Binoculars for astronomy and observing other things in the night sky
 Ed Cannon; March 22, 2006
 
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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".
 
Table of Categories.  In the table below, binoculars in the "In Between" column may be handheld well by some people but may have to be mounted for other persons.  The largest exit pupils (7mm) really can only be used to the fullest by younger people under dark skies. (Note that this is at most only part of each month, due to bright moonlight at least half of every month.)  For people of middle age or older, or who observe from city locations, or in the moonlight, exit pupils of 5mm or even 4mm are likely to be more suitable.
 
 AGE  Suggested
Exit Pupil
(Dark Sky)
Models
Less Magnification More Magnification
Gather Less Light Gather More Light
Lighter Weight Heavier Weight
Less Shaking* More Shaking*
Larger Field of View Smaller Field of View
Handhold Okay In Between Mount Needed
 <40 >7 mm 7x50, 8x56 8x60, 9x63 10x70, 11x80, 14x100, 20x140
 40-50 6-7 mm 7x42, 8x50 9x60, 10x60, 11x70 12x80, 20x120, 25x150, 30x180
 50-60 5-6 mm 7x35, 8x40, 8x42, 8x45, 8.5x44 10x50, 11x56
12x60, 12x63		 15x80, 16x80, 20x100
 60-70 4-5 mm 8x32, 8.5x42 10x40, 10x42, 12x50 15x63, 15x70, 16x70, 20x80,
22x100, 25x100
 >70 3-4 mm 8x25 10x30 16x60, 20x60, 20x70, 30x90
*The "less shaking" and "more shaking" factor does not apply to image-stabilized models.  
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High-power, large-objective binoculars used for astronomy are also called "observation binoculars", e.g., by Monk Optics.
 

Note about the author: My primary use of binoculars is visual satellite observing (or satellite tracking), but I definitely very much enjoy observing bright comets and nebulae, the planets, and even the occasionally bright asteroid or nova -- and I just generally love to be out under a clear night sky. I also greatly enjoy observing meteors -- but that does not involve binoculars.

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