Using Binoculars for Astronomy

By Tim Hornbeck

"Call now for an amazing deal on this telescope capable of powers in excess of 500x. View the planets in our solar system and galaxies on the edge of our universe. All of this for only $200 on the Home Shopping Network."

How many times have you heard this sales pitch and wondered if it was too good to be true at that price? Actually, it probably was too good to be true. But there are quality alternatives to those department store or Home Shopping telescopes that just might be available to you right now; binoculars.

When most people think of binoculars they envision usage for hunting, sporting events, or bird watching. This would be the primary use for binoculars, but take them outside on a clear night and point them up toward the sky.

For the beginning astronomer, binoculars are more beneficial than purchasing a telescope. They are relatively inexpensive, lightweight and offer images that are correctly oriented. Some telescopes invert and flip images, which can be both confusing and a deterrent for the beginner. If you do not currently have a pair of binoculars, ask a relative or a friend if they have a pair you can borrow. Chances are you can find someone with a pair stuffed in the back of a storage closet.

Purchasing a pair of binoculars can be confusing. All of the brands, sizes, features and specifications can make your head spin. Once you become familiar with these variables, you can guarantee that your choice will provide you with years of quality viewing.

Prisms and Glass

Binoculars come in two major designs, the porro-prism and roof-prism. The porro-prisms are easier to align at the factory, are less expensive to manufacture, and produce more of a stereo image than the roof-prism design. This is the style most people think of when discussing binoculars. Roof-prisms are more streamline and compact, but lose more light reflection than the porro-prism. They are more expensive to manufacture than a porro-prism binocular.

Most prisms are made out of either BAK-4 (barium crown) or BAK-7 (borosilicate)glass. BAK-4 prisms are better quality, as they capable of producing brighter images and better edge sharpness. The BAK-7 prism is less expensive.

Magnification
Binoculars sizes vary from 7x35 (the most common) to 20x100 (weighing ~26 lbs.). The first number represents the magnification. For example, 7x35 binoculars will magnify an object 7 times more than the naked eye. This magnification is fixed and cannot be altered.

Field of View (FOV)

Another variable that is relative to magnification is the field of view (FOV). As the magnification increases the FOV will shrink., ultimately reducing what can be seen through the eyepieces. For example, 12x60 binoculars have about a 6-degree FOV, while 20x60 binoculars have only a 3-degree FOV. Mathematically this 3-degree FOV difference is a visual reduction of 156 feet at 1000 yards.

Lens Aperture
The second number represents the aperture or the diameter of the front lens. The 7x35 binoculars have a front lens diameter of 35mm. As the size of the aperture increases, so does the amount of light that can be collected. This leads to a brighter image during astronomical use. A good starting point for aperture size is 50mm. This amount will allow you to see stars that are 3 to 4 times dimmer when viewing with just the naked eye.

Three Types of Coatings
Binocular lenses have a magnesium fluoride coating to help with light transmission and reduce internal reflections that cause image flaring. "Coated" optics have a single layer of magnesium fluoride applied to one surface of the lens. "Multi-coated" optics have from 2 - 16 layers applied to one or more of the lens. "Fully multi-coated" optics have all lens surfaces are multi-coated. This is the most expensive but better performer out of the three coating types.

Zoom Binoculars
"Zoom" binoculars may seem better than fixed-magnification binoculars, because you are not locked into any given magnification. Typically "zoom" style binoculars are not recommended for astronomical use. Zoom optics are generally lower quality than an equally priced fixed-magnification pair of binoculars.

Tripod Adapters
Another variable to consider is whether the binoculars have a threaded tripod adapter hole. These are sometimes located behind a screw-on cap between the two main objectives. Having this feature and a special L-adapter allows you to mount the binoculars to a photographic tripod for hands-free viewing. A pair of 7x35 binoculars mounted on a tripod will typically provide better views than a pair of 10x50 binoculars that are hand held. Cannon currently offers the 15x45 IS (image-stabilization) binocular, which incorporates a special image-stabilization prism that reduces almost all hand-held shake.

What You May See

The moon, planets, star clusters, nebula, and galaxies are all visible with binoculars. The moon looks interesting with any type of binocular. The best time to view the moon is during its waxing and waning phases, which are the phases leading to and away from the full moon. The best location to view is the terminator, which is the line that separates the lighted side from the dark side of the moon. Six of the nine planets are also visible with binoculars. Jupiter and its four largest moons, which change positions nightly, can be seen with a pair 7x35 binoculars. Saturn and its rings can also be seen with a pair of 20x80 binoculars. You can also view the waxing and waning phases of Venus. Star clusters, nebula, and galaxies are some of the most interesting objects to view with binoculars. The Pleiades (M45 or Seven Sisters) and the Double Cluster (NGC869/884) are two of many open clusters that give superb views. The Orion Nebula (M42) and the Lagoon Nebula (M8) are spectacular objects in low to mid sized binoculars. The Andromeda Galaxy (M31) is an elongated patch with central condensation in mid to large sized binoculars.

This article was meant as an introduction to using binoculars for beginners. By educating yourself on the many styles and brands of binoculars, you will make the selection process an enjoyable and rewarding experience. Other resources for selecting and using binoculars for astronomy are located below.

WWW
http://www.telescope.com
http://www.oberwerk.com/bigbinos/default.htm

Books
Touring the Universe through Binoculars : A Complete Astronomer's Guidebook by Phil Harrington (ISBN: 0471513377)
Sky Atlas for Small Telescopes and Binoculars by David Chandler & Billie Chandler (ISBN: 0961320729)
Nightwatch: A Practical Guide to Viewing the Universe by Terence Dickinson (ISBN: 1552093026)

References
Gruan, Ken, 2001, What's Out Tonight; 50 Year Astronomy Field Guide 2000 to 2050, Ken Press, Arizona
Levy, David H., 1994, Skywatching, Time-Life Books, San Francisco
Dickinson, Terence, 2001, NightWatch, Third Edition, Firefly Books Ltd., 1998
Orion Telescope and Binocular, Holiday 2002 cataloge and www.telescope.com (images)