Catalogs
It is easy to forget that the availability of detailed planetarium and catalog data on personal devices was only made possible by the patient and astonishing dedication of generations of astronomers.
Astronomical catalogs are an invaluable resource toto the astrophotographer. They are the à la Carte menu of the cosmos. One can easily imagine, although the first astronomers recorded the very brightest stars onto fanciful charts, as soon as telescopes were used to methodically survey the heavens, the number of objects increased exponentially. This created the need for systematic catalogs by type, position and brightness. One of the earliest catalogs dates from the first millennia and lists more than 1,000 stars in detail, and interestingly includes the fuzzy outlines of the Andromeda Galaxy and the Large Magellanic Cloud.
Classification
As observations became more sophisticated, it was necessary to find ways of classifying stars and organizing them in logical ways. Johann Bayer started the convention of prefixing the constellation name with a letter from the Greek alphabet in the order of their brightness, a system that is still in use today. John Flamsteed, in his star atlas of 1725, listed stars using numbers combined with the constellation in the order of their right ascension. (John Flamsteed was the first Astronomer Royal at the Greenwich Observatory. The observatory was built on the meridian and his telescopes pivoted in altitude only and so it was convenient for him to label stars in the order they crossed the line of sight.)
In 1781 the French astronomer Charles Messier published “Nebulae and Star Clusters”. Crucially, this was not a star catalog but one of deep sky objects. He used a simple index, prefixed with “M” to identify these objects; for example, M31 is the Andromeda Galaxy. Since observations with a telescope at that time only showed the most discernible deep sky objects, it follows that these objects in turn are prime subjects for amateur astrophotography. The Messier catalog is very convenient and arguably the backbone of amateur astrophotography. Indeed, at star parties “The Messier Marathon” is a challenge to see how many of his catalog items (there are 110) you can view in one night.
One hundred years on another significant catalog, the New General Catalog (NGC), compiled by J. Dreyer, listed about 8,000 objects, stars and deep sky objects and remains a useful comprehensive catalog, in use today. It is astonishing to realize that these early catalogs were compiled by hand, without the help of computers or photographic records, but by patient observation and often in poor conditions.
The “Guide Star Catalog” (GSC) is another important catalog, initially compiled to support the Hubble Space Telescope and now also used by amateurs with plate-solving software. (Plate-solving is a technique that recognizes the relative positions and brightness of stars in an image against a catalog database and derives the actual image scale, position and rotation to incredible accuracy.)
In the following century, as telescopes continued to improve and crucially photography allowed astronomers to see fainter objects, the catalogs expanded exponentially. In the early 20th century the Henry Draper Catalog listed more than a quarter of million stars, and later still, using satellite imagery, the Tycho-2 catalog identifies positions and color information of 2.5 million stars in the Milky Way.
In practice, many common objects have several names, corresponding to their listing in each of the popular catalogs, and in addition, descriptive names based on their appearance. Thankfully, we do not need to pore over large books of numbers but can use planetarium programs on computers, smart phones or tablets to select objects for viewing or imaging, display its image and display its relative size and brightness. Many planetarium programs can also command a telescope to point to an object via a number of connections, from external RS232 serial, through Bluetooth and WiFi, wired Ethernet and remotely over the Internet.
Today the main catalogs are available in digital formats and are freely available; for example from U.S. and European Space Agency websites. Clearly in the early days, as new objects were identified, the catalogs expanded and overlapped previous editions. Subsequently, as measurement techniques improved, those with more accurate measurements of position, brightness and color replaced earlier surveys. Even so, stars and galaxies are on the move, relative to Earth and to each other and so any catalog’s accuracy will change in time. This perhaps has less significance for the amateur but for scientific use, renewed surveys are required to update their databases.
Too Much Data?
Several commonly available catalogs are compiled, employing filters to generate data sub-sets for specific purposes, for instance, all stars brighter than a certain magnitude. Even with digital computers, too much data can obscure or slow down the search and display for what you want to view, the proverbial needle in a haystack. It is sobering to realize that the Hubble Space Telescope was upgraded to a ruggedized 486-based PCs running at 25 MHz clock speed and the Chandra X-Ray space observatory, with a VAX computer, is roughly equivalent to a 386-based PC. Hubble’s main computer has just 10 GB of drive space, less than 1/200th of the capacity or speed of the computer writing this! Robustness in this extreme environment is more important than speed.
Catalogs for Astrophotographers
There are two main types of catalog today, the detailed star-based measurement intensive astrometric databases and catalogs of interesting objects. The second form is the most useful for astrophotographers. For deep sky objects, subsequent to the ubiquitous Messier catalog, Sir Patrick Moore generated a supplementary hit list of 109 objects in his Caldwell Catalog. He noticed that Messier had excluded objects that were only visible in the southern hemisphere and had missed quite a few interesting bright deep sky objects too. Since Messier had already taken the “M” prefix, Moore used his middle name Caldwell and used “C” instead. His catalog is listed in numerical order of degrees away from Polaris (declination).
In addition to these two, a group of astronomers selected 400 deep sky objects from the 5,000 listed in John Herschel’s Catalog of 1864, all of which are observable from mid northern latitudes and with a modest telescope. It is called the Herschel 400. About 60 objects in the Herschel 400 also occur in the Messier or Caldwell catalogs.
The astrophotographer has more objects to photograph than a lifetime of clear nights. The choice is bewildering and thankfully many planetarium programs offer recommendations for a given night. The huge astrometric databases are of more importance to the scientific community but can be used for plate solving and supernova detection in amateur systems. Most are available as free downloads from the Internet and most planetarium programs are able to load and access them selectively. If too many are enabled at the same time, the star map is cluttered with multiple names for each object. To add to the fun, several popular objects have multiple common names and their catalog number is useful to remove ambiguity.
| Catalog | Date | Objects | Notes |
| Messier “M” | 1771 | 110 | Deep space objects, including galaxies, nebulae and clusters, visible from Northern Hemisphere |
| Herschel “H” | 1786 1864 |
2,500 5,000 |
Deep space objects, including galaxies, nebulae and clusters, visible from Northern Hemisphere. Later revision by son doubled object count |
| NGC/IC | 1888 | 5,386 | Revised Herschel Catalog but had errors that evaded several attempts to correct. Extensively used. |
| BSC or YBS | 1908 | 9,110 | Bright star catalog, brighter than magnitude 6.5 |
| Melotte | 1915 | 245 | Open and Globular clusters |
| Barnard | ~1923 | 370 | Dark Nebulae |
| Collinder | 1931 | 471 | Open Star clusters |
| ADS | 1932 | 17,000 | Aitkin Double Star Catalog |
| Abell | 1958–89 | 4073 | Galaxy Clusters |
| Sharpless | 1953–59 | 312 | HII and planetary nebula and supernova remnants |
| Herschel 400 | 1980 | 400 | 400 deep space items from the Herschel Catalog - use “NGC” |
| GSC1 GSC2 | 1989 | 20M 1B |
Catalog to magnitude 15 and 21 for space telescope navigation. (Stars) |
| Hipparcos “HIP” | 1993 | 120,000 | Extremely accurate positional and motion star data |
| Caldwell “C” | 1995 | 109 | 109 deep space bright objects missed by Messier or in Southern Hemisphere, by Sir Patrick Caldwell Moore |
| Tycho-2 | 1997 | 2.5M | Star catalog with revised proper motion, brightness and color data |
| USNO-B1 | 2003 | 1B | Stars and galaxies, over 80 GBytes of data |
| NOMAD | 2005 | 1.1B | Merged data from HIP, Tycho-2, USNO-B1 |
| RNGC/IC | 2009 | 5,000 | Revised and corrected Herschel Catalog |
fig.1 The table above lists some of the more common catalogs that one finds in planetarium programs, books and references. Some of these are included since they are used, not necessarily to identify objects to image, but in support of plate-solving software. This accurately locates an image’s center by comparing the star positions and intensities with the catalog database. There are many more specialist catalogs, which can be found on the Internet and imported into planetarium programs, such as comet, satellite and asteroid databases.