May 9, 2008

In Today's One Minute Astronomer...

Binoculars (Part 2)

Here's some plain advice on how to choose a good pair of binoculars for astronomy, and save a bunch of money in the process.

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The "ultimate guide" to choosing a telescope, binoculars, and astronomical accessories. Don't buy your optics before you read this essential resource. Now in it's fourth edition. Learn more...

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The Basics

• Some may disagree, but if you pay less than $75-100 for a new pair of binoculars, you'll be disappointed with what you get. On the other hand, almost no one needs to pay more than $300-$400 for an excellent pair. Between $100-300, you'll be spoiled for choice.

• Stick with Porro-prism binoculars, the classic type of binocular where the objective lens and eyepiece are offset. Binoculars that have a “straight-through” view use roof prisms, and a good pair is expensive. You don't need to pay the premium.

• Avoid binoculars with a zoom feature or a built-in camera. They don't make the grade for astronomical use.

A classic pair of porro-prism binoculars... great for astronomy

A Deeper Look

• When selecting a pair, pick up the binoculars and look at light reflected in the objective lenses. If the lenses have a good anti-reflection coating, they'll appear mostly dark, with a bit of reflected color. If the lenses appear white, or ruby red, don't buy them.

• Look through the lens at the prisms inside. A good anti-reflection coating shows a colored prism surface. A white surface means no AR coating. Not good.

• Now hold the pair away from your face with the eyepieces toward you. Look at the bright disk of the exit pupil. The disk appears round if the prisms use high-grade glass (called BAK-4 glass, if you're interested). If the disk appears squared-off, the prisms are made from lower-grade BK-7 glass. Not terrible, but not optimum.

• If you're near or far sighted, you don't need to wear your glasses when looking through binoculars. But if you have astigmatism, you will need your glasses. Make sure you can see right to the edge of the field of view while wearing your glasses.

• Look through the binoculars, and bring an object into focus at the centre of the field of view. A decent pair of optics will also hold focus out to the edge of the field. It may not be perfectly focused. But if the edge of the field is way out of focus or highly distorted, move on to another pair.

Good To Know

What separates a $200 pair of binoculars from a $2000 pair with the same magnification and aperture? The complexity of the AR coatings, the quality of lenses and prisms, and the precision of the lens shape. An expensive pair gives crisp, high-contrast views without distortion right out to the edge of the field. Nice to have, especially for daylight use, but not critical for your enjoyment.

Personal View

In 1987, I spent $100 on a pair of 7x50's. They work just fine for astronomy. But I've also looked through $1000 binoculars. Yes, they give a wonderful view. But if you want to spend your money elsewhere, you don't need to bother with such high-end optics.