Editing Reflecting telescope (section)

===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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