Friday, April 4, 2008

Schmidt-Cassegrain: High-Performance Hybrid

Also called a catadioptric telescope, the Schmidt-Cassegrain design combines mirrors and lenses. Telescopes of this design are an increasingly popular choice for serious amateurs and introductory astronomy classes. The light passes through a corrector lens before it strikes the primary mirror, which reflects it to a secondary mirror. Since light bounces down the tube an extra time, the focal length of the telescope is effectively doubled, belying the very compact—wide but short and stubby—look of the instrument. A long effective focal length means that these telescopes can have a high magnification (remember that magnification is the ratio of objective focal length to eyepiece focal length) without a cumbersome long tube. Schmidt-Cassegrain telescopes are elegant instruments that offer some of the compactness of rich-field instruments but are much more powerful. The catch?

Diagram of a Schmidt-Cassegrain, or catadioptric, telescope. Light enters from the left and is focused by the primary mirror at the back of the telescope. Then it is refocused by a secondary mirror and sent out through an opening in the primary mirror to an eyepiece at the rear of the telescope.

These are usually more expensive amateur instruments, typically priced from $900 to much, much more, depending on aperture size and features. The portability of the Schmidt-Cassegrain design is a very big plus—not just because a compact telescope is easier to transport, but also because it is easier to keep a small scope stable during use.

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