Maksutov telescope

From The Book of THoTH (Leaves of Wisdom)

Invented by the Russian optician Dmitri Maksutov (1896-1964), the Maksutov reflecting telescope is a type of catadioptric telescope that uses a spherical primary mirror in conjunction with a meniscus-shaped corrector plate at the entrance pupil in order to correct spherical aberration. At the time of his invention, in the 1940s, Maksutov himself hinted at the possibility of a 'folded' Cassegrain-type construction. In 1957, John Gregory published in Sky and Telescope his landmark design for two f/15 and f/23 Maksutov-Cassegrain telescopes to be built by amateurs. The commercial use of the design was explicitly reserved for Perkin-Elmer, the company where Gregory worked at that time.

The key difference from the similar Schmidt telescope design is the meniscus-shaped corrector plate, that has easy-to-make spherical surfaces, and not the complex aspherical form of the Schmidt design.

Most Maksutov-Cassegrains manufactured today use only spherical surfaces. All-spherical 'Cassegrain' designs (as the Gregory telescope cited above) that use, as secondary, a small aluminized spot on the inner face of the corrector are, specially for apertures larger than 150mm, simpler constructions that cannot exploit the Maksutov principle to the full. They are, however, cheaper, and thus extensively used, especially by amateurs. By using the central spot of the corrector as secondary, the construction is easier, but 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.

The best, high-quality makes have all surfaces laser-interferometer tested and even hand-figured to achieve excellent performance. The compact 'Maksutov-Cassegrain' telescope has been mass-produced since the mid-70s, and has proved to be a very convenient instrument for most amateurs. Low-cost Russian and -lately- Chinese mass-production has pushed the prices down to the point that the Maksutov-Cassegrain (and the Schmidt-Cassegrain too) has today become the 'instrument of choice' for the amateur astronomer, if not a 'telescope for the masses'. Something unthinkable in the 60s, when even a small Maksutov-Cassegrain, as the 'Questar', was quite expensive and within the reach of deep pockets only.

The Maksutov design also has excellent correction for off-axis aberrations such as coma, which is a significant problem in the simpler Newtonian reflecting telescope.

In 'folded', Maksutov-Cassegrain or Maksutov-Newton telescopes the diffraction generated by the arms of the 'spider' that holds in place the secondary mirror in 'pure' Newton or Cassegrain designs is eliminated, since the secondary is held by the corrector itself, even being integral with it in some cases, as mentioned above. Hence, image contrast is improved, and the image quality is very high, close to refractors of the same net aperture. This advantage, together with the fact that reflecting telescopes are almost free from chromatic aberration makes this telescope ideal for lunar, planetary and some deep-sky observations.

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. However, specialist manufacturers do create models beyond 14 inches in aperture.

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.

--Angel 16:43, 27 May 2006 (CDT)