Cassegrain reflector
This design puts the focal point at a convenient location behind the primary mirror and the convex secondary adds a telephoto effect creating a much longer focal length in a mechanically short system.
The classic Cassegrain configuration uses a parabolic reflector as the primary while the secondary mirror is hyperbolic.
[2] Modern variants may have a hyperbolic primary for increased performance (for example, the Ritchey–Chrétien design); and either or both mirrors may be spherical or elliptical for ease of manufacturing.
[6] James Gregory's 1662 attempts to create a reflecting telescope included a Cassegrain configuration, judging by a convex secondary mirror found among his experiments.
The closed tube stays clean, and the primary is protected, at the cost of some loss of light-gathering power.
A concave parabolic reflector will reflect all incoming light rays parallel to its axis of symmetry to a single point, the focus.
In most Cassegrain systems, the secondary mirror blocks a central portion of the aperture.
This ring-shaped entrance aperture significantly reduces a portion of the modulation transfer function (MTF) over a range of low spatial frequencies, compared to a full-aperture design such as a refractor or an offset Cassegrain.
Thus, the classical Cassegrain has ideal focus for the chief ray (the center spot diagram is one point).
Finally, and 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.
The Dall-Kirkham Cassegrain telescope 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's astronomy editor at the time.
An unusual variant of the Cassegrain is the Schiefspiegler telescope ("skewed" or "oblique reflector"; also known as the "Kutter telescope" after its inventor, Anton Kutter[9]) which uses tilted mirrors to avoid the secondary mirror casting a shadow on the primary.
[10] Another off-axis, unobstructed design and variant of the Cassegrain is the 'Yolo' reflector invented by Arthur Leonard.
When set up correctly the Yolo can give uncompromising unobstructed views of planetary objects and non-wide field targets, with no lack of contrast or image quality caused by spherical aberration.
The Schmidt-Cassegrain was developed from the wide-field Schmidt camera, although the Cassegrain configuration gives it a much narrower field of view.
An early Schmidt-Cassegrain camera was patented in 1946 by artist/architect/physicist Roger Hayward,[11] with the film holder placed outside the telescope.
In the Argunov-Cassegrain telescope all optics are spherical, and the classical Cassegrain secondary mirror is replaced by a sub-aperture corrector consisting of three air spaced lens elements.
The centrally located sub-reflector serves to focus radio frequency signals in a similar fashion to optical telescopes.
For this antenna, the final focus is in front of the primary, at the top of the pedestal protruding from the mirror.
