Телескоп Максутова: конструкция и история развития

Catadioptric telescope design

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 Soviet optician Dmitri Dmitrievich Maksutov. 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, and building a "Maksutov–Gregorian" style prototype in October 1941. 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.

Maksutov's 1944 design was the first published meniscus telescope design, and was published in the widely read Journal of the Optical Society of America. 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 high power view is a plus, such as resolving tightly packed globular clusters and double stars. The most notable early amateur astronomical type was the Questar 3 1/2 Maksutov Cassegrain introduced in 1954, a small run, expensive model still available on the consumer market. The mid 1970s saw the introduction of mass produced models by some of the major commercial manufacturers. More recently, low cost Russian and, lately, Chinese mass production has pushed the prices down even further. Many manufacturers currently produce Maksutov–Cassegrains, such as Explore Scientific, Intes, Intes Micro, LOMO, Orion Optics, Telescope Engineering Company (TEC), Vixen, the Meade Instruments's ETX line, and the Synta Taiwan produced Celestron, Sky Watcher and Orion Telescopes lines. The spot Maksutov–Cassegrain design has been used extensively in military, industrial, and aerospace applications. Since all of the optical elements can be permanently fixed in alignment and the tube assembly can be environmentally sealed, the design is extremely rugged. That makes it ideal for tracking, remote viewing, and radar calibration / boresighting, where instruments are subjected to severe environments and high g forces.

The Rutten Maksutov–Cassegrain (also called a Rumak or Sigler Maksutov) has a separate secondary mirror mounted on the inner surface of the meniscus corrector, sometimes similar to the corrector/mirror holder configurations found in commercial Schmidt–Cassegrains. This provides an extra degree of freedom in correcting aberration by changing the curvature of the corrector and the secondary independently. Specifically it allows the designer to aspherize the secondary to provide a much wider flat field than traditional spot Maksutovs, with less off axis coma. Mounting the secondary on the corrector also limits diffraction spikes. This version is named after the work of Dutch optical designer Harrie Rutten.

Maksutov noted in his designs that instead of using a full aperture corrector, a small sub aperture corrector could be placed in the converging light cone of the primary mirror and achieve the same effect. In the 1980s Dave Shafer came out with sub aperture Cassegrain designs based on this idea. The design reduces the mass and "cool down time" of a full aperture corrector. It has the drawbacks of an open, unsealed tube and requires a spider assembly to hold the secondary mirror and corrector, which inevitably affects image quality through diffraction artifacts. Also since the light passes through the corrector twice, the number of surfaces involved is increased, making it difficult to achieve good aberration correction. Sub aperture corrector Maksutovs are currently manufactured by Vixen telescopes, their VMC (Vixen Maksutov Cassegrain) models.

Maksutovs optics can be used in Newtonian configurations that have minimal aberration over a wide field of view, with one fourth the coma of a similar standard Newtonian and one half the coma of a Schmidt Newtonian. Diffraction can also be minimized by using a high focal ratio with a proportionally small diagonal mirror mounted on the corrector, allowing this design to achieve contrast and image quality approaching that of unobstructed high end refractors (although with some vignetting when used photographically). Like the Maksutov–Cassegrain, the overall diameter of the optical system is limited, due to the mass of the corrector plate. Synta Taiwan currently produces a 190 mm version under the Sky Watcher brand as does Explore Scientific with a 152 mm version designed in collaboration with astronomer David Levy.

The Maksutov system can be used in a (rare) type of prime focus ultra wide field astronomical camera design similar to the Schmidt camera. Like the Schmidt camera, the Maksutov camera has a curved focal plane.