Optical EngineeringModification of the classic Schmidt telescope design to provide near-stigmatic correction over a very large range of object distances
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The classic Schmidt design has its aspheric corrector plate designed for and located at the center of curvature of its companion spherically concave primary mirror. So configured, it is well corrected for objects at infinity imaged at focus. Any closer object, however, is imaged with increasing spherical aberration as its distance from the corrector plate is diminished. Shifting the unchanged corrector plate toward its primary mirror increases the range of object distances for which the system retains excellent stigmatic correction. At the corrector plate's near-optimum location, this range may extend from a few of the primary mirror's radii in front of the corrector plate to infinity. Coma increases, but likely will remain tolerably small except for very fast systems.