Binocular Advice
Uncle Bob
Binocular Facts and FAQs
[for more info, see the S&T website linked in our links page]
Advantages
Q: Why binoculars instead of a telescope?
A: Expense, weight, portability, use of both eyes, wider field of view and
ease of reverting to naked eye sightings.
Disadvantage? Deep sky objects will be smaller and dimmer than when viewed in a large aperture scope.
Q: I have binoculars for daytime use, like sports- and bird-watching.
Do I need a special kind for astronomy?
A: No. There are thousands of objects in all types accessible to a standard
pair.
By the Numbers
Q: OK, mine are 7x35. What does that mean?
A: The first number is magnification. You will see objects
at 7 times their size and separation. The second number is aperture.
Your front lenses are each 35 mm in diameter. Aperture determines how much
light is captured, and more capture is good for fainter objects.
Caveats: Larger lenses mean heavier; more magnification menas narrower view.
There are a couple of other numbers to note. Exit pupil is the diameter of the beam of light that reaches your eyes. It is found by dividing the first two numbers. Your 7x35 pair have an exit pupil of 35 / 7 or 5 mm, and that is sufficient for the eyes of most observers. Unless you have really dark sky conditions, any more than 7 mm exit pupil is a waste.
Your field of view measured in degrees tells how big a chunk of sky you will see all at once. The most common fields are in the 5-15 degree range, and the more magnification you have the narrower will be your field. Use known separations like in the Big Dipper to guage your instrument’s field.
If your binocs are rated at a number of feet per 1000 yards, you can estimated your field by dividing the feet by 52.
Eye relief is the distance from the eyepiece to the eye. If you wear glases that correct for astigmatism, you will need to wear them while observing. If your field of view narrows significantly because you can’t get close enough to the eyepiece, try another binocular. If you are near- or far-sighted, you can remove your glasses and correct with smple focusing.
Coatings
Good binocs and telescopes are made of quality glass (BAK-4 is better than BK-7) and have magnesium fluoride coatings on the lenses for enhanced transmission of light.
Cleaning. It's best NOT to, unless you swab with a camelhair brush and household alcohol. Don't rub glass.
Focusing
Some binoculars focus each eye separately. If you have central focusing, you will have an independent focus for one eye to accommodate differences. Set this independent dial to zero, or a middle range. Use the central wheel to focus for the other eye or both eyes together. Then adjust the independent focus if necessary. Good binoculars will hold the focus.
Equipment and Accessories [see observing tips]
A New Wrinkle: Image Stabilization (IS)
Some binocs have the ability to suspend the optics in a shock-reducing
gel thus increasing the steadiness in hand-held mode. If you are budgeting
for quality binoculars, you will pay a little more for this very helpful feature.
Tripods have limited usefulness because of the upward direction most often
needed. Some chairs come equipped with brackets to hold binocs. I recommend
a deck chair or a hammock for the most comfortable viewing angles, and hand-held
mode along with taking breaks by using eyes alone.
Shopping
Beware of department store and etail offers for binoculars.
The generalist salesperson will have little advise for you about quality and
use. Even some recognized brand names put out cheap stuff to be sold in this
manner. It is better to send for catalogs from these manufacturers to get
an idea of the range of quality and price. Some well-known makers are Tasco,
Bushnell, Bausch and Lomb, Orion, Celestron and Meade. Quality foreign optics
are made by Zeiss, Leitz, Swarovsky and Fujinon to mention a few.
Trying out binoculars in daytime can be a challenge. Give them a good test
outside by looking at the lettering on a distant sign. The images should be
clear and bright. Look at a wall with parallel lines, such as bricks or clapboards,
in order to detect any distortion going to the edge of your view. Binoculars
should last a lifetime, so take time to make an informed decision.
Source: Deep-Sky Objects for Binoculars by John Kozak.
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