design was the first-published meniscus telescope design, and was published in the widely-read Journal of the Optical Society of America. This led to professional and amateur designers almost immediately experimenting with variations, including Newtonian, Cassegrain, and wide-field camera designs. Maksutov–Cassegrains There are many Maksutov designs that use a Cassegrain configuration, mounting a convex secondary mirror near the focus of the primary mirror. Most types use full-aperture correctors and are therefore not very large, since the corrector plate rapidly becomes prohibitively large, heavy and expensive as the aperture increases, with very long cool-down times to reach optimal optical performance. Most commercial manufacturers usually stop at 180 mm (7 in). Gregory or "spot" Maksutov–Cassegrains Maksutov's design notes from 1941 explored the possibility of a 'folded' Cassegrain-type construction with a secondary silvered "spot" on the convex side of the meniscus facing the primary mirror. He thought this would create a sealed and rugged optical system suitable for use in schools. This design appeared commercially in Lawrence Braymer's 1954 Questar telescope and in PerkinElmer designer John Gregory's competing patent for a Maksutov–Cassegrain. Commercial use of Gregory's design was explicitly reserved for Perkin–Elmer but was published as an amateur telescope design in a 1957 issue of Sky and Telescope in and variations. Most Maksutovs manufactured today are this type of 'Cassegrain' design (called either a "Gregory–Maksutov" or "Spot-Maksutov") that use all-spherical surfaces and have, 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 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 () 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 type of Maksutov-Cassegrain's high focal ratio and narrower field of view makes them more suitable for lunar and planetary imaging and any other type of observing where a narrow field