The following sky codes are used as part of AOML. In particular these codes are used in the <rating> element. It is my intention to define here the meaning of the various values that I use. Note that I also have a numeric coding system for rating deep sky objects. These codes are unrelated to my deep sky codes. In fact, my deep sky codes are likely to be replaced (or at least changed) when I get around to defining an XML schema for deep sky observations.
| Rating | Value | Criteria |
|---|---|---|
| 0 | Unobservable | Nothing can be seen at all. |
| 1 | Very Poor | Only a few bright objects can be seen, perhaps with difficulty. Clouds obscure the sky or the light pollution is so excessive that at best only a handful of stars are visible. This rating might also be appropriate for the case where the sky is almost completely cloudy but has a few small breaks in the clouds here and there. Observing some objects might be possible but probably not worth the time to set up the telescope. Rating 1 is not that different than rating 0. |
| 2 | Poor | The bright stars can be seen but no fainter stars. Clouds obscure part of the sky or perhaps cover most of the sky thinly. This rating might also be appropriate for heavily light polluted skies, for example from a city, even if perfectly clear. Observing of bright objects possible. |
| 3 | Fair | Mediumly bright stars can be seen and faint stars as well in some areas. This rating is appropriate for a bright moonlite night with no significant clouds in an otherwise good location. This rating might also be applied to a sky that is partly cloudy but for which the viewing is good to very good between the clouds. Observing of many objects possible, including some faint objects. |
| 4 | Good | Full range of star brightness visible. Light pollution or moonlight minimal. No signficant clouds (perhaps a few minor cloudy patches). Milky way visible. Observing of all object classes appropriate. |
| 5 | Very Good | Extremely dark with limiting magnitude 6th or greater. No clouds or other obscuring atmospheric phenomemon. No light pollution. Milky way exhibits complex structure to the naked eye. Observing extremely difficult objects appropriate. |
Note that these codes are based largely on sky clarity, limiting magnitude, and darkness. While this criteria is often appropriate there are certain kinds of observations that are more concerned with atmospheric stability ("seeing"). There are cases where the sky is not very clear but for which the seeing is excellent. Similarly many of the clearest and darkest nights are often accompanied by poor seeing. A more comprehensive rating system would combine both sky clarity and seeing. For example: "3/4" might specify a rating of 3 for clarity and 4 for seeing. The seeing rating would have to be based on different criteria than those shown below—a second table would need to be defined.
At this point in time I am not going to define a seeing criteria table. Observing the seeing normally requires optical aid; seeing can't be easily assessed with the naked eye. I thus don't anticipate making regular observations of seeing at this time (Jan 2003) although that may change in the future. When and if I do start using a two part rating system, I will define the first (and default if there is only one number) to be the clarity rating. Thus ratings such as "4" or "4/-" both imply that no seeing rating exists. This is an area to expand on in the future.