The number-one decision with respect to binoculars for night-sky watching is (1) handheld or (2) mounted. Binoculars that will be handheld need to be lighter and have less magnification, but they have larger fields of view, taking in more of the sky at a time. Binoculars that will be mounted can have much larger front (objective) lenses and higher magnification and weigh a lot, but they have smaller fields of view and take more skill to use to their fullest.

      Exit Pupil. The exit pupil is simply the objective (front) lens diameter divided by the magnification. Because the pupils of the eyes dilate in the dark, binoculars for night use need larger exit pupils than binoculars that will only be used in daylight.

      Field of View. In general, a larger field of view is better, but more magnification reduces field of view. So the observer must balance the factors. If the target or the observer will be in motion, a larger field of view will make it easier to keep the target in view.

      Night and Day. As a general and important rule, binoculars that are good for night viewing are fine for daylight use ᾿but NOT vice versa. The difference, in technical terms, is that binoculars with small exit pupils (roughly, less than 3 mm) can be fine in daylight but not good for night use. (High-power binoculars are less subject to this generality.)

      Another very important consideration is whether the binoculars will be used with or without eyeglasses. For use with eyeglasses, more "eye relief" (or "exit pupil distance" or "high eye point") is needed; in this case, binoculars called "long eye relief" are appropriate. However, long eye relief is unhelpful at best if the binoculars are used without eyeglasses. On the other hand, long eye relief is good for daytime use with sunglasses. Long eye relief generally is at least 18 mm.

      Prisms. There are two types of prisms used in binoculars:porro and roof. In general, maybe about the only generality is that for equal optical quality, porro prism binoculars do not cost as much. People with large hands may tend to prefer porros, which are wider, as a matter of ergonomics. Here's a good article.

      Glass."BaK-4" (barium crown) glass is better quality than "BK-7" ᾿although the latter is not necessarily bad (based on my experience of a few years using nice 10x50 binoculars with BK-7 prisms).

      Coatings."Fully multi-coated" (FMC) is best.Roof prism binoculars are better if they are also "phase coated".