Astronomical Binoculars

For the faint fuzzies outside the solar system, large aperture
binoculars offer something a telescope can almost never give you -- a
wide field of view. They let you see the big objects all at once, such
as the three degree width of the Andromeda galaxy that has to be seen in
segments in the one degree field of the average telescope. Generally
speaking, binoculars with apertures of 50mm and larger, and
magnifications of 10x and higher, are best suited for astronomical
observing outside the solar system. 50mm binoculars have the same light
gathering capacity as a 70mm refractor at the same power. 80mm
binoculars have the same light gathering as a 4.5" reflector. You should
aim for a binocular exit pupil of 4mm to 5mm if you're in your 40's or
older (10 x 50mm, 16 x 70mm, or 20 x 80mm binoculars, for example). Exit
pupil is found by dividing the binocular aperture by the magnification.
Binoculars with 6mm to 7mm exit pupils (a 7 x 50mm or 8 x 50mm
binocular, for example) are best suited for teenagers and those in their
early 20's, who have eyes with pupils that can still expand wide enough
in dark skies to take in all the light of the large binocular exit