Editing Schmidt camera (section)

====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>