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Compound-catadioptric Telescopes

         Above is a fine drawing by the architect of the 200" Palomar Hale telescope, Russell W. Porter. The actual telescope tube is the vertically suspended cut-away cylinder in the center, with room enough for an astronomer to sit inside the tube at the top Prime Focus!


         A compound telescopes originally referred to a telescope in which a secondary mirror makes a substantial change in the focal length, as in the first diagram below. All variants of the Cassegrainian and Gregorian telescope are considered to be compound telescopes.  The word "catadioptric" comes from the Greek prefix "cata" meaning through, away, against or backward, and "diopter," meaning "of optical lenses."  So catadiopters are telescopes using both lenses and mirrors in their design; that is, they are a mixture of both reflector and refractor elements. The Schmidt-Cassegrain and Maksutov telescopes fall into this category, as in the second diagram below. The distinction between compound and catadioptric seems to have been abandoned in the modern literature, however, so I shall use the two words synonymously here.

         In the diagram of a basic compound telescope below, light enters the telescope tube from the left, hits the primary mirror at the bottom of the tube, is reflected back to the face of a secondary mirror, and exits through a hole in the primary mirror where it is either magnified by an eyepiece or falls upon CCDs or a photographic plate.





         The catadiopter's secondary mirror is held in place by either thin vanes (as in the Palomar Hale telescope above), by being affixed to the back of a transparent "corrector plate" (as suggested in the diagram below), or supported at the end of a tube rising from the central hole in the primary mirror (not shown).

         Amateur catadiopters differ nowadays from large, research ones in that the former usually have a correcting plate (shown above) across the front of the tube to the back of which is affixed the secondary mirror. Since parabolic surfaces are very hard to grind and figure and spherical ones much easier, primary catadioptric mirrors are usually ground to a spherical shape, then the corrector plate in the front of the tube optically corrects it to a parabolic shape.

         The first catadioptric telescope was made by the German astronomer Bernhard Schmidt in 1930. It had a spherical primary mirror and a glass corrector plate to remove the spherical aberration. Designed primarily for photography because it had no secondary mirror or eyepiece, photographic film instead was placed at the prime focus of the primary mirror. The most popular type of telescope today is the Schmidt-Cassegrain design invented in the 1960s. It uses a secondary mirror that bounces light through a hole in the primary mirror to an eyepiece.

         The second type of compound telescope was invented by a Russian astronomer, D. Maksutov, although a Dutch astronomer, A. Bouwers, came up with a similar design in 1941, before Maksutov. The Maksutov telescope is similar to the Schmidt design, but uses a more spherical corrector lens. The Maksutov-Cassegrain design is similar to the Schmidt Cassegrain design.





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telescopes in general
reflecting telescopes
refracting telescopes
charge-coupled devices (CCDs)
adaptive optics





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