnitude is 10.4 and the diametcs 5 ase-minutcs. This corresponds to a mean surface es1ght1css of 22.6 magnitudes per square arcsecond. The main spiral arms are oily 3 arcminutes across, which is often as large as the galaxy appears. Thc arms beyond 3 arcminutes are quite faint and low in eo1tsast even uider excellent skics.

In the 8-iich, io spiral stsuctnsc could bc detected uider modcsatc to good skics. Some observers have scposted hints of spiral arms ii medium size telescopes.

Drawing of M100.

Scali:1.2 ase-m11 /cm     Viewing Distaicc (cm)

8'iich f/11.5 Casscgrali ------------------------

20mm Erfle (11 7X)        25x：115 200 x：14

12.4mm Erfle (188 X ,      50X： 57 300 x ：10

best view)                 100X： 29 400X： 7

air mass:12 & faintest       1 3.8 at zeniih,188 X：

no tracklig

5/15/83 9:55-10:12 UT at Barbers Point, Hawaii;

R. Clark

Surface brightnesses are in magnitudes per square arc-second.