Any optical aid will bring deeper views of the sky than the naked eye, and any pair of binoculars that happen to be available, no matter how poor or small, are enough to launch a rewarding observing program. But some kinds are much better for astronomy than others.
The variety of brands and models on the market can be bewildering. Prism binoculars of the same basic type made today have been sold for 100 years — so manufacturers have long since discovered and incorporated every easy improvement possible. This is a very mature technology (with one big exception: new image-stabilized binoculars let you see as much in a small pair as you can see in a conventional large pair, though at a high price.)
Therefore, when a particular model offers special advantages, you can expect these to be offset by corresponding disadvantages, either in performance, convenience, or price. Choosing the right binoculars for astronomy is a matter of choosing where to compromise. The following guidelines will help.
Magnification and Aperture
Every binocular has a two-number designation, such as 6×30 or 10×50. The first number is the magnifying power or magnification. The second is the diameter of the objective (front) lenses in millimeters — the aperture of each lens.
Beginners usually assume that the higher the power the better. Higher powers are indeed generally preferable; they penetrate light pollution more effectively and are especially desirable for double stars, star clusters, and certain other objects such as the moons of Jupiter. But high power also narrows the field of view (making it harder to find your way among the stars), and, worst of all, magnifies the dancing of the stars when the instrument is held by hand. For this last reason, 10 power (10×) is the maximum usually recommended for hand-held binoculars (except for image-stabilized binoculars, which contain special mechanisms to counteract the jiggling).
As for aperture: the bigger the objective lenses the brighter the stars, and here the astronomer should compromise least. Most astronomical objects are hard to see not because they are small and need more magnification, but because they are faint and need more aperture. A pair of 8×50s collects twice as much light as all-purpose 8×35s, and hence makes everything appear about 0.7 magnitude brighter. The corresponding disadvantage of the 8×50s (aside from higher price) is that they are bigger and heavier, making them less appropriate for prolonged daytime use. For hikes or birdwatching the 8×35s would be the better choice — or even 7×30s or 6×24s, sacrificing both power and aperture for light weight and convenience. And image-stabilizing technology enables small binoculars (10×30s, say) to perform as well as larger handheld ones (10×50s, say).
Most binoculars are "center-focus," meaning you turn a knob in the center to focus both eyes at once. The right-hand eyepiece is also individually focusable so you can correct for differences between your two eyes; in theory this only has to be done once. Center-focus binoculars are convenient for birdwatchers and others whose targets often shift from near to far.
But astronomers don't need this feature. Everything in the sky is at the same "infinity" distance for focusing purposes. So you can save both money and mechanical complication (with its increased likelihood of problems) by choosing individual focus binoculars for stargazing. With these, you focus each eyepiece separately.
Quality vs. Price
Suppose you've decided on 8×50s — a fine all-around choice for astronomy. You may find three similar-looking instruments offered for $49, $190, and $1,000. Do these prices really reflect the range of value?
This is a matter of opinion, though a pair costing 20 times more than another certainly won't show 20 times as much. Away from the price extremes, say in the $75 to $400 range, you basically get what you pay for. (As in all consumer markets, a good rule of thumb is that the very bottom end is likely a poor value because it's a junky imitation, and the very top end is likely a poor value because its price is specially set for the people who will pay anything.)
Some manufacturers offer different lines of binoculars having poor, moderate, and good quality (in sales talk: "good," "better," and "best") to provide a selection of prices and values. A cheap instrument may be okay for a casual user. But quality is very important in the stringent applications of astronomy, so the amateur should stick to the better grades of binoculars for stargazing.
Once you decide on a make and model, shop around for prices. The differences may surprise you. However, we think it's worth paying more to buy from an outfit that specializes in astronomy, knows its customers and products, and can talk to you intelligently if the need arises.
Used binoculars can be bought at huge savings at yard sales, second-hand stores, and pawn shops, but you risk getting stuck with a lemon — unless you know how to check for quality.
The following tests, which can be done in less time than it takes to read them, will enable you to judge the value of any binoculars, new or used.
1. Pick up the instrument and compare its overall workmanship with other brands; some will seem better made than others. Hold the two barrels and try to twist them slightly. If there is any play in the joints or anything rattles, reject the pair. Move the barrels together and apart; the hinges should work smoothly, with steady resistance. So should the focusing motions for both eyepieces. On center-focus binoculars, the eyepiece frame should not tilt back and forth when you turn the focus in and out.
2. Next, look into the large objective lenses with a light shining over your shoulder so the inside of the barrel is illuminated. Reject the pair if a film of dirt or mildew is visible on any glass surface. (Dust on the outside is not a problem.) Look at the two reflections of the light from the front and back of the objective lens; the reflections will appear to float a little above and behind the lens. If the lens is antireflection coated — as it should be — both reflections will have a blue, purple, amber, or greenish cast, instead of white.
Move the binoculars around until you see a third reflection deep inside, from the first surface of the prisms. This too should be tinted, not white.
Then, still looking in the front, aim the eyepiece at a nearby light bulb and move the glasses around to view a row of internal reflections. The ratio of colored to white images suggests the ratio of coated to uncoated surfaces.
The coatings increase light transmission and especially image contrast ("clarity"), both of which are especially important in astronomy. "Multicoating" is the best kind. In very good models, all glass-to-air surfaces are multicoated.
Don't take vague advertising terms such as "fully coated" too literally; in the past, shady makers have used this term to mean that one lens is "fully" coated, and the rest are not.
3. Turn the binoculars around and repeat your examination of lenses and coatings from the eye end.
Then, holding the glasses a foot or so in front of you, aim them at the sky or at a bright wall. Look at the little disks of light that are floating just outside the eyepieces. These are the exit pupils. If they have four shadowy edges, rendering them squarish instead of round, the prisms are not the best and are cutting off some light. In good binoculars the exit pupils are uniformly bright to their round edges. Also, they should be surrounded by darkness, not by reflections from inside the barrels.
4. Finally, look through the binoculars. Adjust the separation of the barrels to match the separation of your eyes, then focus each side separately. A noticeably filmy or gray image indicates an unacceptable contrast problem. If you have to wear glasses to correct for astigmatism, make sure you can get your eyes close enough to see the whole field of view with the glasses on. If your glasses do not correct for astigmatism, you can take them off.
Each barrel should point in the same direction! If you see signs of a double image or feel eyestrain as your eyes compensate for the binoculars' misalignment, you have a reject. The eyestrain would soon become a real headache.
For a better test, first make sure the barrels are adjusted exactly for the separation between your eyes, then look at something distant through the binoculars. Slowly move them a few inches out from your eyes while still viewing the object. It should not become double. This test is tricky because your eyes will automatically try to fuse a double image, yet even a correctly aligned pair of images will look double for a brief moment before your eyes get them into register.
Misalignment due to flimsy prism supports is the worst problem of cheap binoculars; even a small knock can render a working pair worthless. More expensive instruments should survive minor bumps better.
Notice the size of the field of view: the wider the better. But the edges of a wide field may have poor optical quality. Is the view near the edge blurry when the center is sharp? Also, sweep the field at right angles across a straight line, such as a door frame or telephone wire. Watch whether the line bows in or out near the edges. This distortion should be only slight.
Look at sharp lines dividing light and dark, such as dark tree limbs or the edge of a building against a bright sky. Do they have red or blue fringes? No instrument is perfectly free of this chromatic aberration, but some are better than others.
Under the Stars: Testing Binoculars for Stargazing
A star at night is the most stringent test of optical quality, so try the binoculars on real stars if you get a chance. If not, look for an "artificial star" such as sunlight glinting off a distant insulator on a telephone pole or piece of shiny metal. Center it in the field of view. Looking with one eye at a time, can you bring it to a perfect point focus? Or, as you turn the knob, do tiny rays start growing in one direction before they have shrunk all the way in the direction at right angles? This astigmatism is especially bothersome when viewing stars, and binoculars that are completely free of it can be forgiven some other faults.
If, as you turn the focus, little rays start growing out of the star in all directions before the rest of the star reaches focus, you're looking at spherical aberration. This problem may be in your own eye, even if you're wearing your glasses. If it is, all binoculars with a given size exit pupil will show the same problem. To reduce it, choose higher-power binoculars (which yield a smaller exit pupil with a given aperture). Unfortunately, your spherical aberration cannot be corrected with glasses.
Now move the star from the center of the field to the edge. It will go out of focus unless you have a perfectly flat field and freedom from various other aberrations. As a rule of thumb, no degradation should be visible until the star is at least halfway to the edge of the field.
After running through these tests with several binoculars, you will have an excellent idea of their relative value.
One last word: Don't be discouraged if you can't find (or can't afford) perfection. Successful amateur astronomy depends on attitudes, not instruments. This was driven home to me some years ago after I moved into downtown New Haven, Connecticut. The sky seemed hopelessly light polluted, my pair of 7×50s was mediocre, and there was no place to use them except through a plastic bubble skylight in the roof of our apartment. The plastic turned star images into shapes I felt no proper amateur would even deign to look at. But they were there, all right, and I was so intrigued at being able to see stars at all under such conditions that I kept at it. It turned out that stars could be detected down to 8th magnitude, and I wound up spending nearly a year following variable stars, hunting clusters and doubles, comparing stellar colors, and becoming more familiar with the sky than ever before. So take what you've got and enjoy it.