Function
Schmidt corrector plates work because they are aspheric lenses with spherical aberration that is equal to but opposite of the spherical primary mirror they are placed in front of. It is placed at the center of curvature "C" 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 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.
Read more about this topic: Schmidt Corrector Plate
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