The Maksutov (also called a "Mak") is a catadioptric telescope design that combines a spherical mirror with a weakly negative meniscus lens in a design that takes advantage of all the surfaces being nearly "spherically symmetrical". The negative lens is usually full diameter and placed at the entrance pupil of the telescope (commonly called a "corrector plate" or "meniscus corrector shell"). The design corrects the problems of off-axis aberrations such as coma found in reflecting telescopes while also correcting chromatic aberration. It was patented in 1941 by Russian optician Dmitri Dmitrievich Maksutov. Maksutov based his design on the idea behind the Schmidt camera of using the spherical errors of a negative lens to correct the opposite errors in a spherical primary mirror. The design is most commonly seen in a Cassegrain variation, with an integrated secondary, that can use all-spherical elements, thereby simplifying fabrication. Maksutov telescopes have been sold on the amateur market since the 1950s. Invention Dmitri Maksutov may have been working with the idea of pairing a spherical primary mirror in conjunction with a negative meniscus lens as far back as 1936. His notes from that time on the function of Mangin mirrors, an early catadioptric spotlight reflector consisting of negative lens with silvering on the back side, include a sketch of a Mangin mirror with the mirror part and the negative lens separated into two elements. Maksutov seems to have picked up the idea again in 1941 as a variation on an earlier design that paired a spherical mirror with a negative lens, Bernhard Schmidt's 1931 "Schmidt camera". Maksutov claimed to have come up with the idea of replacing the complex Schmidt corrector plate with an all-spherical "meniscus corrector plate" while riding in a train of refugees from Leningrad. Maksutov is described as patenting his design in May, August, or October 1941 and building a "Maksutov–Gregorian"-style prototype in October 1941. Maksutov came up with the unique idea using an "achromatic corrector", a corrector made of a single type of glass with a weak negative meniscus shape that departed from the pure concentric spherical symmetrical shape to correct chromatic aberration. Similar independent meniscus telescope designs were also patented in 1941: Albert Bouwers (his 1941 concentric meniscus telescope), K. Penning and Dennis Gabor (a catadioptric non-monocentric design). Wartime secrecy kept these inventors from knowing about each other's designs, leading to each being an independent invention. Derivative designs Maksutov's 1944