Use Binoculars to Sweep the Night Sky

By Stephen P. Maran

Owning a good pair of binoculars is a must. Buy or borrow a pair and observe with them before you get a telescope. Binoculars are excellent for many kinds of observation, and if you give up on astronomy (sigh), you can still use them for many other purposes. And if you borrowed the binoculars, be sure to return them before you are viewed with suspicion.

Binoculars are great for observing variable stars, searching for bright comets and novae, and sweeping the sky just to enjoy the view. You may never discover a comet yourself, but you’ll certainly want to view some of the brighter ones as they appear. Nothing works better for this purpose than a good pair of binoculars.

The following sections cover the way binoculars are specified according to their capabilities and the steps to take as you figure out what kind of binoculars to buy. The figure takes you inside a pair of binoculars.

astronomy-bionoculars
Binoculars are like a pair of telescopes coordinated for your eyes.

Prisms, glass, and shapes

Binoculars contain prisms to bend the light coming from the two large lenses (objective lenses) to the two smaller lenses (eyepieces) that you look through. It’s necessary because the eyepieces can’t be farther apart than the distances between your eyes, or you won’t be able to look through both eyepieces at once. The objectives are bigger than your eyes, so they need to be farther apart; therefore, the light paths through the binoculars have to be bent.

The two basic prism types in binoculars are as follows:

  • Roof prisms are used in binoculars that are relatively straight and narrow; these binoculars are favorites among bird-watchers.
  • Porro prisms are used in binoculars that are relatively wide and short; they are the better type for stargazing because they give brighter images for the same-size lenses. It’s also easier to hold wide binoculars steady.

Binoculars use two main types of glass:

  • BK-7 glass, a trade term for garden-variety borosilicate glass, is often used in cheap binoculars.
  • BaK-4 glass, or barium crown glass, is used in fine binoculars and often yields brighter images of dim astronomical objects.

Deciphering the numbers on binoculars

Binoculars come in many sizes and types, but each pair of binoculars is described by a numerical rating — 7×35, 7×50, 16×50, 11×80, and so on. (Note that the ratings read “7 by 35, 7 by 50, 16 by 50, and 11 by 80.” Don’t say “7 times 35.”) Here’s how to decode these ratings:

  • The first number is the optical magnification. A 7×35 or 7×50 pair of binoculars makes objects look seven times larger than they do to the naked eye.
  • The second number is the aperture, or diameter, of the light-collecting lenses (the big lenses) in the binoculars, measured in millimeters. An inch is about 25.4 millimeters; thus, 7×35 and 7×50 binoculars have the same magnifying power, but the 7×50 pair has bigger lenses that collect more light and show you fainter stars than the 7×35 pair.

Also keep the following considerations in mind:

  • Bigger binoculars reveal fainter objects than smaller ones do, but they weigh more and are harder to hold and point steadily toward the sky.
  • Higher-magnification binoculars, such as 10×50 and 16×50, show objects with greater clarity, provided that you can hold them steady enough, but they have smaller fields of view, so finding celestial targets is harder than with lower-magnification binoculars.
  • Giant binoculars — 11×80, 20×80, and on up — are heavy and hard to hold steady; many people can’t use them without a tripod or stand. You can use some really big binoculars only with a heavy stand that may come with them, and they cost thousands of dollars. They’re definitely not for beginners.
  • Many intermediate sizes are available, such as 8×40 or 9×56.

7×50 is the best size for most astronomical purposes and certainly the best size to start with. If you purchase binoculars much smaller than 7×50, you really equip yourself for bird-watching rather than astronomy. Most astronomers can use 7×50 binoculars without a tripod or stand, although some people may need to brace themselves to hold these binoculars steady. Buy a much larger size than 7×50, and you may be investing in a white elephant that you’ll rarely use.

Making sure your binoculars are right for you

First and foremost, don’t buy binoculars unless you can return them after a trial run. Here’s how to make the basic check to determine whether a pair of binoculars is worth keeping:

  • The image should be sharp across the field of view when you look at a field of stars.
  • You should have no difficulty focusing the binoculars for your eyesight, with a separate adjustment for at least one of the eyepieces (the small lenses next to your eyes when you look through the binoculars).
  • When you adjust the focus, it should change smoothly. Stars’ images should be sharp points when in focus and circular in shape when not.
  • Special transparent coatings are deposited on the objective lenses (large lenses) of many binoculars. This feature, called multicoating, results in a clearer, more contrasting view of star fields. Binoculars that are fully multicoated are even better; in them, the coatings are applied to all the lenses and prisms.

Some astronomers wear their eyeglasses when observing with binoculars. Others, like me, are more comfortable putting away the glasses when using binoculars. But if you don’t have your eyeglasses on, you may have a problem writing notes, reading a star chart, and so on. Your choice of the best binoculars for you may depend on whether you will keep your glasses on.

If you plan to wear your eyeglasses when you observe with binoculars, you need binoculars that have enough eye relief. Eye relief is the distance (measured in millimeters) from the outer surface of a binocular eyepiece to the focal point, where the binoculars focus an image. If your eye is beyond that distance from the eyepiece, you can’t see the whole field of view. That circumstance happens when the thickness of your eyeglasses keeps your eye from getting to the focal point. Here’s my advice: Disregard salesperson assurances and binoculars manufacturers’ specs on eye relief. Instead, perform this simple test on binoculars that you are considering buying:

  1. Take off your glasses and focus the binoculars on a scene a block away (or in the sky).

    Note how much of the scene is in view.
  2. Put on your glasses.

    If the eyepieces have rubber eye cups, fold down the cups so that you can get closer to the eyepieces while wearing glasses.
  3. Focus the binoculars on the same scene as before.

    If you don’t see as much of the scene with your glasses on, the binoculars don’t have enough eye relief.

Good binoculars are sold in optical and scientific specialty stores. Some large camera stores have decent binocular selections. But I recommend avoiding department stores. You may get low-grade merchandise in some department stores or pay exorbitant prices for fancy binoculars in others. And you can bet that the salespeople peddling them know less than you do.

You can pay hundreds of dollars or even a few thousand bucks for a good pair of 7×50 binoculars, but if you shop around, you can find a perfectly adequate pair for $120 or less. (Pawn shops and military surplus stores are excellent places to look.) Used binoculars are often a good deal, but you must try them before you buy them because they may be out of whack.

Many astronomers buy their binoculars from specialty retailers and manufacturers that advertise in astronomy magazines and on the web. If you must order your binoculars online or by mail, first ask experienced amateurs you meet at an astronomy club or consult a staff member at a planetarium to find a reliable dealer.

Reputable makers of binoculars include Bushnell, Canon, Celestron, Fujinon, Meade, Nikon, Orion, Pentax, and Vixen. Some high-end Canon and Nikon binoculars have image stabilization, a high-tech feature that makes the image much steadier. They come in handy on a boat rocking at sea and are often a great help on land as well.