Editing Reflecting telescope (section)

== Reflecting telescope designs ==

===Gregorian===

{{Main|Gregorian telescope}}

[[File:Gregorian telescope.svg|thumb|500px|Light path in a Gregorian telescope.]]

The '''[[Gregorian telescope]]''', described by [[Scotland|Scottish]] astronomer and mathematician [[James Gregory (mathematician)|James Gregory]] in his 1663 book ''Optica Promota'', employs a concave secondary mirror that reflects the image back through a hole in the primary mirror. This produces an upright image, useful for terrestrial observations. Some small [[spotting scope]]s are still built this way. There are several large modern telescopes that use a Gregorian configuration such as the [[Vatican Advanced Technology Telescope]], the [[Magellan telescopes]], the [[Large Binocular Telescope]], and the [[Giant Magellan Telescope]].
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===Newtonian===

[[File:Newtonian telescope2.svg|thumb|500px|Light path in a Newtonian telescope.]]

{{Main|Newtonian telescope}}

{{See also|Schmidt–Newton telescope}}

The '''[[Newtonian telescope]]''' was the first successful reflecting telescope, completed by [[Isaac Newton]] in 1668. It usually has a paraboloid primary mirror but at [[F-number|focal ratio]]s of about f/10 or longer a spherical primary mirror can be sufficient for high visual resolution. A flat secondary mirror reflects the light to a focal plane at the side of the top of the telescope tube. It is one of the simplest and least expensive designs for a given size of primary, and is popular with [[amateur telescope making|amateur telescope makers]] as a home-build project.
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===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.

===Off-axis designs===

There are several designs that try to avoid obstructing the incoming light by eliminating the secondary or moving any secondary element off the primary mirror's [[optical axis]], commonly called [[off-axis optical system]]s.

==== {{anchor|Herschelian telescope}}Herschelian ====

{{multiple image
 |direction = vertical
 |header=Light paths
 |align = right
 |width = 250
 |image1=Herschel-Lomonosov reflecting telescope.svg
 |image2=Off-axis optical telescope diagram.svg
 |caption1=Herschelian telescope
 |caption2=Schiefspiegler telescope
}}

The '''Herschelian''' reflector is named after [[William Herschel]], who used this design to build very large telescopes including the [[40-foot telescope]] in 1789. In the Herschelian reflector the primary mirror is tilted so the observer's head does not block the incoming light. Although this introduces geometrical aberrations, Herschel employed this design to avoid the use of a Newtonian secondary mirror since the [[speculum metal]] mirrors of that time [[tarnish]]ed quickly and could only achieve 60% reflectivity.<ref>[http://catalogue.museogalileo.it/indepth/Telescope.html catalogue.museogalileo.it - Institute and Museum of the History of Science - Florence, Italy, Telescope, glossary]</ref>

====Schiefspiegler====
{{Main|Schiefspiegler}}
A variant of the Cassegrain, the Schiefspiegler telescope ("skewed" or "oblique reflector") uses tilted mirrors to avoid the secondary mirror casting a shadow on the primary. However, while eliminating diffraction patterns this leads to an increase in coma and astigmatism. These defects become manageable at large focal ratios — most Schiefspieglers use f/15 or longer, which tends to restrict useful observations to objects which fit in a moderate field of view. A 6" (150mm) f/15 telescope offers a maximum 0.75 degree field of view using 1.25" eyepieces. 
A number of variations are common, with varying numbers of mirrors of different types. The Kutter (named after its inventor [[Anton Kutter]]) style uses a single concave primary, a convex secondary and a plano-convex lens between the secondary mirror and the focal plane, when needed (this is the case of the ''catadioptric Schiefspiegler''). 
One variation of a multi-schiefspiegler uses a concave primary, convex secondary and a parabolic tertiary. One of the interesting aspects of some Schiefspieglers is that one of the mirrors can be involved in the light path twice — each light path reflects along a different meridional path.

====Stevick-Paul====

Stevick-Paul telescopes<ref>[http://www.amsky.com/atm/telescopes/spscopes/spt.html Stevick-Paul Telescopes by Dave Stevick]</ref> are off-axis versions of Paul 3-mirror systems<ref>{{cite journal
 |last=Paul |first=M.
 |year=1935
 |title=Systèmes correcteurs pour réflecteurs astronomiques
 |journal=[[Revue d'Optique Théorique et Instrumentale]]
 |volume=14 |issue=5 |pages=169–202
}}</ref> with an added flat diagonal mirror. 
A convex secondary mirror is placed just to the side of the light entering the telescope, and positioned afocally so as to send parallel light on to the tertiary.
The concave tertiary mirror is positioned exactly twice as far to the side of the entering beam as was the convex secondary, and its own radius of curvature distant from the secondary. Because the tertiary mirror receives parallel light from the secondary, it forms an image at its focus.
The focal plane lies within the system of mirrors, but is accessible to the eye with the inclusion of a flat diagonal.
The Stevick-Paul configuration results in all optical aberrations totaling zero to the third-order, except for the Petzval surface which is gently curved.
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====Yolo====

The Yolo was developed by Arthur S. Leonard in the mid-1960s.<ref>[https://web.archive.org/web/20030302111137/http://www.atmsite.org/contrib/Holm/yolo/index.html Arthur S. Leonard THE YOLO REFLECTOR]</ref> Like the Schiefspiegler, it is an unobstructed, tilted reflector telescope. The original Yolo consists of a primary and secondary concave mirror, with the same curvature, and the same tilt to the main axis. Most Yolos use [[toroidal reflector]]s. The Yolo design eliminates coma, but leaves significant astigmatism, which is reduced by deformation of the secondary mirror by some form of warping harness, or alternatively, polishing a toroidal figure into the secondary.
Like Schiefspieglers, many Yolo variations have been pursued. The needed amount of toroidal shape 
can be transferred entirely or partially to the primary mirror. In large focal ratios optical assemblies, both primary and secondary mirror can be left spherical and a spectacle correcting lens is added between the secondary mirror and the focal plane (''catadioptric Yolo''). The addition of a convex, long focus tertiary mirror leads to Leonard's ''Solano'' configuration. The Solano telescope doesn't contain any toric surfaces.
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===Liquid-mirror telescopes===

{{Main|Liquid-mirror telescope}}

One design of telescope uses a rotating mirror consisting of a liquid metal in a tray that is spun at constant speed. As the tray spins, the liquid forms a paraboloidal surface of essentially unlimited size. This allows making very big telescope mirrors (over 6 metres), but they are limited to use by [[zenith telescope]]s.