Observing With Binoculars

© 1985, 2002 by Robert C. Moler

If you are looking for a comparatively inexpensive but useful telescope, consider a pair of binoculars. They are portable, easy to set up, and good for astronomical as well as terrestrial observation.

A good pair of binoculars will allow its user to explore vast areas of the sky. This is particularly important to the beginner who most become acquainted with the patterns of the constellations before a larger telescope can be made useful. I wonder how many telescopes are stored in closets and attics gathering dust because their owners didn't know the sky well enough to find anything other than the moon and a few bright planets.

What can be seen with a pair of binoculars? Here are a few examples

Buying a pair of binoculars can be confusing. There are so many types and the prices range from less than $50 to over $300. Let's leave the types to later. Every pair of binoculars is stamped with two cryptic numbers such as 6X30, 7X35, 11X80, etc. The first number is the magnification, while the second is the diameter the front lens or objective in millimeters. So a 7X50 binocular has 7 power and an objective of 50 millimeters or about 2 inches diameter.

As a rule 'the greater the aperture the brighter the image. But it is also affected by magnification. A 7X50 binocular gives as bright an image on diffuse light sources as an 11X80 since both power and aperture are increased by the same factor. The latter will give brighter star images and allow fainter stars to be observed because they are point sources.

There are three types of binoculars. The first is very cheap and usually advertised in Sunday supplements of newspapers. It is a field glass. The rating is never mentioned, but it is around 3X30. The field of view is narrow and it sells for $10 or less. It doesn't use prisms to turn the image right side up but uses a simple negative eye lens as Galileo did with his primitive telescopes. The manufacturers generally add a dummy bulge to the side of the barrels to simulate the prism housing, but the light path is straight through (the objective and eyelens line up). It is in a word; junk.

The second type is the most popular, and until most recently the only type. It uses two prisms, called porro prisms, in each barrel to erect the image. The objective is offset from the eyepiece.

The third type is the roof prism binocular. They don't come in very large sizes. They have a straight through design and are proud of it. No phony prism housings.

Binoculars are relatively fragile. It is quite easy to get the optics out of alignment by dropping them. This is especially true of the porro prism type. Here again the higher the price the better the Prism mounting, perhaps.

There are several things I'd like to add about binoculars and my preferences. Those who wear glasses are best served with a pair of binoculars with rubber eyecups that can be folded down so the eye can get close to the eyepiece. Additionally it would be nice if the binocular can be focused to allow viewing without eyeglasses. This is tough when your eyes are 20/400. The fast focus type binoculars are a minor bother when used in astronomy where everything is near infinity. For bast viewing binoculars should be mounted on something steady. Some binoculars have the threaded socket for mounting on a photographic tripod, Others can be fitted with tripod adapters. Binoculars over 10 power should be mounted on a tripod even for general use.

All in all a good pair of binoculars make a fine choice as a first or any telescope.


Questions? Send Email to me at bob@bjmoler.org

Updated: 01/21/02