All-spherical catadioptric telescope with small corrector lenses.

A well-corrected telescope with a large spherical primary mirror can be realized by using three or four small, widely spaced, corrector lenses. An interesting solution uses the lenses in a relay configuration which results in an erect image, accessible real exit pupil, and an internal field stop. Lateral color is corrected by adding a field lens at an intermediate focus. Polychromatic axial correction is excellent despite use of only one type of glass. The useful field of view is typically limited to 1 deg. Examples are presented.