Binoculars for Astronomy

Examining the universe.
Night Sky: Chuck Baker

If you're new to astronomy, you've probably been flipping through magazines like Sky & Telescope and Night Sky and marveling at the pictures of celestial objects. Surely you want to see these objects in real life. But what if you don't have a telescope?

What if I told you that, within limits, you can find galaxies, star clusters, and nebulae without a telescope? Sound too good to be true? Well, if you've got an ordinary pair of binoculars lying around, astronomical observing is just a clear night away.

Beginning stargazers often overlook binoculars, but experienced observers keep them close at hand. Compared to a telescope, binoculars actually have certain advantages. Granted, they're smaller and give lower magnification. But they're lighter, much easier to take outside, use, and put away, and less expensive. They also give a much wider view than a telescope does, making celestial objects easier to find. They let you use both eyes, providing surer, more natural views. Moreover, in binoculars everything is right-side up and presented correctly, not upside down and/or mirror-reversed.

Finally, there's another big plus for binoculars: you may already have a pair in the back of a closet (or know someone who can loan you a pair). If so, stop reading and go get them now. I'll wait.

Engraved on the back of virtually all binoculars, you'll find the magnification (10x, in this case) and the diameter of the front lenses in millimeters (50, in this case). There may be also information on the width of the field of view.
Alan MacRobert

Now look them over. On the back you'll see two numbers separated by an "x". Common combinations are 8x40, 7x35, 7x50, and 10x50. The first number is the magnification, or "power." The second is the aperture, the diameter of the large front lenses measured in millimeters (see the example at right).

You may also find more numbers below the magnification-x-aperture rating. These give the field of view, which is how wide a scene you'll see. It's expressed in feet at a distance of 1,000 yards, or, more commonly these days, in degrees. (The conversion is simple: 1° is 52 feet at 1,000 yards.) Binoculars' fields of view vary from about 10° (the size of the Big Dipper's bowl, or the size of your fist held at arm's length) for wide-angle models, to a mere 2° (the width of your thumb at arm's length) for high-power models. Most of the time, though, the field of view is about 5° to 8° wide: about as much sky as is covered by a golf ball or squash ball held at arm's length.

What Can I See?

No matter what binoculars you just dug out of the closet, they'll be a great addition to your stargazing sessions. Consider this: on a clear, dark night out in the country, your eyes can see up to 3,000 stars, give or take. But when you use even modest 7x35 binoculars, that number rises to roughly 100,000 stars!

Similarly, in a light-polluted suburb you may be able to see only a couple hundred stars unaided, but binoculars will cut through the murk to show you more than you could see with your unaided eye from the top of Mount Everest.

And there's much more to look at in the night sky than random stars. Scores of double stars, rich Milky Way star clouds, star clusters of various sizes and types, stars that vary in brightness from month to month or even hour to hour, a smattering of ghostly nebulae and dim, distant galaxies — all are waiting for you to track them down with binoculars and suitably detailed sky maps and guidebooks.

Granted, most of the interesting astronomical objects that binoculars can show will appear very faint in them. But most objects that a telescope will show also appear very faint in it — certainly much fainter than you would like. Moreover, the map-using skills that you'll gain using binoculars to hunt out these dim, distant things in the dark wilds overhead are exactly the skills that you will need in order to put a telescope to good use.

And the sky is always changing. Summertime offers such showpiece sights as Mizar and Alcor, the famous pair of stars at the bend in the Big Dipper's handle, and the perfectly round little fuzzball of M13, the Great Cluster in Hercules. Sweep the summer Milky Way from Cygnus overhead through Sagittarius low in the south, looking for knots of stars and luminous bubbles of interstellar gas. Some sections of the Milky Way look, to me, better in binoculars than through any telescope.

In autumn, don't miss the Great Andromeda Galaxy, which looks like a dimly glowing little oval cloud. Contrast its smoothness with the delicate sparkle of the Double Cluster in Perseus. Winter's crisp skies are great for scanning the Hyades and Pleiades star clusters, then sweeping down to gaze at the Great Orion Nebula in Orion's Sword. Spring brings the unique Beehive star cluster in the constellation Cancer.

If you don't already have binoculars (or if you're outgrowing the ones you've got), it's time to buy. But there are so many choices. Are some better for astronomy, as opposed to birdwatching or baseball games? You bet!

Astronomy is done in the dark, so you really want big aperture: big front lenses. These collect lots of light so you can see fainter things. This doesn't matter so much in the daytime, when there's plenty of light and you can get by with small front lenses — allowing daytime binoculars to be smaller, lighter, and less expensive. But for astronomy, the bigger the aperture the better.

Binoculars come in two basic styles: the step-sided Porro-prism design here, and the H-shaped roof-prism design shown below. Porro-prism instruments are less expensive and excellent for astronomy.
Night Sky: Chuck Baker

You also want high optical quality. Stars and faint celestial objects seen against a dark sky are much more demanding than daytime scenes, so mediocre optics display their flaws much more obviously when you're observing the night sky rather than eatching the pitcher's mound. In general, price is a pretty good indicator of optical quality. The best optics are not going to be cheap.

Binocular Styles

Binoculars come in two body styles: the familiar "stepped" look of the Porro-prism design (right), and the sleeker "H" profile of the roof-prism design (below right). Most binoculars marketed for astronomy use Porro prisms, named for the Italian optician who invented them in the 1850s.

Roof-prism binoculars are a good choice when lightness and compactness matter more than high cost. Beware of cheap versions (and ultra-cheap fakes).
Night Sky: Chuck Baker

Roof-prism units are smaller and lighter-weight but have a more complicated, touchy optical design, which makes them more difficult and expensive to manufacture well. As a result, roof-prism binoculars tend to cluster at the high end of the market and, inexplicably, at the bottom end too — but not so often in between. A saying around my local astronomy club is that if your roof-prism binoculars don't seem to be performing well, you didn't spend enough money!

Giant Binoculars

In recent years there's been a new twist on the bigger-is-better theme: giant binoculars. These are impressive, even imposing-looking devices. It's as if someone put your friendly 7x50s on steroids. Common apertures include 70, 80, and even 100 mm. (Fujinon even makes a 150-mm model — the equivalent of two 6-inch telescopes, one for each eye!) For such beasts a tripod, preferably with a special binocular mount that allows you to aim upward, is mandatory. While they're a good supplement to your gear collection, I don't recommend giant binoculars as your primary instruments — they're just too unwieldy. Get more modest 7x40s or 10x50s first, and consider buying these big shots later.

Big, high-power binoculars need a proper mount. Parallelogram-style mounts, like the two shown here on tripods, are far and away the best design.
Sky & Telescope: Dennis di Cicco

Image-Stabilized Binoculars

Another recent innovation is image-stabilized binoculars. These employ the same ingenious mechanisms found inside the best video cameras. Push a button and the shaky magnified view suddenly calms down, almost freezing in place. The result is that you can use higher magnifications, get away with slightly less aperture, and yet still see more than with conventional binoculars.

Image-stabilized binoculars. Push the button on top, and the view holds miraculously still. Image-stabilized units are, however, expensive and a bit bulky, and they eat batteries.
Night Sky: Craig Michael Utter

I was skeptical when these little marvels were introduced, but I was quickly won over. When I first used Canon's compact little 10x30 image-stabilized binoculars (shown at left) from a dark-sky site, I had no trouble picking out the galaxies M81 and M82 near the Big Dipper. In fact, I actually tried to jiggle the view, but it refused to budge!

What Kind of Magnification is Best?

You may be thinking that more magnification is better, but in practice once you get to 10x and above, it gets harder (and more tiring) to hold binoculars steady if they're not on a mount or equipped with image stabilization.

There's some debate on what magnification is best. Low-power advocates recommend staying with 7x or 8x, whereas the high-power types say that the increased detail and darker sky background provided by 10x units are worth the narrower field of view and extra jiggling. (For tips on stopping the jiggling, see the next page of this article.)

My take on this? I'm in the low-power camp. I don't find the visual difference between 7x and 10x all that great, but I'm noticeably less fatigued when using low-power glasses. If you get the chance, give both a good try and see which you prefer.

The exit pupils are the little disks of light you see floating behind the eyepieces.
Night Sky: Craig Michael Utter

A related consideration is the exit pupil, the size of the little round disk of light that you see floating in the air behind the eyepieces when you hold the binoculars out in front of you toward a bright sky or a bright indoor wall. The size of the exit pupil is an important factor that's often overlooked. To determine it, just divide the aperture by the magnification — and luckily these are inscribed right there on the back facing you. For example, 7x50 binoculars have about a 7-mm exit pupil, while 10x50s have a 5-mm exit pupil.

Why is this important? Because the bright disk of the exit pupil should fit inside the pupil of your eye. And not everyone's eyes open to the same diameter in the dark. Young people (under age 30 or so) have pupils that open to about 7 millimeters across. While individuals vary a lot, the rule of thumb is that after age 30 you lose 1 mm of exit pupil every 10 or 15 years. So older eyes can't take advantage of binoculars with large exit pupils and, as a result, might see no difference between 7x35s and 7x50s. The extra light collected by the bigger 7x50s isn't fitting into your eyes; it's just going to waste. Score a big point for the high-power camp, at least if you're getting on in years; the higher the power, the smaller the exit pupil.

Testing Binoculars for Stargazing

Of course, the best way to see if a binocular model suits you is to give it a good tryout at night. Do stars focus down to pinpoints better in one pair than another? Your local optical shop, however, may not be thrilled with the idea of letting you play with lots of equipment overnight on a loaner basis.

Luckily there are ways to tell right in the shop how well binoculars will likely perform. These tests really work (I've done them for years), and if you learn them well you'll gain a reputation as an expert on binocular optics. Ready?

Inspecting the front lenses. The darker the reflections in the lenses look, the better their optical coatings. This means more light is transmitted through the glass to your eyes, and less light is scattered around adding haze to the view.
Night Sky: Craig Michael Utter

First, pick up several binoculars and look at their objective (front) lenses. Do this with a bright white light coming over your shoulder from behind. You'll notice right away that in some objective lenses, the reflection of the light will be brighter than in others. Pick the models with the reflections that look darkest (and no doubt deeply colored); this is a sign of quality lens coatings. Good coatings increase the transmission of light through the glass and reduce the amount of scattered light hazing the view.

Now, while still looking in the big front lenses, tilt the binoculars around a bit and look for more reflections deeper inside. They should all be colored, not white. A white reflection is the sign of a glass surface that has no coating at all.

You might think you could tell the quality of the coatings from designations such as "coated," "multicoated," or "fully multicoated," but in practice these terms can be next to meaningless. The proof is in the looking, so look. Note: Don't be taken in by models hyping "ruby-coated lenses."

Now turn the binoculars around and repeat the procedure, looking for colored versus white reflections in the eye lenses.

Next, face well-lit wall and hold the binoculars nearly at arm's length, with the eyepieces pointed at you. You'll see the exit pupils (disks of light) floating just behind the eyepieces, as was illustrated above. You might think that exit pupils would always be perfectly round, but this isn't so. The ones on cheaper binoculars often have a slightly "squared off" look, as if someone shaved off, or dimmed, two or four edges. This is a sign of manufacturer's corner-cutting that will slightly dim all the images you see.

Pick the units with round exit pupils; this tells you that quality prisms were used and that you're getting all the light you should. (You can also check the specification sheet: the best prisms are made from BAK-4 glass, while others use BK-7 glass.) Since they're hidden inside, the prisms are one of the first things manufacturers skimp on when trying to lower the price. Seeing "shaded" or "squared off" exit pupils is a sign of lesser-quality or undersized prisms.

Next, if you wear glasses for astigmatism, make sure you can see the entire field of view with your glasses on. If you're merely near-sighted or far-sighted, you can observe with your glasses off and just refocus as needed. If you're astigmatic, sorry — you'll have to use the binoculars with your glasses on, so this test will be important.

Next: see if you can detect whether the binoculars' two barrels are out of optical alignment, or "collimation." Experienced users can pick up on this relatively quickly, but beginners have a harder time of it, because your eye and brain automatically try to compensate for any misalignment. The best way I can describe this is that out-of-alignment binoculars will make you feel slightly "seasick." In really bad cases you may have trouble merging the two images into one, at least right away. Or maybe you'll have a mild sense of relief when you stop looking through them. Reject such units.

Finally — at last! — look through the binoculars.

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? Some binoculars are much better in this regard than others.

Keep 'em dry! The most common binocular problem, aside from getting knocked out of alignment, is moisture-related stains (often fungus) forming on the internal prisms and lenses. So be sure to let yours dry thoroughly after stargazing on a dewy night, or after bringing them into a humid house from the outdoor cold. If you expect dew and humidity to be a problem, consider sealed, waterproof binoculars such as these.
Night Sky: Craig Michael Utter

Sweep the field at right angles across a straight line, such as a door frame or a 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.

Follow these steps and you'll go a long way toward selecting quality binoculars right there in the store. If you're ordering from an online or mail-order distributor, this kind of test drive won't be possible before you make the purchase. So make sure the sales policies allow you to return defective or unsatisfactory units, then check the pair thoroughly once they arrive.

Testing Under the Stars

If and when you do get a chance to test binoculars under the stars, take it. A star at night is the most stringent indicator of optical quality. You may even find a daytime "artificial star" such as sunlight glinting off a distant insulator on a power pole or a distant 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. If you have astrimatism in your eyes, be sure to wear your glasses when doing this test.

If, as you turn the focus, little rays start growing out of the star in all directions before the rest of the star comes down to focus, you're looking at spherical aberration. This problem too 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; these yield a smaller exit pupil for a given aperture. Unfortunately, your eye's 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.

Keep It Simple

Let me offer one final caution: Keep it simple. As a rule you should avoid zoom binoculars, at least for astronomical use. Good zoom eyepieces require a very high (and expensive) standard of precision manufacture that's usually not met.

Likewise, binoculars with built-in cameras may be okay for casual daytime use, but not at night. They're gimmicky toys.

Most binoculars have center focus, meaning that you focus both barrels at once by turning a knob or a rocker in the center. This is great for when the distance of your target often changes, such as in birdwatching, or when you often pass the binoculars back and forth between people. But the night sky always stays at infinity focus, and you're probably observing it alone. So <individual-focus eyepieces will do just fime. This not only saves you money but provides better mechanical stability and gets rid of a common cause of eyepiece misalignment (slight tipping of the center-focus mechanism).

But no matter what imperfections you have to live with (and face it, optical perfection does not exist), you'll find that binoculars can show you more of the universe than you probably imagined.

Based in Amherst, New Hampshire, Ed Ting reviews stargazing equipment for his website, www.scopereviews.com.