May 2, 2008

In Today's One Minute Astronomer...

Binoculars (Part 1)

A good pair of binoculars gives you grab-and-go convenience on nights when you don't have much time to observe. Binoculars give you an expansive view of the sky, let you see in “3D” with both eyes, and cost far less than a telescope. Even a modest pair lets you see as many as 100,000 stars, hundreds of star clusters and nebula, supernovae remnants, and dozens of galaxies. Here are some binocular basics.

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The Basics

• All binoculars are marked with two key numbers, magnification and aperture. A pair markeed "7x50", for example, magnifies 7 times and has objective lenses 50 mm in diameter. Bigger lenses let you see dimmer objects.

• Some binoculars are marked with the field of view, either in degrees or “feet at 1000 yards”. This tells you how wide a scene you'll see. A typical pair lets you see 5 to 8 degrees, about the width of a golf ball held at arms length. In comparison, a telescope lets you see a field of view of 1 degree or less, which is like looking at the sky through a straw.

• For astronomy, more aperture is better. So a 10x80 pair lets you see fainter objects than a 10X50 pair. The trade-off? Bigger lenses means more weight, which makes them harder to hold for any length of time.

• High power means more detail and a darker background sky. But it's harder to keep a high-power pair of binoculars steady enough to see fine detail, since the shaking of your arms also gets magnified

A Deeper Look

• Another key measure of binoculars is the “exit pupil”, the size of the bright disks of light you see in the eyepieces when you hold the binoculars at arms length. For astronomy, you want the size of these disks to be no larger your eye's pupils when dark-adapted. Otherwise the light collected by the lenses doesn't enter your eye.

• The exit pupil is simply the ratio of aperture to magnification. So a 7x50 pair has an exit pupil of 50/7 = 7 mm (approx), and a 7x35 pair has a 35/7=5 mm exit pupil

• Under age 30, most people have a dark-adapted exit pupil of 7 mm. But we lose about 1 mm every 10-15 years. At age 50, for example, it may not make sense to use binoculars with an exit pupil larger than 5-6 mm. So if you're older, a pair of 7x35's might be just as good (and less expensive) than a pair of 7x50's.

Good To Know

if you have a friend with a good pair of binoculars, by all means try them out get an idea of what you can see. But because most people have their binoculars carefully focused for their own eyes, borrowing binoculars from a stranger at a star-party is considered bad etiquette... like borrowing someone's toothbrush.

Personal View

Back in the day, as summer approached, I used to strengthen my arms and shoulders for holding up my 7x50's by doing daytime endurance exercises, sometimes while holding small weights. Now I just mount my binoculars on a tripod.