Schmidt camera
The Schmidt corrector, the first full-diameter corrector plate, was used in Bernhard Schmidt's 1931 Schmidt camera. The Schmidt camera is a wide-field photographic telescope, with the corrector plate at the center of curvature of the primary mirror, producing an image at a focus inside the tube assembly at the prime focus where a curved film plate or detector is mounted. The relatively thin and lightweight corrector allows Schmidt cameras to be constructed in diameters up to 1.3 m. The corrector's complex shape takes several processes to make, starting with a flat piece of optical glass, placing a vacuum on one side of it to curve the whole piece, then grinding and polishing the other side flat to achieve the exact shape required to correct the spherical aberration caused by the primary mirror. The design has lent itself to many Schmidt variants.