It is said that “binoculars are halfway to a telescope” for observing
the night sky. Not only this, but they are easier to use than a
telescope and far less than half the cost. So, binoculars make the
ideal “first scope” for someone newly interested in amateur
astronomy.

So, which binoculars should you buy?

First of all, if you already have a pair, go ahead and use them! Very
few pairs of binoculars are so bad as to be useless, and this way you
can start tonight (if it’s clear). If you have a starmap (you can see
one online at Sky and
Telescope or download a 55-page .pdf of a full sky atlas for free
at Guide to Backyard
Astronomy) take it outside and start finding things. Depending on
the time of year, good objects to look at are M42 (the Great Orion
Nebula) in Orion, the Double Cluster in Perseus, the Coathanger
Cluster in Vulpecula (between Altair and Vega), M13 (the Great
Hercules Cluster) in Hercules, M31 (the Andromeda Galaxy) in
Andromeda, M44 (the Beehive Cluster) in Cancer (Between Leo and
Gemini), MEL 111 (the Coma Star Cluster) in Coma Berenices at Leo’s
tail, or if you can see the Milky Way, just scan that for lots of good
sights.

If you wish to purchase a pair of binoculars, there are several things
to consider. Holding them steady can be a challenge, particularly if
they are heavy. High magnification amplifies the shaking, so that is
a consideration. To start with (especially a child) one would want to
limit himself to about 10x magnification and about 50mm for lens
diameter (written 10×50 in a binocular’s specs). In fact, 7×45 or
10×50 is an ideal place to start for most people. If you can handle
it, 11×70 is a good size that lets you see a bit more. (I most often
observe with my Oberwerk 11×70 binoculars). If possible, borrow a
pair of binoculars and try them to see what you can handle.

If you wear eyeglasses, you need at least 10mm, and preferably 15mm,
of eye relief (sometimes called “exit pupil distance”). This is how
far you hold your eye from the eyepiece lens to see the best image.
If you wear glasses, you need that extra distance. Many binoculars
have foldable eyecups–leave them unfolded if you do not wear glasses,
or fold them in if you do, to properly place your eyes.

Any type prism would work, but BAK4 is the best for astronomy. There are
differences of opinion on whether Porro prisms is better than Roof
prisms, but the difference seems slight to me. Roof prisms mean more
compact binoculars, while Porro prisms cost less.

You will want a pair with a focusing control. Fixed-focus or
focus-free binoculars are usually a little out of focus for
astronomy.

Either separate focusing knobs for each eyepiece, or a single focusing
knob and a diopter adjustment knob are helpful if one eye focuses
differently from the other.

Also, “zoom binoculars” often leave something to be desired in
performance, and add to the price.

Now, you generally want bright, high-contrast images, subject to
constraints above and to what you are willing to pay. Higher
magnification for a given lens size means better contrast for stars
(but not for extended objects like galaxies or nebulae–they get
larger but dimmer with higher magnification). Larger lens size for a
given magnification means brighter images.

You can get a feel for the brightness of an image through “exit pupil
diameter”. If not printed in the specs, you can compute this by
dividing the lens diameter, in millimeters, by the magnifcation. So
for 10×50 binoculars, the exit pupil diameter is 5mm. A child’s
pupils dilate to about 9mm, and an adults to 7mm, or maybe only 5mm
with old age. Generally, larger exit pupil means brighter
images…until the exit pupil is the size of your pupils. After that,
more exit pupil diameter doesn’t help.

If you do get big binoculars, you may want a tripod. Many find they
need a tripod for 11×70 binoculars. Most people would want one for
20×80 binoculars, and everybody would want one for 25×100 binoculars.
Make sure the tripod is sturdy enough for the weight of the
binoculars. A tripod made for a tiny video camera would probably be
insufficient. Also, check that the binoculars have a standard
mounting thread. You might have to purchase an L-shaped mounting
bracket separately depending on the type of tripod or where the
mounting thread is on the binoculars. Nearly all giant binoculars
have the mounting thread.

For (lots) more money, you can get electronic stabilizing binoculars,
but I have never tried them myself. In theory, they should allow you
to use larger binoculars while still getting a steady image.

Then, there are high-end brands that cost several thousand and give
really clear images. These are for the serious binocular astronomer.
But then by that time, you might be wanting a telescope instead.