The Maksutov is a catadioptric telescope design that employs a full diameter meniscus lens (commonly called a "corrector plate") to correct the problems of off-axis aberrations such as coma found in reflecting telescopes while avoiding chromatic aberration. The design is most commonly seen in a Cassegrain variation, with an integrated secondary, that can use all-spherical elements, thereby simplifying fabrication.
The design is named after Russian optician Dmitri Maksutov who invented it in 1941 (although it was independently invented by Dutch optician Albert Bouwers that same year). The design uses a spherical primary mirror in conjunction with a "meniscus corrector shell" at the entrance pupil in order to correct spherical aberration, which is a significant problem in other types of reflecting telescopes. The chief disadvantage of the Maksutov design is that it does not scale up well to large apertures (>250mm/10 inches), since the corrector plate rapidly becomes prohibitively large, heavy and expensive as the aperture increases – most commercial manufacturers usually stop at 180mm (7 inches).
At the time of his invention, Maksutov himself hinted at the possibility of a 'folded' Cassegrain-type construction. John Gregory, a designer for Perkin-Elmer, developed a Maksutov-Cassegrain from Maksutov's ideas. Gregory later published his landmark design for two f/15 and f/23 Maksutov-Cassegrain telescopes in a 1957 issue of Sky and Telescope. Commercial use of the design was explicitly reserved for Perkin-Elmer.
Most Maksutovs manufactured today are this type of 'Cassegrain' design (sometimes called a Spot-Maksutov) that may use all spherical surfaces and has, as secondary, a small aluminized spot on the inner face of the corrector. This has the advantage of simplifying construction. It also has the advantage of fixing the alignment of the secondary and eliminates the need for a 'spider' that would cause diffraction spikes. The disadvantage is that, if all spherical surfaces are used, such systems have to have focal ratios above F15 to avoid aberrations . Also a degree of freedom in correcting the optical system by changing the radius of curvature of the secondary is lost since that radius is the same as that of the rear meniscus face. Gregory himself, in a second, faster (f/15) design resorted to aspherization of the front corrector surface (or the primary mirror) in order to reduce aberrations. This has led to other designs with aspheric or additional elements to further reduce off-axis aberration.
This design was originally envisaged by G. I. Popov with a practical implementation by Yuri A. Klevtsov.