Cassegrain reflector

The Cassegrain telescope [ kɑsɡʀɛ ] is a reflecting telescope. In the literature it is sometimes known as Jean or Giovanni telescope. It was in 1672 by the French scholar Laurent Cassegrain (ca. 1629-1693 ) presented to the public. Only a few years earlier Since the Newtonian telescope and the Gregory telescope were invented, a European debate on the advantages and disadvantages of these systems took place.

Similar to the Newton's telescope and the Gregorian telescope from the main mirror, the collimated beams on a sub-mirror side (Newton) or directed through a hole in the primary mirror to the receiver.

The incident light falls on a concave parabolic primary mirror ( the primary mirror ). This reflects the light to a convex hyperbolic secondary mirror, the secondary mirror. It is arranged so that its focal point lying on its concave side coincides with that of the large parabolic mirror. His lying on its convex side focal points in the direction of the primary. Either the receiver is prior to the primary mirror (see picture of the radar antenna ), or the reflected rays reach him through a hole in the primary mirror. Alternatively, the beams can be thrown before reaching the primary mirror by means of a plane mirror at the side of the tube. This position of the focal plane is called Nasmythfokus.

Unlike the Newtonian telescope ( auxiliary mirror is ground flat ) and as with the Gregory telescope ( auxiliary mirror is an ellipsoid ), extended the hyperbolic secondary mirror, the focal length and allows compact designs. In contrast to the Gregory telescope, the secondary mirror is but closer to the primary mirror as the focal point, which allows a shorter design.

The frame is curved, and other optical errors occur. This is especially true when a spherical primary mirror is used for small telescopes. These errors can be reduced by retouching the secondary mirror to a certain degree.

Other mirror telescopes rely on in their execution on the Cassegrain telescope and try on different ways to reduce image error:

• Nasmyth telescope
• Schmidt- Cassegrain telescope
• Maksutov telescope
• Hypergraph telescope
• Ritchey -Chrétien Cassegrain telescope (RC telescope)

The RC system has prevailed in the construction of very large telescopes.