Maksutov telescope

A 150mm aperture Maksutov-Cassegrain telescope.

The Maksutov is a catadioptric (mirror-lens) telescope that is designed to minimize off-axis aberrations such as coma.

Invention and Design

The design is named afer Russian optician Dmitri Maksutov who invented it in 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).

Applications

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.

Light path in a Maksutov-Cassegrain

Most Maksutovs manufactured today are this type of 'Cassegrain' design (sometimes called a Spot-Maksutov) that use, as secondary, a small aluminized spot on the inner face of the corrector. This 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 a degree of freedom (the radius of curvature of the secondary) is lost, that radius being 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.

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 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.

Derivative Designs

The Klevtsov-Cassegrain

A variation on the Maksutov is the Klevtsov-Cassegrain. Its corrector is much smaller than that of a Maksutov, consisting of a meniscus lens and a Mangin mirror that serves also as the secondary . The corrector and secondary are 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.

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

See also

(commercialy produced models)

• Questar
• Celestron
• Meade
• Orion Telescopes & Binoculars
• Vixen
• Orion Optics

External links

• Evolution of the Maksutov design