Editing Schmidt camera (section)

==Invention and design==
The Schmidt camera was invented by Estonian-German optician [[Bernhard Schmidt]] in 1930.<ref>[http://www.ast.cam.ac.uk/about_ioa/history/schmidt ast.cam.ac.uk (The Institute of Astronomy (IoA), at the University of Cambridge (UoC)) – ''The Schmidt Camera'']{{dead link|date=January 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Its optical components are an easy-to-make [[Sphere|spherical]] [[primary mirror]], and an aspherical correcting [[Lens (optics)|lens]], known as a [[#Schmidt corrector plate|Schmidt corrector plate]], located at the center of curvature of the primary mirror. The film or other detector is placed inside the camera, at the prime focus. The design is noted for allowing very fast [[focal ratio]]s, while controlling [[Coma (optics)|coma]] and [[Astigmatism (optical systems)|astigmatism]].<ref>{{cite book 
|last=Wright 
| first=Franklin B. 
|editor-first=Albert G.
| editor-last=Ingalls 
|editor-link=Albert Graham Ingalls
|title=Amateur Telescope Making Advanced 
|publisher=[[Scientific American]] 
|date=1959
|pages=401–409 
|chapter=Theory and Design of Aplanatic Reflectors Employing a Correcting Lens 
| title-link=Amateur Telescope Making 
}}</ref>

Schmidt cameras have very strongly curved [[focal plane]]s, thus requiring that the film, plate, or other detector be correspondingly curved. In some cases the detector is made curved; in others flat media is mechanically conformed to the shape of the focal plane through the use of retaining clips or bolts, or by the application of a [[vacuum]]. A [[Field flattener Lens|field flattener]], in its simplest form a planoconvex lens in front of the film plate or detector, is sometimes used. Since the corrector plate is at the center of curvature of the primary mirror in this design the tube length can be very long for a wide-field telescope.<ref>{{cite web|url=http://www.astronomical.com/TelescopeOptics.htm|title=Telescope Optics – Schmidt |access-date=1 October 2014|url-status=dead|archive-url=https://web.archive.org/web/20091020143019/http://www.astronomical.com/TelescopeOptics.htm|archive-date=20 October 2009}}</ref> There are also the drawbacks of having the obstruction of the film holder or detector mounted at the focus halfway up the tube assembly, a small amount of light is blocked and there is a loss in contrast in the image due to [[diffraction]] effects of the obstruction and its support structure.<ref>[http://seds.org/~spider/scopes/obstruct.html "Obstruction" in optical instruments] {{webarchive|url=https://web.archive.org/web/20100620195356/http://seds.org/~spider/scopes/obstruct.html |date=2010-06-20 }}</ref>

===Schmidt corrector plate===
[[File:Schema lame de Schmidt.svg|thumb|250px|Exaggerated cross section of a Schmidt corrector plate. The real curves are hard to detect visually, giving the corrector plate the appearance of being an optically flat window.<ref>{{Cite book | last = Malacara  | first = Daniel | title = Handbook of Lens Design | publisher = [[Marcel Dekker|Marcel Dekker, Inc.]] | location = New York | year = 1994 | page = 468 | isbn = 0-8247-9225-4 }}
</ref>]]
A '''Schmidt corrector plate''' is an [[aspheric lens]] which corrects the [[spherical aberration]] introduced by the spherical [[primary mirror]] of the [[Schmidt telescope|Schmidt]] or [[Schmidt–Cassegrain telescope|Schmidt–Cassegrain]] telescope designs. It was invented by [[Bernhard Schmidt]] in 1931,<ref>{{cite book 
|last=Wright 
| first=Franklin B. 
|editor-first=Albert G.
| editor-last=Ingalls 
|editor-link=Albert Graham Ingalls
|title=[[Amateur Telescope Making|Amateur Telescope Making Advanced]] 
|publisher=[[Scientific American]] 
|year=1959
|pages=401–409 
|chapter=Theory and Design of Aplanatic Reflectors Employing a Correcting Lens 
}}</ref> although it may have been independently invented by Finnish astronomer [[Yrjö Väisälä]] in 1924 (sometimes called the [[Schmidt–Väisälä camera]] as a result).<ref>[http://www.telescope-optics.net/Schmidt-camera.htm telescopeѲptics.net, 10.2.2. - Full-aperture Schmidt corrector: Schmidt camera]</ref> Schmidt originally introduced it as part of a wide-field photographic [[catadioptric telescope]], the Schmidt camera. It is now used in several other telescope designs, camera lenses and image projection systems that utilise a spherical primary mirror.

====Function====
[[File:Aberration de sphéricité d'un miroir sphérique concave.svg|thumb|left]]
[[File:Correction par lame asphérique de Schmidt.svg|thumb|left|Example of an optical system using just a spherical mirror (''top'') and a spherical mirror combined with a Schmidt corrector plate (''bottom'').]]
Schmidt corrector plates work because they are aspheric lenses with spherical aberration that is equal to but opposite of the spherical primary mirrors they are placed in front of. They are placed at the center of curvature "'''C'''" of the mirrors for a pure Schmidt camera and just behind the prime focus for a [[Schmidt–Cassegrain telescope|Schmidt–Cassegrain]]. The Schmidt corrector is thicker in the middle and the edge. This corrects the light paths so light reflected from the outer part of the mirror and light reflected from the inner portion of the mirror is brought to the same common focus "'''F'''". The Schmidt corrector only corrects for spherical aberration. It does not change the focal length of the system.

====Manufacture====
Schmidt corrector plates can be manufactured in many ways. The most basic method, called the "classical approach",<ref name="patent124">{{Cite patent|number=3837124|title=Method for Making Replica Contour Block Masters for Producing Schmidt Corrector Plates|gdate=1974-09-24|invent1=Johnson|invent2=O'rourke|inventor1-first=Thomas J.|inventor2-first=John F.|url=https://www.freepatentsonline.com/3837124.html}}</ref> involves directly [[figuring]] the corrector by grinding and polishing the aspherical shape into a flat glass blank using specially shaped and sized tools. This method requires a high degree of skill and training on the part of the [[optical engineer]] creating the corrector.<ref name="patent124"/><ref name="DWL">[http://uncle-rods.blogspot.com/2010/02/down-with-love.html Rod Mollise, Down with Love, uncle-rods.blogspot.com, Sunday, February 21, 2010]</ref>

Schmidt himself worked out a second, more elegant, scheme for producing the complex figure needed for the correcting plate.<ref>{{Citation | last = Hodges | first = Paul C. | title = Bernhard Schmidt and his Reflector Camera | journal = The American Journal of Roentgenology and Radium Therapy | volume = 59 | date = January 1948 }}</ref>  A thin glass disk with a perfectly polished accurate flat surface on both sides was placed on a heavy rigid metal pan. The top surface of the pan around the edge of the glass disk was ground at a precise angle or [[bevel]] based on the [[Elasticity Coefficient|coefficient of elasticity]] of the particular type of glass that was being used.  The glass plate was sealed to the ground edge of the pan. Then a [[vacuum pump]] was used to exhaust the air between the pan and glass through a small hole in the center of the pan until a particular negative pressure had been achieved. This caused the glass plate to warp slightly. The exposed upper surface of the glass was then ground and polished spherical.<ref name="patent124"/> When the vacuum was released, the lower surface of the plate returned to its original flat form while the upper surface had the [[aspheric]] figure needed for a Schmidt corrector plate. Schmidt's vacuum figuring method is rarely used today. Holding the shape by constant vacuum is difficult and errors in the o-ring seal and even contamination behind the plate could induce optical errors.<ref name="patent124"/> The glass plate could also break if bent enough to generate a curve for telescopes of [[focal ratio]] f/2.5 or faster.<ref>{{Citation | last = Everhart | first = Edgar | title = Making Corrector Plates by Schmidt's Vacuum Method | journal = Applied Optics | volume = 5 | pages = 713–715 | date = May 1966 | issue = 5 |bibcode = 1966ApOpt...5..713E |doi = 10.1364/AO.5.000713 | pmid = 20048933 }}</ref> Also, for fast focal ratios, the curve obtained is not sufficiently exact and requires additional hand correction.

A third method, invented in 1970 for [[Celestron]] by Tom Johnson and John O'rourke,<ref name="patent124"/><ref>[http://www.universetoday.com/guide-to-space/telescopes/celestron-telescope/ Tammy Plotner, universetoday.com, Celestron Telescope]</ref> uses a vacuum pan with the correct shape of the curve pre-shaped into the bottom of the pan, called a "master block". The upper exposed surface is then polished flat creating a corrector with the correct shape once the vacuum is released.<ref name="patent124"/> This removes the need to have to hold a shape by applying an exact vacuum and allows for the mass production of corrector plates of the same exact shape.<ref name="DWL"/>

The technical difficulties associated with the production of Schmidt corrector plates led some designers, such as [[Dmitri Dmitrievich Maksutov]] and [[Albert Bouwers]], to come up with alternative designs using more conventional [[meniscus corrector]] lenses.<ref>[http://www.weasner.com/etx/guests/mak/MAKSTO.HTM John F. Gills, Ph.D, From James Gregory to John Gregory - The 300 Year Evolution of the Maksutov-Cassegrain Telescope]</ref>