a wide-angle optical system having a concave, spherical mirror whose aberration is neutralized by a correcting lens: often used in special, photographic reflecting telescopes to obtain clear pictures of large areas of the celestial sphere
See Schmidt system in American Heritage Dictionary 4
noun
A system consisting of a concave spherical mirror and a transparent plate of glass at its center of curvature, used in reflecting telescopes to offset spherical aberration and coma.