Schmidt corrector plate
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A Schmidt corrector plate, invented by Bernhard Schmidt in 1931, is a lens used to correct spherical aberration in a reflecting telescope that uses a spherical primary mirror. The corrector is placed in front of the mirror and intercepts the light as it enters the telescope. It is placed at the center of curvature of the mirror 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 focuses at the same point as light reflected from the inner portion of the mirror. The Schmidt corrector only corrects for spherical aberration. It does not change the focal length (or magnification) of the system.
The curve of the Schmidt corrector is complex but is produced using a fairly simple method. The thin glass is figured flat on one side. The plate is placed in a vacuum device, figured-side-down, and a vacuum is used to distort the plate making the unfigured side concave to a precise curve. The unfigured side is then figured flat. When the vacuum is released the plate springs back to its original shape but now the originally flat side is flat again and the other side has the complex shape needed for the corrector. The corrector is mounted with the flat side toward the mirror.