Cassegrain reflector

The Cassegrain reflector is a combination of two coaxial reflectors used in Cassegrain telescopes and 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 parabolic while the secondary mirror is hyperbolic.

Of the three basic types of telescopes: refractors, reflectors and catadioptrics, the Cassegrain reflector falls under the categories of reflecting and Catadioptric designs.

Cassegrain designs are also utilized in satellite telecommunications earth station antennas, ranging in size from 6.3 metres to 14 metres. The centrally located sub-reflector serves to focus radio frequency signals in a similar fashion to optical telescopes.

Cassegrain designs

The "Classic" Cassegrain

The "Classic" Cassegrain has a parabolic primary mirror, and a hyperbolic secondary mirror that reflects the light back down through a hole in the primary. Folding the optics makes this a compact design. On smaller telescopes, and camera lenses, the secondary is often mounted on an optically flat, optically clear glass plate that closes the telescope tube. This support eliminates the "star-shaped" diffraction effects caused by a straight-vaned support spider. The closed tube stays clean, and the primary is protected, at the cost of some loss of light-gathering power.

It makes use of the special properties of parabolic and hyperbolic reflectors. A concave parabolic reflector will reflect all incoming light rays parallel to its axis of symmetry to a single point, the focus. A convex hyperbolic reflector has two foci and will reflect all light rays directed at one of its two foci towards its other focus. The mirrors in this type of telescope are designed and positioned so that they share one focus and so that the second focus of the hyperbolic mirror will be at the same point at which the image is to be observed, usually just outside the eyepiece. The parabolic mirror reflects parallel light rays entering the telescope to its focus, which is also the focus of the hyperbolic mirror. The hyperbolic mirror then reflects those light rays to its other focus, where the image is observed.

Ritchey-Chrétien

The Ritchey-Chrétien is a specialized Cassegrain reflector which has two hyperbolic mirrors (instead of a parabolic primary). It is free of coma and spherical aberration at a flat focal plane, making it well suited for wide field and photographic observations. Almost every professional reflector telescope in the world is of the Ritchey-Chrétien design. It was invented by George Willis Ritchey and Henri Chrétien in the early 1910s.

Dall-Kirkham

The Dall-Kirkham cassegrain telescope's design was created by Horace Dall in 1928 and took on the name in an article published in Scientific American in 1930 following discussion between amateur astronomer Allan Kirkham and Albert G. Ingalls, the magazine editor at the time. It uses a concave elliptical primary mirror and a convex spherical secondary. While this system is easier to grind than a classic Cassegrain or Ritchey-Chretien system, it does not correct for off-axis coma and field curvature so the image degrades quickly off-axis. Because this is less noticeable at longer focal ratios, Dall-Kirkhams are seldom faster than f/15.

chiefspiegler

An unusual variant of the Cassegrain is the Schiefspiegler telescope ("skewed" or "oblique reflector"), which uses tilted mirrors to avoid the secondary mirror casting a shadow on the primary. However, while eliminating diffraction patterns this leads to several other aberrations that must be corrected.

Catadioptric Cassegrains

chmidt-Cassegrain

The Schmidt-Cassegrain was developed from the wide-field Schmidt camera, although the cassegrain configuration gives it a much narrower field of view. The first optical element is a Schmidt corrector plate. The plate is figured by placing a vacuum on one side, and grinding the exact correction required to correct the spherical aberration caused by the primary mirror. Schmidt-Cassegrains are popular with amateur astronomers. An early Schmidt-Cassegrain camera was patented in 1946 by artist/architect/physicist Roger Hayward [ [http://www.freepatentsonline.com/2403660.pdf US Patent 2,403,660, Schmidt-Cassegrain camera] ] , with the film holder placed outside the telescope.

Maksutov-Cassegrain

The Maksutov-Cassegrain is a variation of the Maksutov telescope, invented by Dmitri Dmitrievich Maksutov. It starts with an optically transparent corrector lens that is a section of a hollow sphere. It has a spherical primary mirror, and a spherical secondary that in this application is usually a mirrored section of the corrector lens.

Argunov-Cassegrain telescope

In the Argunov-Cassegrain telescope all optics are spherical, and the classical Cassegrain secondary mirror is replaced by three air spaced lens elements. The element farthest from the primary mirror is a Mangin mirror, in which the element acts as a second surface mirror, having a reflective coating applied to the surface facing the sky.

See also

* Reflecting telescope
* Refracting telescope
* Celestron (Schmidt-Cassegrains, Maksutov Cassegrains)
* Meade Instruments (Schmidt-Cassegrains, Maksutov Cassegrains)
* Questar (Maksutov Cassegrains)
* Vixen (Cassegrains, Klevtsov-Cassegrain)

References