Hey, Don't Stick With Just the "Usual" Stuff!
Here's a quiz: What do Trumpler, Stock, Ruprecht, Melotte, Harvard, Dolidze, and Collinder have in common? No, it's not the name of a law firm ! Before I answer that question, let's take a look at what kind of deep sky objects we generally observe. (Oops...I just gave you a big hint.)
When it comes to searching out deep sky objects, most of us start with the Messier list. They're generally the brightest of such objects, and if Messier could locate them with 18th Century telescopes, then we certainly should be able to find them with ours. As our confidence builds, we sometimes venture off to the never-never land of NGC objects, and are often surprised to find that some of these are so bright that we wonder why Messier never cataloged them. (Case in point: the Double Cluster in Perseus.) But generally, we use our star atlases as something of a safety net. Those who start off with Norton's Star Atlas or the Edmund Mag. 6 feel a certain assuredness - anything shown there should be well within the range of most of our instruments. Then we decide to go big time and invest in a Tirion Sky Atlas 2000, which includes about 2500 deep sky objects. But we discover that some of those shown are fainter than 13th magnitude and perhaps begin to realize the frustration of less than dark skies or a telescope that doesn't quite have enough light grasp. If you get hooked on this deep sky stuff, you eventually wind up in the really big time...the Uranometria. Now all of a sudden, you wonder where all these deep sky objects came from, but soon learn that many of the 10,300 objects shown are of 14th magnitude and beyond. Then there are those other objects with weird designations such as Cr89, Do10, Tr1, Mel 15, St10, and even crazier designations such as PK204+4.1. (The Sky Atlas 2000 begins to show a tantalizing few of these "weird" objects.) Welcome to the universe beyond Messier !
You might be wondering whether all such objects have some sort of designation. Not by a longshot! There are billions of objects out there (to quote Carl Sagan). As we approach the limits of our vision into the universe, astronomers refer to objects based on their coordinates. The most distant galaxies, which appear as no more than fuzzy stars in the largest instruments, are simply called "anonymous". For example, the IAU circular which announced the discovery of Supernova 1990a, stated it was in one of two anonymous galaxies which lie adjacent to one another. The supernova was spotted at roughly magnitude 19.5; it was impossible to determine which was actually the host galaxy. This leaves the field wide open for the naming of faint and distant deep sky objects. Let's say George Murphy wants his own form of immortality; he might designate one of the many globular clusters around the Sombrero Galaxy (M104) as "Murphy 1". Of course, like the International Star Registry, this would have no scientific validity. It is rather humbling to ponder that perhaps in one of these far flung galaxies there is an intelligence that looks out on their night sky and regards the Milky Way - us - as "anonymous".
Where all these objects came from is explained by the fact that as astronomical equipment and techniques have become more sophisticated, many heretofore unknown objects were identified. J. L. E. Dreyer based his New General Catalog on the observations of the Herschels, then objects identified later were added in an Index Catalog to the NGC (hence the "IC" designations for many objects). With larger telescopes and photographic imaging, new objects were continually being identified. The designations given to them generally reflect the names of their discoverers or those who first cataloged them. Sometimes the designations refer to the location from which they were cataloged, such as the Uppsala General Catalog (UGC) of galaxies and the United Kingdom Schmidt (UKS) globular clusters. As you might expect, many of these objects are beyond the capabilities of all but the very largest amateur telescopes. For example, the very brightest of the UGC galaxies is magnitude 11.3, with most ranging between 13th and 15th magnitude. The PK (for Perek & Kohoutek) designations are applied to planetary nebulae, the brightest of which is magnitude 8.6, but the bulk of them are much fainter (many beyond 17th magnitude).
But don't throw in the towel ! There's still a large number of these objects well within our grasp. Some dark nebulae, often called "coalsacks", reveal themselves in a thoroughly dark sky as star-poor areas in a rich star field or as dark spots in nebulae. Many have Barnard designations, such as B33, the Horsehead Nebula in Orion. But the easiest of the lot are the open clusters. Modern techniques have identified many stellar groupings as true galactic clusters, inasmuch as they are composed of stars that have a connection with one another. This connection is a physical proximity, along with similar stellar types and proper motion. The "Coathanger Asterism", also called "Brocchi's Cluster", in Vulpecula is an easy naked eye object known as Cr 399 (a Collinder object). The Hyades in Taurus is a true cluster known as Mel 25 (a Melotte object). Of course, these examples are rather unusual - most such objects require a telescope in order to be seen.
Quite a few of these open clusters don't require a large telescope. I've seen many with a copyscope; in some cases, a good pair of binoculars will suffice. And as mentioned above, even your naked eyes may do just fine. While some objects stand out so well that they clearly look like clusters, there are many that appear as nothing more than a few scattered stars. It's often hard to separate these clusters from the rest of the field stars. As an example of an object that doesn't look like a cluster, yet is easily seen with the naked eye, we can refer to the cluster Cr 70. You know the components of this cluster more commonly as the three belt stars in Orion ! Now take that same cluster and hypothetically reduce the stellar components to about tenth magnitude and superimpose it on a fairly rich star field; you can then appreciate just how difficult it often becomes to visually identify some of these objects as true clusters through your telescope.
The following is a sample of a few identifiable clusters.
Berk 86 (Berkeley) - Cygnus: With a 98mm copyscope appears as a small hazy spot with one star clearly resolvable roughly at center and a few more glimpsed with averted vision
Cr 106 - Monoceros: A loose grouping of about twenty fairly bright stars that covers an area almost a degree in diameter.
Cz 20 (Czernik) - Auriga: Four barely visible stars in the 98mm, surrounded by seven brighter ones which may not be part of the cluster. Suspected more unresolved members.
Harvard 20 - Aquila: In the 10", it's an elongated grouping of faint stars with sparse concentrations at each end.
Ru 173 (Ruprecht) - Cygnus: In the 98mm scope, twelve brighter stars were visible, along with many fainter ones. It lies in a rich star field, and since it lacks any concentration, it doesn't stand out well.
Stock 2 - Cassiopeia: A very large cluster with an abundance of fairly bright stars; loose, without any concentration. A fine, easy object.
Tr 1 (Trumpler) - Cassiopeia: The 10" shows two lines of stars at a slight angle to one another, in a roughly east-west direction. Composed of seven brighter stars, of which four appear double; also a few fainter ones with a background glow that suggests some unresolved members.Published in the May 1993 issue of the NightTimes