Deep-Sky Companions The Messier Objects by Stephen James OMeara

This book is your companion under the stars. Similar to a field guide to birds, insects, or flowers, this book will help you locate and identify the Messier objects - the most famous deep-sky splendors in the universe. It

tographs, pencil drawings of their telescopic appearance, detailed descriptions, and descriptions of other interesting celestial sights near the Messier objects. I have also included a lineup of some of my favorite non

Each Messier object section opens with a photograph and some essential data, including the object's coordinates, magnitude, apparent size, and distance. Descriptions of the object from a translation of Charles Messiers original catalogue and from the New General Catalogue of Nebulae and Clusters of Stars (1888) or the two subsequent Index

Catalogues (1895 and 1908) follow. These data and descriptions should help you realize what to expect when you first glance at a Messier object through the eyepiece.

The objects are ordered as Messier catalogued them. However, you can view them in any order you wish, as long as a particular object is above the horizon when you plan to observe it. Use a star wheel to determine which constellations and, therefore, which Messier objects are visible on the date and time of night you want to look.

This book was conceived at the May 1994 Texas Star Party, where I spent several nights observing the Whirlpool Galaxy (M51) with a Tele Vue Genesis 4-inch refractor. Until that time I had spent most of my life observing the moon and planets with permanently mounted, long-focus, observatory-class refractors with old, high-quality glass. Any views of the deep sky were confined to high powers and narrow fields. For instance, lhe lowest magnification I normally used with the 9-inch f/12 Clark refrac-^tor at Harvard College Observatory was 137 x, which offered me a field of view. Only one-ninth of the Andromeda Galaxy (M31) would fit in that held! And when I used the 18-inch f/16 Clark refractor at Amherst College, a typical low power was 365 x and the field of view was even smaller. Even ^{as a} young Messier-object hunter in the 1960s, I used long-focus reflectors and refractors and never saw the objects in a field greater than 1°. The Genesis refractor, when coupled with a 22-mm (2-inch) Nagler eyepiece, will show the Andromeda Galaxy at 23 x in a field of view close to 3°; Nearly the entire galaxy and its companions fill the field. Furthermore, changing eyepieces and adding a Barlow lens allows me to study the galaxy's nuclear region with 10 times that magnification.

When it came time to decide which telescope to use for this project, I had no hesitation in choosing the 4-inch f/5 (500-mm) Genesis refractor. The unobstructed optics in the Genesis are of unquestionable quality. Al Nagler (formerly a NASA optical engineer who designed the wide-field optics for the Apollo Lunar Landing Simulator) created a special four-element optical primary for the Genesis refractor. When coupled with his Tele Vue eyepieces (which have an equally revolutionary optical design), the Genesis transcends the traditional limitations of both the long-focus and rich-field refractors by combining the best qualities ofboth.

For this study I used only two eyepieces： a 22-mm Panoptic (adaptable 1%-inch and 2-inch) and a 7-mm Nagler (1%-inch); these provided magnifications of 23 x and 72 x, respectively. A 1.8X Barlow lens with the 7-mm eyepiece gave me a "high" power of 130 x, which is a magnification of about 32 x per inch of aperture. As a rule, 50 x per inch of aperture is considered the maximum useful limit under ideal atmospheric conditions. Although the Genesis can perform beyond that theoretical limit (especially under very dark and stable skies at high altitudes) _FI restricted myself to the above magnifications, because the vast majority of us constantly deal with less-than-perfect atmospheric conditions.

I did not use any filters when making the observations for this book. Observers who live under light-polluted skies, however, should consider using light-pollution or skyglow-reduction filters to sharpen the contrast between the diffuse Messier objects and the background sky. Hydrogen-beta and oxygen-III filters work wonders on emission nebulae, but be aware that they also dim starlight by about one magnitude.

The Genesis is cradled in the altazimuth yoke of a Gibraltar mount. This heavy, ash-wood tripod certainly lives up to its name and holds the telescope rock steady. Sometimes when I am out observing on the summit of Kilauea volcano, the winds will suddenly gust to gale force, yet the tele・ _sCOpe hardly jiggles. The mount has also performed quite heroically during several minor earthquakes (a frequent occurrence when you live on an active volcano!); I never lost my field of view during these events, and sometimes the small volcanic quivers helped me to confirm the existence of some faint details in a nebula.

I should add that no one twisted my arm, dangled a carrot, or offered me the key to Manhattan Island to use this particular telescope for this project. The Genesis and I met serendipitously under dark Texas skies, where I realized its quality and potential. The rest is history. Naturally, you can enjoy the Messier objects through any telescope, large or small. And most of the objects are even visible in 7x35 binoculars.

After the 1994 Texas Star Party, I returned home to Boston, Massachusetts, where city lights wash out all but the brightest stars from the night sky. Unquestionably, if this project were to go forward, I would have to travel to complete the book. My plan was to observe from the darker skies of western Massachusetts on weekends and augment these observations with any I could make at various star parties. By a twist of fate, in the fall of 1994 my wife, Donna, was offered a job on the Big Island of Hawaii, and I followed her there that December.

A better stage could not have been set for performing the observations for this book. We purchased a house in Volcano, Hawaii, at an altitude of3,600 feet. It is some 3 miles east of the summit of Kilauea volcano, which rises 4,200 feet above sea level. The nearest streetlight to our house is 1 mile to the north. Beyond that some 300,000 acres of Hawaii Volcanoes National Park and reserved forest border our subdivision to the north, south, and west. Hilo, the nearest city to the east, is about 45 miles away. The massive, swollen back of Mauna Loa volcano, which rises nearly 14,000-feet above sea level, blocks our view of Kona, 100 miles to the west. All around us for 3,000 miles is the vast Pacific Ocean. Adding to the visual pleasure, the entire island has a lighting ordinance to help keep the sky dark for astronomers at the 14,000-foot-high Mauna Kea observatory to the north.

enough to cast shadows, and Venus can "pollute" the sky with its brilliance. The bright haystack of the zodiacal light, the dim, dusty alley ofthe zodiacal band, and the much fainter elliptical glow of the gegenschein are all visible from our front yard. These large, faint features of the night -created by sunlight reflecting off dust-size particles in the plane of our solar system - are the hallmarks of a truly dark observing site. In their book, Observing Handbook and Catalogue of Deep-Sky Objects, Brian Skiff of Lowell Observatory and Christian Luginbuhl write, "Frequently the sole requirement for the visibility of a notoriously difficult nebula is not a large telescope or a special eyepiece, but a truly dark sky." Although none of the Messier objects are notoriously faint or difficult, the exotic details of each -a spiral arm, a dark lane, a tenuous wisp of nebulosity - can be extinguished in all but the darkest of skies.

To push the limits of vision in deep-sky exploration (which I have tried to do for the observations in this book), you need to know the quality of your observing site. The following formula created by Tim Hunter, president of the International Dark-Sky Association, will help you rate your site:

where OSI is the observing site index, Tis the transparency (limiting magnitude), IV is the weather (number of clear nights per 100 days), A is the altitude (in thousands of feet), S is the seeing (1 to 10; 1 = worst), Vis the visibility (percentage of unobstructed sky), and C is the convenience factor (0=a very inconvenient site, costs a lot to get there; 100 = a very convenient site).

The OSI of The Ka'u desert area ofVolcano, where I do most of my observing, for example, would work out to be the following:

In this formula, a site at 10,000 feet altitude with a magnitude limit of 8.5, perfect weather conditions and horizons, and a perfect convenience factor, would rate 100. As fate would have it, the observations for this book were made during an exceptional period of atmospheric clarity. Since 1985, Kevin Krisciunas of the Joint Astronomy Center in Hawaii has measured the natural brightness of the night sky over Mauna Kea. The brightness varies during cycles of sunspot maxima and minima; heightened solar activity energizes earth's upper atmosphere, causing it to glow feebly but perceptibly. His data, plotted in the accompanying figure, show a significant decrease in sky brightness in 1995. In fact, in mid-1995, the sky over Mauna Kea was the darkest it had been in the last 10 years. The observations in this book were made over the seven-month period between December 1994 and June 1995. The sun experienced sunspot maximum in 1986 and probably reached minimum in 1995.

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That truly great skies are truly dark skies is a myth. What is really meant by dark skies is that the atmosphere is so free of man-made pollutants and nighttime lighting that multitudes of fainter stars can be seen. The more starlight you see, the brighter the night sky appears. From the darkest sites, the zodiacal light and band, the gegenschein, and the Milky Way add illumination to the celestial backdrop. (This dark-sky myth is deeply rooted. Sometimes, visitors atop Mauna Kea - the world's best observing location, where magnitude 8.5 stars are within grasp of the naked eye -complain that the site is substandard, because the sky does not look dark!) Limiting magnitude is itself a very good indicator of a sky's quality. But another myth can blind us from determining that limit accurately. Many popular amateur astronomy books state that 6th magnitude is the limi【of naked-eye vision and that if you have good eyes and a good site you can probably see about 0.5 magnitude fainter. Magnitude limits are also attributed to telescopes of given apertures: a 4%・inch aperture will

show stars to magnitude 12; an 8-inch will reveal stars to magnitude 14. What these general rules fail to consider is the human contribution in seeing faint objects. Several years ago I researched the origins of the magnitude formula. Most profound is the classic work by the nineteenthcentury English astronomer Rev; N. R..Pogson (Monthly Notices of the Royal Astronomical Society, Vol. XVII, 1857). His words, which reveal the individuality oflimiting magnitudes, seem to have been forgotten:

I selected [the ratio] 2.512 for the convenience of calculation.... If then any observer will determine for himself the smallest of Argelandefs magnitudes just discernible by fits, on a fine moonless night, with an aperture of one inch, and call this quantity L, or the limit of vision for one inch, the limit l_t for any other aperture, will be given by the simple formula, Z=L + 5x log aperture. Numerous comparisons, made with various telescopes and powers, at different seasons of the year, have furnished me with the value L=9.2 for my _own sight, which is, I believe, a very average one, and therefore suitable for such a determination.

AH emphases are mine. Pogson created a formula that worked for him. He encouraged others to discover their own limits with this formula. Pogson clearly states that his eyesight was average, and he was keenly aware that limiting magnitude varies with aperture, season, and atmospheric condi-rior to that employed by today's amateurs. A better formula is the "brat" equation

tions. Also, consider that the glass he used for his observations was infe-

where N is the number of photons available per unit time to the eye; b is the most effective bandpass for the human eye (100 nm); ris the transmission through the atmosphere, optics, and reflective coatings; A is the lightcollecting area of the aperture (n r²); t is the eye's storage time for collecting photons (0.1 second);/Tm； is the decrease in stellar magnitude, m_t being discussed (f(m) = 2.512 ~^m); and Cis the value of incident stellar radiation of a zero-magnitude standard star beyond the earth's atmosphere. C= 10,000 photons/second/nanometer per square centimeter.

The origin and significance of this formula is explained in more detail in an article entitled "Some Thoughts on Limiting Visual Stellar and

Cometary Magnitudes With Various Apertures,⁰ by Daniel W E. Green, in the Intemational Comet Quarterly (April 1985). The equation demonstrates that under ideal conditions, the naked eye can, theoretically, detect enough photons to see a 9th-magnitude star! It is also not impossible to see stars fainter than 15th magnitude with a 6-inch reflector, as the keen-eyed variable star observer E. H. Mayer has done from his dark-sky site in Ohio.

In 1901 Heber Curtis of Lick Observatory wrote a memoir "On the Limits of Unaided Vision⁰ (Lick Observatory Bulletin Number 38). He began with the following comment: "It is generally stated that stars of the sixth magnitude are as faint as can readily be seen by the unaided eye, though it is well known that under conditions of exceptional clearness favorably placed stars from a half to a whole magnitude fainter can be made out." Curtis proved this in a visual experiment conducted at the observatory in which he detected with the unaided eye stars of magnitude 8.2 without difficulty and stars of magnitude 8.3 with difficulty. He also glimpsed a star of magnitude 8.5. (Brian Skiff notes that Curtis's data are ^{not on} lhe new standard magnitude system. It might be interesting, Skiff suggests, to refer to the original paper and do this study again with new magnitudes for the stars Curtis saw.) Regardless, Curtis*s observations support my own limiting-magnitude studies at the 9,000-foot level of Mauna Kea, where I consistently detected stars as faint as 8.4 magnitude with the unaided eye. In another study performed at the 1994 Texas Star Party, Florida amateur astronomer Jeannie Clark reported seeing a 7.9-magnitude star with her unaided eye. (For another discussion on the power of vision, see "Telescopic Limiting Magnitudes/* by Bradley E. Schaefer, a pioneer in the study of human perception in astronomy, which appeared in the Publications of the Astronomical Society of the Pacific, volume 102, February 1990.)

Although "conventional wisdom" says you need an 8-inch telescope to resolve 14th-magnitude stars in globular clusters, in fact it is possible io reach that magnitude with a 4-inch telescope. The same misconceptions apply to naked-eye and binocular magnitude limits as well. Of course, there are factors that influence the limit of our vision, the most debilitating of which is light pollution - that ugly glow over our cities and suburbs that robs us of our views of the stars. Light pollution is amateur astronomy's greatest nemesis. If writing this book has done anything for me, it has made me appreciate how beautiful the night really can be. I fear that if we do not act now to preserve the few untainted sites we have left for amateur astronomy, night itself may become a myth.

At the 1994 Texas Star Party Brian Skiff and I used a 7-inch refractor to do some visual tests of limiting magnitudes. When we turned it on the globular cluster M3 and compared what we saw to a magnitude sequence published in the Observing Handbook and Catalogue of Deep-Sky Objects (see photo opposite), we independently recorded a 15.1-magnitude star and concluded we could have gone fainter. That same magnitude sequence is reproduced here, to help you determine your own limits. And do not forget that using averted vision - looking slightly off to the side of the object of interest - can be very helpful when viewing faint objects. Skiff concludes from naked-eye tests that he can see three magnitudes fainter with optimally averted vision than he can with direct vision when his eyes are fully dark-adapted.

Altitude also has a significant effect on how faint you can see. The higher you are the fainter you can see, up to a point. I gain roughly % to I magnitude for every 3,000 feet above sea level. The improvement levels off at about 9,000 feet, for two reasons. First, you're above the densest layers of the earth's atmosphere (which greatly affect the visual limit), and second, the relative lack of oxygen in the air at that altitude means less oxygen gets to the eyes, thus adversely affecting vision.

Estimating the brightness of diffuse objects is not easy. The traditional method, first employed by J. B. Sidgwick, is to compare the size and brightness of the diffuse object with a selection of similarly bright stars that have been racked out of focus until they appear the same size as the

diffuse object. In another method, both the star and the diffuse object are simultaneously defocused and their brightnesses and sizes compared. The results of these methods can be quite accurate, but it has been my experience, particularly in comet studies, that they are inadequate for objects with strong central condensations surrounded by faint extended halos. Many Messier objects, especially the galaxies and globular clusters, fall into this troublesome category.

Simple in-and-out focusing methods lead one to underestimate the intensity of these awkwardly diffuse objects, because the contribution of light from the extended halo is either negated (in the first method) or lost (in the second).

T^venty years ago. I devised my own method for estimating the mag-nimde of diffuse objects, which factors in the light loss of this outer enve-lope. This is accomplished by racking the diffuse object out of focus until it is a uniform glow. I then defocus my eyes to select candidate comparison stars. The selected stars are racked well out of focus, and I compare the glow from each stellar disk to that of the out-of-focus diffuse object. I then move back and forth between the diffuse object and the comparison stars until a reasonable match is obtained in both intensity and size. The entire procedure can take more than an hour! IYy this method and see if you come up with magnitudes similar to those in this book.

Charles Morris, a well-known comet observer, has described a similar method in which the entire diffuse glow of the defocused comet is compared to a star image racked out to the same size (meaning the star is defocused even more than the comet). His method is published in the International Comet Quarterly, volume 2,1980.

Nearly all my magnitude estimates for the Messier objects were made with the unaided eye or with binoculars. I included them in the list of data for each object only if I found a marked disagreement with published values. Otherwise, the magnitude estimates and other object data provided are from the following sources.

Stellar magnitudes, spectra, and double star information were gleaned from Sky Catalogue 2000.0, volumes 1 and 2 (Sky Publishing Corp., 1982 and 1985, respectively). Magnitudes and dimensions of diffuse nebulae are from Luginbuhl and Skiff's Observing Handbook and Catalogue of Deep-Sky Objects (Cambridge University Press, 1990); planetary nebula data were extracted from the Catalogue of the Central Stars of True and Possible Planetary Nebulae, by Acker and Gliezes (Observatory of Strasbourg, France, 1982). The sizes, magnitudes, distances, and star counts in open clusters are from Star Clusters (Willmann-Bell, 2000), by Steven Hynes and Brent A. Archinal. Accurately determining the number of stars in an open cluster, or its diameter, is virtually impossible because of the difficulty in discerning where the cluster proper ends and the background stars begin. Also, counts are sometimes based on measured, rather than only counted, stars. Therefore, published counts tend to vary.

Globular cluster magnitudes are from "Integrated Photometric Properties of Globular Clusters/ an article by C. J. Peterson published in the Astronomical Society of the Pacific^ conference series (volume 50, 1993), and •their sizes and distances are from Skiff's "Observational Data for Galactic Globular Clusters/* in the January 1995 issue of the Webb Society's Quarterly Journal. As for open clusters, determining the number of stars in globular clusters is an inexact science. The star counts given for globulars in this book are probably on the high side.

Magnitudes and diameters of galaxies are from the NASA Extragalactic Database, which was kindly provided to me in part by Barbara Wilson and Kevin Krisciunas. Galaxy velocities and distances are from the Nearby Galaxies Catalog, by R. Brent Tully (Cambridge University Press, 1988). Astronomers continue to debate galaxy distances.

Defensible estimates for the distance of the Virgo Cluster of galaxies, for example, range from 49 million to 72 million light years. The galaxy dis-tances in Tully's work (and in this book) assume a Hubble constant of 75 ldlometers/second/megaparsec and velocity perturbations in the vicinity of the Virgo Cluster. This model also assumes that the galaxy is retarded by 300 kilometers/second from universal expansion by the mass of the Virgo Cluster - which is some 55 million light years distant.

The physical diameters of the galaxies were calculated using the followingformula:

where D is the object's diameter in arc minutes and R is the object^ distance in megaparsecs. The physical diameters of all the other Messier

where D is the object's apparent diameter in arc minutes and R is the object's distance in light years.

Much of the data in this book differs from those appearing in older but popular references such as Burnham's Celestial Handbook, The Messier Album, and Messier's Nebulae and Star Clusters. This book contains the most up-to-date astronomical data for the Messier objects of any book in the popular literature.

All the photographs of the Messier objects in this work appear in black and white. Through the telescope, some stars, clusters, and nebulae do display subtle hues (though some astronomers would argue otherwise). But for the most part, the objects appear as grayish white hazes or clusters of stars projected against a black background. Color photographs, while often beautiful, may mislead beginners into thinking they can expect to see vivid reds, greens, and blues; such expectations would only lead to disappointment when the real thing is seen. Because my mission is to excite you about what you can see through a telescope, I decided to stick with black and white images only

The photographs are meant not only to inspire you to look at these objects but also to help you confirm your visual impressions of them -especially when you think you're seeing an elusive detail in a nebula, ora pattern of faint stars in a cluster. For example, if you study the globular cluster M13 at high magnification, you might suspect several dark lanes that slice across the stars near the cluster's center. By orienting the photo-graph to match your visual impression, you will discover that the dark lanes really do exist. These features were first seen by Lord Rosse with his 72-inch speculum-mirror telescope in the 1800s, but theyYe within reach of moderate-size amateur telescopes today.

The photographic appearance of stars and nebulosity depends on the length of the exposure, the sensitivity of the film, and how the film is developed. Many of the photographs in this book show more stars and nebulosity than you can expect to see with your telescope. Yet there is also a frequent and unfortunate tradeoff in deep-sky photography; To expose the faint outer arms of a spiral galaxy, the photographer must overexpose the brighter core or nuclear region. Therefore, it is possible you will see more detail in these areas through the eyepiece than is apparent in the photographs. The same tradeoff applies to highly condensed globular clusters. In photographs, the tightly packed stars in a cluster's center often blend into an unresolvable mass.

For ease of orientation, all photographs in this book have south up and east to the right. This will match the view in a simple inverting telescope without a diagonal.

Of course, no photograph can record emotion. When I finally pick out a faint blur in the sky and realize that it is a galaxy 40 million light years away, well, the visual image might not be all that impressive, but 1 am in

All the drawings in this book are composites, based on several observations with magnifications of 23x, 72 x, and 130x. They are shown with south up to match the photographic view; the finder charts have north up. I spent a minimum of six hours on each object over three nights. The observations were made only on the clearest, moonless nights, at alti* tudes no lower than 3,500 feet. The drawings represent what good observers from dark, sea-level sites might expect to see with an 8- to 10-inch Schmidt-Cassegrain, a popular backyard telescope.

I did not try to accurately position every resolvable star in a globular cluster. I did, however, try to plot, to the best of my ability, the locations of any particularly bright members, especially if they would help you orient the drawing to the photograph for comparison. Otherwise, the drawings of the globular and open clusters reveal the patterns of major star streams, dark lanes, and irregularities in shape. Diffuse objects, such as the Orion Nebula, were extremely challenging and required longer dedicated efforts. I treated the Orion Nebula, for example, as if it were a dozen individual objects, and focused on one area at a time over the course of several weeks. (For a revealing look at how increased time spent behind the eyepiece can enhance the amount of detail you can see, refer to the sequence of drawings of the galaxy M101 on page 24.)

_n0( perfect, and my interpretations of what I see may be different from those of others. For instance, my eye and mind might work together to follow a particular pattern of stars in a globular cluster - say, a counterclockwise spiral of stars - but other observers might see a perfectly sonable clocku/ise spiral of stars in the same region.

per some objects, open clusters in particular, I took the liberty of

drawing the whimsical creatures (e.g., bats, alligators, fireflies) that I visu-alized in the patterns of the stars during moments of fancy. An example is

M41, whose stars form an outline of a fruit bat reaching for a bite to eat. I

have also highlighted certain geometrical patterns in the drawings for emphasis. The Spanish surrealist artist Salvador Dali referred to drawing

_as honesty of art, saying "there is no possibility of cheating. It is either

good or bad." With that in mind, I hope you enjoy my renditions of the

Messier objects and find them useful in lending perspective to your obser

The constellation map at the back of the book is sufficient for locating most, if not all, of the Messier objects, which are fairly bright and easy to spot once you're looking in the general vicinity But the smaller-scale finder charts provide additional detail. They can be particularly helpful in identifying other objects that lie near to the Messier objects. The scale and magnitude limit of the finder charts vary. For some, a smaller scale (and higher magnitude limit) may have been needed to distinguish several close-together objects, whereas a wider scale (and lower magnitude limit) was appropriate to show objects separated by greater distances or to show an object's location relative to a large pattern of stars in a constellation.

The following symbols are used to represent the different object types on the finder charts:

Messier's own descriptions of the objects he catalogued are included in chapter 4. The translation of the French catalogue in the Connaissance des 九mps for 1874 was expertly done by Storm Dunlop, author and translator of numerous books on astronomy and a fellow of the Royal Astronomical Society. Storm gained access to the catalogue in the library of the RAS in London, where it is preserved. His is the most precisely interpreted and smoothest-reading English translation of the catalogue that I have seen, and I am grateful to him for bringing his linguistic gifts and knowledge of astronomy to bear on this important task.

There are a few things to note about Messier's catalogue and its translation. His object descriptions appeared on right-hand pages, while the facing left-hand pages contained columns listing the object number, the right ascension and declination, his estimate of the objecfs angular diameter, and the date of his observation (which I have put in brackets preceding each description). This edition of the catalogue, the last one published before Messiefs death, included 103 objects; numbers 104 through 110 were added later.

Messier used the third-person grammatical form when referring to himself, saying ofM32, for example, "M. Messier saw it for the first time in 1757, and has not noted any change in its appearance/* This style was not imposed by the translator. Bracketed [ ] words or phrases within the descriptions were added in translation for purposes of clarification. Messier also made frequent use of the term "ordinary telescope/* which has been translated literally by others in the past. The more correct meaning, "simple refractor/* is used here. The implication is that the telescope used was not a compound (achromatic) refractor. When Messier describes an observation made with a ^Hone-foot telescope,^M or a "simple three-foot telescope/* he is referring to the scope's length, not its aperture. Messier's use of the word "parallel" was translated literally, e.g., "the cluster is close to Antares and on the same parallel?* Dunlop suggests that Messier might have meant something more like "zone of declination/* because he often referred to an object being on the same parallel, but slightly above (or below) it. Messier occasionally refers to an object being plotted on the English Atlas Celeste, by which he means an original English edition of Flamsteed^ Atlas Coelestis. A French-language edition, Atlas Celeste, was published in 1776, several years before Messier's catalogue.

The Latin names of constellations are given throughout, rather than the vernacular names that Messier used. Punctuation has been modernized.