Sometimes Binoculars are Best

I’ve always thought it’d be just a matter of time until I’d write an article in praise of using binoculars for amateur astronomy, something not immediately thought of by many who enjoy stargazing. The notion of trying binoculars on the night sky seems to be by no means an automatic consideration to a fair percentage of amateurs, unfortunately. Among this group are some of the very people, ironically, who stand to benefit the most from using “binos”—newcomers to our astronomy fraternity who may not be familiar with the night sky and who could really use all the help they can get in gradually easing into simple, straightforward star watching and constellation recognition. To some, binoculars just don’t seem, somehow, appropriate for looking up at night to the stars, something mistakenly considered to be properly reserved for astronomical telescopes. A bino is such a common item in many households that daytime use on traditional terrestrial scenes such as sporting events, bird and animal watching, ships at sea, distant mountains or hills, etc. registers on the mind of the user as the normal and “correct” way to use such an instrument. Perhaps the thinking is that a binocular is simply too small, basic and underpowered to meet astronomical viewing criteria, so why bother? I can’t overemphasize to those who hold this opinion just how much you’re missing! There are four basic points I should make before getting into specifics on reasons for using binoculars. I’ll do a follow-up piece explaining many of these reasons in next month’s issue, due to space limitations here. First, we recognize that telescopes have their obvious applications in astronomy and can show the observer a number of things not revealed at all in binoculars. (Separation of tight double stars, resolution of planetary details, highly magnified enhancement of tiny angular-size deep-sky objects so as to enable resolution, far greater light-gathering ability in medium-to-larger aperture scopes, and so on.) Second, no writer can write for everyone—there’ll be many among you who are already well aware of this subject and need no introduction. I chose this topic for those of you who might simply enjoy a piece on binocular viewing and perhaps could be encouraged to give it a try, if you’ve not done so already. Third, this article is concerned with hand-holdable binoculars of 50 to 60mm aperture or under and magnifications of approximately 10 power maximum. Fourth, so-called “giant” binos—owing to their size, weight, and the necessity of attaching them to some kind of mount—are not mentioned here because they’re deserved of a separate article all their own and are not as quick and easy to use as smaller, hand-held types. (The giant types, if of good quality and in proper collimation, can yield superb views that might be classed roughly midway as an observing experience between, say, a 7x35mm bino and a good 6 to 8-inch telescope. Should you have an opportunity to try one sometime, there’ll be a good chance you’ll be so impressed that you might end up reviewing your budget for new toys.)

Plenty of good information on the most important features that binoculars for astronomy should have is readily available in a number of books and magazine articles, and some publications have fine, comprehensive listings of recommended objects for observing. Phil Harrington’s Touring the Universe Through Binoculars is an absolute treasure trove in this respect. Phil has put together object listings for each of the 88 constellations that are so extensive that his book can easily serve as a good observing guide for telescope users, although his choices are things that can actually be seen in binoculars. If I were to compile a “short list” of a dozen or so of the best books for an amateur astronomer’s library, this splendid book would have to be included.

Although the information is available elsewhere, it won’t hurt to go over some recommendations here. Due to the nature of the images presented by stars and other celestial objects, the optical criteria for viewing them well is much higher than for, say, watching players on a football field or spotting a hawk in a distant tall tree. (Remember: stars themselves, other than our own Sun, are technically “point sources” of light because of their fantastic distances.) Should you happen to own a binocular now, I hope it’ll be in collimation, meaning that the two barrels are parallel and therefore give one clear image when using both eyes simultaneously, which is what a binocular is intended to do. (You’d be surprised at how often the “double image” problem comes up!) I also hope that your binocular has sufficiently long eye relief that you can see the entire width of field, even when wearing eyeglasses. Eye relief is simply the distance-expressed in millimeters- between your eye and the point at which the full field of view is rendered in an optical instrument. The more generous this important value is, the better. An eye relief of 20mm, for example, is vastly superior to one of only 12mm—it’s critical that you be able to comfortably and effectively see your binocular’s entire field without having to just about “glue” your eyeballs up against the ocular (eyepiece) lenses, particularly when wearing glasses.

Maybe you don’t have a bino but might consider buying one? The following points can help greatly in guiding you; for brevity’s sake I’ll skip a few lengthy technical explanations and hope you can accept what I say as a given.

1. Coatings: Avoid any bino having red (so-called “ruby”) lens coatings; same goes for gold-colored coatings— these can be cheap gimmicks intended to impress the uninitiated and optical performance will be poor on stars. Buy a bino with the highest grade of coatings you can afford, multicoated (MC) or, better still, fully multicoated (FMC). The best coatings will show a deep greenish-violet color when examining the lenses at an angle under light.

2. Collimation: We covered this but I’m repeating it here for emphasis. A bino has to be in proper collimation to be of any use to you, so be fussy when checking one out. 3. Eye relief: Same story—eyeglass wearers should be able to leave glasses on and still make easy use of a binocular. Get the longest eye relief rating possible.

4. Variable (“zoom”) power binoculars: Don’t buy one of these for astronomy purposes! Stick with a fixed-power instrument. I’ll forego explaining why; just trust me.

5. Tapped hole: Any binocular purchased for astronomy should have a tapped hole in its central pivot brace at the front end—standard size is 1/4x20 thread. The hole enables you to easily attach the bino to a bracket which, in turn, goes on a mount head. Even small, lightweight binos can benefit by the ability to be used on a mount of some kind, although one of their primary advantages over telescopes is hand-holdability.

6 “BAK-4” or “BK-7” prisms? These code designations refer to special kinds of optical glass used in the internal prisms. Always choose a bino having BAK-4 prisms; various optical qualities inherent in BAK-4 make them superior for astronomy viewing to BK-7. You can usually check this feature by just holding a bino up to light at about one foot away from your eyes and studying the appearance of the light dots emitted through the ocular (eyepiece) lenses. If they’re nice and round, the prisms probably are BAK-4. If, however, they’re slightly squared-off and appear a bit diamond-shaped instead of truly round, then the prisms are the inferior BK-7 type.

7. “Roof” or “Porro” prism type? Some might argue this point, but in general, you’ll be better off with a Porro prism binocular, the basic type having 90-degree offsets in the barrels. Because of this design feature the distance between the centerlines of the barrels will always be greater than your eyes are apart. This enhances stereoscopic depth perception, even if only by a slight margin. Porro prism binos are simpler and easier to assemble and test by a manufacturer, meaning they’re less expensive than a roof prism type of comparable quality. They’re often a bit shorter in overall length, too. Roof prism binos employ a more complicated arrangement of their internal prisms, a design making it possible for the barrels to be straight all the way though with no side displacement. This makes them more compact, on average, than the Porro type, a good reason for their recent increase in popularity. Quality roof prism binos tend to be appreciably higher in price than a Porro instrument of the same quality level. Also, there tends to be greater loss of light in the image due to the complexity of the prism arrangement, a condition minimized in very high quality models of extreme price.

8. Power (magnification) and aperture (diameter of main lens) selection: A binocular will always be defined by two very important numbers, power being first and aperture being second in the description of any given model. Examples are 7x35, 8x42, 10x50, and so on. You might find the variety available to be confusing, especially if you’re surfing the websites or looking through, say, Orion’s catalog. We’ll narrow things down rather quickly, though. Many experienced bino users would say that the ideal “one size fits all” astronomy model is 10x50mm, followed closely by 8x42mm or 8x40mm. Ten power is the best compromise in magnification between higher powers with images that are hard to hold steady by hand and lower powers that often don’t magnify quite enough for pleasing resolution. Fifty millimeter aperture seems about ideal between the size and weight concerns you’d have with a model larger than 60mm, and the reduced light-gathering ability that a 35mm gives. A good bino of 10x50mm type should yield an apparent width of field of about 5 degrees; some models will do even better. An 8x42mm should, because of the lower power, yield 5.5 degree to 6.5 degree fields. By the way, you can easily figure a binocular’s “exit pupil” rating by simply dividing the aperture (in millimeters) by the power number, as in this example: 10x50 works out to a 5mm diameter exit pupil, which is ideal for most people. The exit pupil is simply the diameter of the light beam emitted though the eyepiece into your eye, always expressed in millimeters. This value should approximate the diameter of your dark-adapted pupils at their full dilation, and not be substantially larger or smaller. (I’ll skip a detailed reason.)

Next month in this space look for part 2 of this article, which will take you through reasons why binoculars are sometimes a better choice for certain kinds of observing than telescopes.

In line with that thought, here’s a brief preview of my main reason—it’ll dramatize beautifully why no telescope is the equal of a suitable binocular for viewing large celestial objects (or rich starfields) that can truly only be seen in their entirety and appreciated fully in a binocular. If you’ve got a bino, train it on the Pleaides, M45. Never seen this cluster in a bino? You’ll be a believer! Also, sweep the central Cygnus Milky Way just below Gamma Cygni (Sadr)—only a binocular can show this spectacle the way it should be seen.