How To Choose Binoculars For Astronomy

In this article, you learn how to choose binoculars for astronomy, and you get some good bets for binoculars that are ideal for looking at the night sky.

Choosing Binoculars

When you select binoculars, stick with Porro-prism binoculars, the classic type of binocular where the objective lens and eyepiece are offset.  Binoculars that have a “straight through” view use roof prisms, and a good pair is expensive. You don’t need to pay the premium. Avoid binoculars with a zoom feature or a built-in camera. They don’t make the grade for astronomical use.

Binoculars with Porro-prisms (right) and preferred for astronomy compared to straight-through binoculars (left)

Avoid binoculars with a zoom feature or a built-in camera. They don’t make the grade for astronomical use.

If you can, shop for a pair of binoculars in person and go through the following steps when selecting a good pair…

Pick up the binoculars and look at light reflected in the objective lenses. If the lenses have a good anti-reflection coating, they’ll appear mostly dark, with a bit of reflected color. If the lenses appear white, or ruby red, don’t buy them.

Look through the lens at the prisms inside. A good anti-reflection coating shows a colored prism surface. A white surface means no AR coating. Not good.

Now hold the pair away from your face with the eyepieces toward you. Look at the bright disk of the exit pupil. The disk appears round if the prisms use high-grade glass (called BAK-4 glass, if you’re interested). If the disk appears squared-off, the prisms are made from lower-grade BK-7 glass. Not terrible, but not optimum.

If you’re near or far sighted, you don’t need to wear your glasses when looking through binoculars. But if you have astigmatism, you may need your glasses. Make sure you can see right to the edge of the field of view while wearing your glasses.

Finally, look through the binoculars, and bring an object into focus at the centre of the field of view. A decent pair of optics will also hold focus out to the edge of the field. It may not be perfectly focused. But if the edge of the field is way out of focus or highly distorted, move on to another pair.

Cross-section of binoculars using Porro prisms

Cross-section of binoculars using Porro prisms

Good Bets for Astronomy Binoculars

What separates a $200 pair of binoculars from a $2000 pair with the same magnification and aperture? The complexity of the AR coatings, the quality of lenses and prisms, and the precision of the lens shape. An expensive pair gives crisp, high-contrast views without distortion right out to the edge of the field. Nice to have, especially for daylight use, but not critical for casual astronomical use.

As far as brands, there are many good one.  Orion, Bushnell, Nikon, Celestron, and so on, all make quite good optics.  Just follow the steps and suggestions in this article and the last one.

Some may disagree, but if you pay less than $75-100 for a new pair of binoculars, you’ll be disappointed with what you get. On the other hand, almost no one needs to pay more than $300-$400 for an excellent pair. Between $100-300, you’ll be spoiled for choice.

As a rule of thumb, get stargazing binoculars with an aperture of 35 mm to 60 mm aperture and a magnification of 7x to 10x.  So a pair of 7×35’s is about the minimum acceptable for astronomical observing.  A pair of 7×50’s is better… this will give you the same magnification but a wider field of view.

If you are older than 40, you may not need the 7 mm exit pupil provided by 7×50’s, so you might consider 8×42’s which give slightly larger images.  If you don’t mind a little more weight, a pair of 8×56’s or 10×50’s are superb choices for stargazing.  Also, 9×63’s are excellent, though they are getting a little on the heavy side.  Any more than 10x, and the image will get a little shaky.  Any bigger than 63 mm, and the binoculars will get too heavy to hold for long periods.   You want to stick with a pair that weighs less than 2.5 lbs (1.1 kg).