CHOOSING
BINOCULARS FOR ASTRONOMY
By Alan MacRobert
Adapted from Sky &
Telescope
ANY OPTICAL AID
will bring deeper views of the sky than the naked eye, and any 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 commercially for more than 100 years
-- so manufacturers have long since discovvered and incorporated every easy
improvement that is possible. This is a very mature technology. 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 instrument for your purpose is a
matter of choosing where to compromise. The following guidelines will help.
Power. Every binocular has a two-number
designation, such as 6 × 30 (pronounced "six by thirty") or 8 × 50.
The first number is the magnifying power. The second is the diameter of the
objective (front) lenses in millimeters.
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. Even worse, it magnifies the dancing
of the stars when the instrument is held in the hands. For this last reason, 10
power is the maximum usually recommended for hand-held
binoculars. (Recently several vendors have introduced image-stabilized
binoculars. They're pricey, but they permit steady views at magnifications up
to 18× while conveniently hand-held.
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 7 × 50s collects twice as much
light as all-purpose 7 × 35s, and hence makes everything appear about 0.7 magnitude brighter. The corresponding disadvantage of the 7
× 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 7 × 35s would be the better choice -- or
even 6 × 24s, sacrificing both power and aperture for light weight and
convenience.
Focusing. 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
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 as far as focusing is concerned. So you can save
both money and mechanical complication (with its increased likelihood of
problems) by choosing individual focus binoculars. With these you focus each
eyepiece separately.
Quality vs. price. Suppose you've decided on 7 ×
50s -- a fine all-around choice for astronomy. You may find three
similar-looking instruments offered for $49, $180, and $1,000. Do these prices
really reflect the range of value?
This is a matter of
opinion, though it's certainly true that a pair costing 20 times more than
another 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.
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 the best
buy for a casual user. But quality is very important in the stringent applications
of astronomy, so the amateur should consider the better grades. However, having
decided on a make and model, you may get a bargain on it by checking with
discount stores and dealers.
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. 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.
Testing Binoculars
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,
which will appear to float a little above and behind it. 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 colored, 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 percentage of coated
to uncoated surfaces.
The coatings
increase light transmission and contrast, both of which are especially
important in astronomy. "Multicoating" is
the best kind. In top-notch models, all glass-to-air surfaces are multicoated.
Don't take vague
advertising terms such as "fully coated" too literally; this could
mean 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 a bright wall.
Look at the little disks of light seen 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 view
the full field 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 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 a bit tricky because
your eyes will automatically try to fuse a double image. At the same time, 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 accidents better.
Notice the size of
the field of view: the wider the better. But the edges of a wide field usually
have poor optical quality. 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 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.
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 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.
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. Success in
amateur astronomy depends more on attitudes than instruments. This was driven
home to me some years ago after I moved into downtown