Cassegrain / Rtichey-ChretienClick on a link above to see telescopes in a particular category The Schmidt-Cassegrain was developed from the wide-field Schmidt camera, although the cassegrain configuration gives it a much narrower field of view. The first optical element is a Schmidt corrector plate. The plate is figured by placing a vacuum on one side, and grinding the exact correction required to correct the spherical aberration caused by the primary mirror. Schmidt-Cassegrains are popular with amateur astronomers as they pride long focal lengths in a short body. The Maksutov-Cassegrain is a variation of the Maksutov telescope named after the Russian optician and astronomer Dmitri Dmitrievich Maksutov. It starts with an optically transparent corrector lens that is a section of a hollow sphere. It has a spherical primary mirror, and a spherical secondary that in this application is usually a mirrored section of the corrector lens. The Ritchey-Chrétien telescope or RCT is a specialized Cassegrain telescope designed to eliminate coma thus providing a large field of view compared to a more conventional configuration. An RCT has a hyperbolic primary and a hyperbolic secondary mirror. It was invented in the early 20th century by American astronomer George Willis Ritchey and French astronomer Henri Chrétien. Although usually associated with expensive observatory applications recent advances have seen the RC design become available to the amateur astronomer. APM offers the Astronomy Technologies range of RC telescopes. The Modified Dall-Kirkham telescope utilizes an elliptical primary and spherical secondary mirror as in the conventional Dall-Kirkham configuration, but also includes two lens elements ahead of the focal point to improve off-axis image quality. The primary mirror conic constant is slightly different than that for a conventional Dall-Kirkham and must be optimized along with the lenses during design. Usable fields is much better than the RC telescope.
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