Cassegrain telescope
Sieur Guillaume Cassegrain was a French sculptor who invented a form of reflecting telescope in 1672. A Cassegrain consists of primary and secondary reflecting mirrors. In a traditional reflecting_telescope, light is reflected from the primary mirror up to the eye-piece and out the side the telescope body. In a Cassegrain telescope, there is a hole in the primary mirror. Light enters through the aperture to the primary mirror and is reflected back up to the secondary mirror. The viewer then peers through the hole in the primary reflecting mirror to see the image.
There are three basic types of telescopes: refractors, reflectors (aka Newtonian-reflectors) and combined lens-mirror systems or catadioptric sensors. Catadioptrics are a combination of a refractor and reflector telescope, using both mirrors and lens to focus the incoming light. There are two popular catadioptrics designs: the Schmidt-Cassegrain and the Maksutov-Cassegrain, both are similarly designed.
Catadioptics are the one of the best all-around design for telescopes. They are used in all modern installations, such as the Hubble Space Telescope and the Keck telescopes.
See also
Cassegrain Telescope:
First developed in 1672 by the French sculptor Sieur Guillaume Cassegrain (1625?1712), this type of reflector is a combination of a prime concave and a secondary convex mirror, both aligned axially. The prime mirror is usually featuring a hole in the center thus permitting the light to reach an eyepiece, a camera, or a light detector.
Concave Mirror: Hyperboloid type
