The National Aeronautics and Space Administration in 1968 undertook the development of a very large and complex interplanetary spacecraft. Each spacecraft consisted of an orbiter and a lander. Each orbiter was to provide support for the landers as well as independent scientific studies. In support of these objectives each orbiter carried twin camera systems having identical 475mm f/3,5 telescopes. The telescopes were all spherical catadioptric cassegrains in in design using Mangin primary mirrors. The telescopes had high theoretical and measured performance and many high quality images of the Martian surface were obtained.
Leonard Larks, Leonard Larks,
"Optical System Of The Visual Imaging Subsystem On The Viking Orbiter Spacecraft", Proc. SPIE 0172, Instrumentation in Astronomy III, (3 May 1979); doi: 10.1117/12.957106; https://doi.org/10.1117/12.957106