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.
A general algebraic theory based on first and third order equations has been developed for systems composed of two separated spherical mirrors. The name inverse cassegrainian systems has been provisionally assigned by the author to this general arrangement. One subgroup is the aplanatic Schwarzschild mirror system. The theory not only includes previously known examples of this subgroup, but has revealed the existence of others. In addition, nonaplanatic systems of major potential interest are discussed. These include systems of predecided geometrical arrangement, telecentric systems, inside-out systems, parfocal systems, and others. Means of extending the theory to include nonspherical surfaces are discussed.
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