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Your Home Observatory Help (page 4)

By — McGraw-Hill Professional
Updated on Sep 19, 2011

Field Of View

When you look through a properly adjusted pair of binoculars, with the focus and barrel spacing optimized for your vision, you should see a single, large circular region. The absolute field of view is the angular diameter, in degrees of arc, of this region as measured against the background of distant stars. In some sets of binoculars, the absolute field of view is expressed in terms of feet at 1,000 yards instead of degrees.

The absolute field of view, which is defined in the same way as it is with telescopes (see Chapter 17), depends on the magnification and also on the apparent diameter of the circular region—the apparent field of view—as seen through the eyepiece. Binoculars with large apparent fields of view offer a more pleasant viewing experience than those with narrower apparent fields regardless of the absolute field of view.

Even if a pair of binoculars has a wide apparent field of view, the image quality won’t necessarily be good. How well do objects near the outer periphery of the field stay in focus compared with objects near the center? Are stars near the edge distorted or blurred? To what extent do “little rainbows” appear around stars, especially near the edge of the field? You can’t easily test these things in a hobby store when you are deciding which pair of binoculars to buy. Therefore, it’s a good idea to check out the return policy of any store with which you do business. If the binoculars prove unsatisfactory, you should be able to return them to exchange for a better pair or to get a full refund within a few days of purchase. Keep the sales receipt!

Practice problems of this concept can be found at: Your Home Observatory Practice Problems

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