The Schmidt—Cassegrain is a catadioptric telescope that combines a Cassegrain reflector 's optical path with a Schmidt corrector plate to make a compact astronomical instrument that uses simple spherical surfaces.
In this Cassegrain configuration the convex secondary mirror acts as a field flattener and relays the image through the perforated primary mirror to a final focal plane located behind the primary. Some designs include additional optical elements such as field flatteners near the focal plane.
The Schmidt—Cassegrain design is very popular with consumer telescope manufacturers because it combines easy-to-manufacture spherical optical surfaces to create an instrument with the long focal length of a reclecting telescope with the lower cost per aperture of a reflecting reflecting telescope advantages.
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The compact design makes it very portable for its given aperture, which adds to its marketability. Their high f-ratio means they are not a wide-field telescope like their Reflecting telescope advantages camera predecessor, but they are good for more narrow-field deep sky and planetary viewing. While there are many variations, both mirrors spherical, both mirrors asphericalor one of each they can be divided into two principal types: compact and non-compact. In the compact form, the corrector plate is located at or near the focus of the primary mirror.
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In the non-compact, the corrector plate remains at or near the center of curvature twice the focal length of the primary mirror. Compact designs combine a fast primary mirror and a small, strongly curved secondary.
This yields a very short tube length, at the expense of field curvature. Non-compact designs keep the corrector at the center of curvature of the primary mirror.
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One very well-corrected type is the concentric or monocentric Schmidt—Cassegrain, where all the mirror surfaces and the focal surface are concentric to a single point: the center reflecting telescope advantages curvature of the primary. Optically, non-compact designs give better aberration correction and a flatter field than most compact designs, but at the expense of longer tube length.
Media related to Schmidt-Cassegrain telescopes at Wikimedia Commons.]
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