Large objective for night observation.

A relatively large (23-cm) aperture system working at f/1.5 has been designed for low-light-level application. A catadioptric system, utilizing only spherical surfaces as described by M. Shenker, serves as a starting point. It consists of three large correctors in front of the primary, and a single element located near the focal plane. All elements are made of BK-7 glass. The system presented incorporates one of the large corrector elements with the primary to form a Mangin mirror, and the secondary has been made coincident with the rear surface of the second element. A different glass has been selected for this objective, yielding a system with improved performance, greater rigidity, and higher transmittance. The optical evaluation, thermal properties, and mechanical mounting are reported with a comparison of the theoretical and actual performance.