Cassegrain reflector

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The Cassegrain reflector is a combination of two mirrors used in some telescopes, which are then known as Cassegrain telescopes. It is also used in very high gain radio antennas.

First developed in 1672 by Laurent Cassegrain, this reflector is a combination of a primary concave mirror and a secondary convex mirror, both aligned symmetrically about the optical axis. The primary mirror usually contains a hole in the centre thus permitting the light to reach an eyepiece, a camera, or a light detector. The primary mirror is of paraboloid type, while the secondary mirror is of hyperboloid type.

Light path in a Cassegrain reflector

There are three basic types of telescopes: refractors, 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.

Another descendant of the Cassegrain design is the Ritchey-Chrétien telescope, which uses a hyperboloid primary and secondary mirror, eliminating the corrector plate needed for the catadioptic telescopes. Most modern telescopes, including the Hubble Space Telescope, use the Ritchey-Chrétien design.

The term Cassegrain has also been associated with a number of science fiction novels, such as David Drake's Igniting the Reaches and Patriots. The term is used in reference to a kind of laser weapon, called a flashgun, which has interchangeable batteries used to power the devestatingly powerful weapon, and uses the same principle as the telescope mentioned above. Drake either meant to refer to this kind of structure, or merely gave a name to the gun's type.

Cassegrain reflector

See also