With regard to telescopes, “catadioptric” means “due to both the reflection and refraction of light.” These instruments also are known as “compound” telescopes and are hybrids that have a mix of refractor and reflector elements in their design. German astronomer Bernhard Schmidt made the first compound telescope in 1930. The Schmidt telescope had a spherical primary mirror at the back of the telescope and a glass corrector plate in the front. The Schmidt telescope was the precursor of today’s most popular design, the Schmidt-Cassegrain telescope, or SCT. It also incorporated elements by French professor Laurent Cassegrain. In the SCT, light enters the tube through a corrector plate and then hits the primary mirror at the tube’s base, which reflects the light to a secondary mirror mounted on the corrector. The secondary reflects light through a hole in the primary mirror to the eyepiece, which sits at the back of the scope.
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