Prime focus correctors for the spherical mirror.

The aberrations of a spherical mirror of moderate aperture ( approximately f/5) can be corrected by a lens system near the focal plane. A four-element lens system is necessary to provide a satisfactory state of correction over a full field of 0.5 deg. The design of such a lens system is described with emphasis on the intrinsic properties of each component and the logic behind the choice of layout.