Deep Sky Codes

In an effort to summarize the visibility of deep sky objects, I sometimes use the simple coding system described here. This allows me to characterize an object very quickly. The intent is to make it easier to evaluate an object later when I review my old observations to prepare new ones.

The system described here was created by myself and a friend of mine (Joe Roberts) in the mid 1970s. It was used to good effect for several years. I then stopped using this system for many years but have just recently (September 2002) started using it again. It is my hope to eventually enhance this system and perhaps modernize it to make it more suitable for electronic archiving and searching.

The system works by rating several "key" characteristics of an object: brightness, field, condensedness (for open clusters), diffuseness (for diffuse objects such as galaxies or unresolved clusters), and size. The summary is then summarized by assigning each object to a class. See the detailed descriptions below for more information.

Brightness

Almost all objects can have their brightness rated. However, very open clusters where the stars are spread widely can be difficult to rate in this way. In that case, the brightness of the stars can serve to form the brightness rating for the object. Typically such clusters are very loose and that information is evident from the condensedness rating.

Field

This rating characterizes the field in which the object is located. This information can be especially important for clusters since it can, at times, be difficult to distinguish clusters from the background. However, this rating can also be important for diffuse objects since faint star associations often look like such objects. A rich field would, one assumes, contain many such associations.

Note that an "average" Milky Way field is probably "Rich" by the criteria above.

Condensedness

This rates the degree of compaction of an open cluster. This rating does not apply to diffuse objects (see "Diffuseness" below). Highly condensed clusters tend to stand out against even a rich field. On the other hand, very loose clusters may be obscure even against an average field.

Note that many clusters are partially resolved due to the faintness of their members, not their proximity to each other. Thus it is dangerous to use the degree of resolution too much when judging condensedness. For example, an "Average" condensed cluster might well appear partially resolved.

It's not entirely clear to me how to best deal with this issue. You should keep in mind, however, that these ratings are not an attribute of just the object. The ratings will be different in different instruments. That is clearly true for the brightness rating but it also applies here. One person might see a cluster as being highly condensed while another, with a larger telescope and a higher power, might disagree. Nevertheless, use caution when ranking the condensedness of faint clusters to avoid giving them a ranking that is too condensed.

Diffuseness

This rates the degree of contrast between a diffuse object and the sky background. This applies to nebula, galaxies, most globular clusters, and some open clusters (or partially resolved open clusters). Thus some objects can carry both a condensedness rating and a diffuseness rating but for many objects having both ratings makes very little sense.

You will have a tendency to rate bright objects as having high surface brightness. While this is fine to a point, be aware that in theory the two concepts are somewhat different. A (large) bright object might fade away into the background so gradually that it is never clear where the object is and is not (particularly if the object is larger than the field). Thus "bright" but "diffuse" objects are possible. For example, M31 is very bright but is also relatively diffuse in most instruments. Similarly faint objects that are very distinct once seen are also possible.

Size

This rates the apparent size of the object.

Obviously the size of an object will be a function of the magnification used to observe it and, based on the criteria above, on the size of the field of view used. This is appropriate since the purpose of this rating is to describe the difficulty in seeing an object. It is understood that you can make small objects bigger by magnifying them, but if you are searching for an object using the same magnification and/or field size that was used to make a size rating then the rating will give you some idea of what to expect.

Class

The class rating attempts to embody, in a single number, how difficult an object is to observe.

Instrument Codes

Many of the rates above will be different depending on the instrument used to observe the object. The ratings are not intended to be an absolute property of the object but rather a a property of the combination of object, observer, instrument, and sky conditions.

In the current edition of these codes, I've assigned letters for each instrument that I own and use those letters in the deep sky codes. I'm not sure this is really the most appropriate thing to do. Obviously it keeps the deep sky codes (or at least this part of them) from being useful to others. It may make more sense to just ignore the issue of which instrument is used and let that information be provided by other context. I already do that with the observer and sky conditions information. However, for now I present the instrument codes that I have been using.

The order of the designators doesn't entirely make sense but that's because I can't reuse or relabel instruments once I've specified designators for them. To do otherwise would cause some of my observations to become invalid.

Examples

To see how the deep sky codes work, I will include a few examples here. First notice how in all cases the higher the rating value the more difficult it is to observe the object. For example, a high brightness rating implies that the object is faint. A high field rating implies that the field is very rich and thus the object is more likely to be hidden or obscured. A high condensedness rating implies that the object is very loose and thus difficult to notice. This allows you to quickly look at a deep sky code and get a sense of how "hard" the object was to observe.