Schmidt corrector plate

A Schmidt corrector plate is an aspheric lens which is designed to correct the spherical aberration in the spherical primary mirror it is combined with. It was invented by Bernhard Schmidt in 1931, although it may have been independently invented by Finnish astronomer Yrjö Väisälä in 1924 (sometimes called the Schmidt-Väisälä camera). Schmidt originally designed it as part of a wide-field photographic catadioptric telescope, the Schmidt camera, and is also used in other telescope designs, camera lenses and image projection systems.

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. 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.

Schmidt corrector plates can be manufactured in many ways. The most basic method, called the "classical approach", 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.

Schmidt himself worked out a second, more elegant, scheme for producing the complex figure needed for the correcting plate. A thin glass disk with a perfectly polished accurate flat form was placed on a heavy metal pan. The upper edge of the pan was ground at a precise angle or bevel based on the coefficient of elasticity of the particular type of glass plate 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 until a particular negative pressure had been achieved. This caused the glass plate to warp slightly. The exposed side was then ground and polished to a perfect flat. When the vacuum was released, the plate sprang back until its bottom surface was again plane, while the upper surface had the correct figure. Schmidt's vacuum figuring method is rarely used today. The glass plate will usually break if bent enough to generate a curve for telescopes of focal ratio f/2.5 or faster. Also, for fast focal ratios, the curve obtained is not sufficiently exact and requires additional hand correction.

...
Wikipedia