Editing Reflecting telescope (section)

===The Cassegrain design and its variations===

[[Image:Cassegrain Telescope.svg|thumb|500px|Light path in a Cassegrain telescope.]]

{{Main|Cassegrain reflector}}

The '''Cassegrain telescope''' (sometimes called the "Classic Cassegrain") was first published in a 1672 design attributed to [[Laurent Cassegrain]]. It has a parabolic primary mirror, and a hyperbolic secondary mirror that reflects the light back down through a hole in the primary. The folding and diverging effect of the secondary mirror creates a telescope with a long focal length while having a short tube length. 
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====Ritchey–Chrétien====

{{Main|Ritchey–Chrétien telescope}}

The '''Ritchey–Chrétien''' telescope, invented by [[George Willis Ritchey]] and [[Henri Chrétien]] in the early 1910s, is a specialized Cassegrain reflector which has two hyperbolic mirrors (instead of a parabolic primary). It is free of [[coma (optics)|coma]] and spherical aberration at a nearly flat focal plane if the primary and secondary curvature are properly [[Figuring|figured]], making it well suited for wide field and photographic observations.<ref name="Sacek1">{{Cite web|work=Notes on AMATEUR TELESCOPE OPTICS|last=Sacek|first=Vladimir|title=8.2.2 Classical and aplanatic two-mirror systems|date=July 14, 2006|url=http://www.telescope-optics.net/classical_and_aplanatic.htm|access-date=2009-06-22}}</ref> Almost every professional reflector telescope in the world is of the Ritchey–Chrétien design.

====Three-mirror anastigmat====

{{Main|Three-mirror anastigmat}}

Including a third curved mirror allows correction of the remaining distortion, astigmatism, from the Ritchey–Chrétien design. This allows much larger fields of view.

====Dall–Kirkham====
{{See also|Modified Dall–Kirkham telescope}}
[[File:Large 1987 0528 0001 .jpg|thumb|Dall-Kirkham reflecting telescope, built by Horace Edward Dall]]
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 [[ellipse|elliptical]] primary mirror and a convex [[spherical]] secondary. While this system is easier to grind than a classic Cassegrain or Ritchey–Chrétien system, it does not correct for off-axis coma. Field curvature is actually less than a classical Cassegrain. Because this is less noticeable at longer [[focal ratio]]s, Dall–Kirkhams are seldom faster than f/15.