An Aplanatic Reflector with a Flat Field Related to the Schmidt Telescope

The theory of the Schmidt telescope has been based on consideration of the properties of a spherical mirror.1 It develops that a spherical mirror, in conjunction with a correcting lens of little or no magnification mounted at the center of curvature of the mirror, provides a system with properties unique among reflectors, a system altogether free from the monochromatic aberrations of the third order with the exception of curvature of field. If the theory is developed along more general lines with no apriori assumptions as to the shape of the mirror, or the distance between mirror and correcting lens, a whole family of telescopes may be obtained, all members of which are free from spherical aberration and also obey the sine condition for freedom from coma.