Compound or Catadioptric

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.