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photovisual

Maksutov telescope

[mak-soo-tawf, -tof]

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.

Invention and Design

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).

Derivative designs

The Maksutov-Cassegrain

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.

The Klevtsov-Cassegrain

Another variation on the Maksutov is the Klevtsov-Cassegrain. Its corrector is much smaller than that in Dmitri Maksutov's design, consisting of a Mangin mirror that serves also as the secondary, and a meniscus lens of approximately the same diameter placed between the secondary and primary mirror. The assembly of meniscus and Mangin mirror is held in place by a spider vane and the front of the telescope tube is otherwise open. Advantages of this system over the Maksutov include a lower overall weight and lower cost of manufacture due to the smaller optical surfaces that need to be figured. Furthermore, temperature gradients between a telescope and its environment tend to affect the image quality due to thermal tensions within the optical elements; an open-tube design is beneficial in this respect because of a shorter cool-down time in a cool environment. One disadvantage lies in the necessity of a spider to hold the corrector, which inevitably affects image quality through diffraction artifacts .

This design was originally envisaged by G. I. Popov with a practical implementation by Yuri A. Klevtsov.

Applications

Astronomical uses

The focal ratio of the Maksutov-Cassegrain design provides high powers and a narrower field of view. This makes them unsuitable for wide-field astrophotography but superb at lunar and planetary imaging. They are also very adept at imaging tightly packed formations such as globular clusters and at splitting double stars. Maksutov-Cassegrain telescopes have been sold on the amateur market since the 1950s. Most early models were small run prestige models that were very expensive. The mid-70s saw the introduction of mass-produced models by some of the major commercial manufacturers. More recently low-cost Russian and, lately, Chinese mass-production have pushed the prices down even farther. Today the design has become a popular choice for the amateur astronomer, if not a 'telescope for the masses', something unthinkable in the 60s when even a small Maksutov-Cassegrains such as the 'Questar 3.5' were quite expensive and within the reach of deep pockets only.

Industrial/Aerospace uses

The 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 them ideal for tracking, remote viewing, and radar calibration/boresighting where instruments are subjected to severe environments and high g-forces.

References

See also

(commercially produced models)

External links

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