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.