Above: Optical layout of a typical Maksutov-Cassegrain telescope
This section details the optical design and inherent aberrations of Maksutov-type catadioptric telescopes. For a more basic overview of these designs please see the Maksutov-Cassegrain page and the Maksutov-Newtonian page. For a review of the optical design terms, see the Optical Aberrations and Optical Design sections.
Maksutov Camera Design
The Maksutov camera is rarely, if ever, seen any longer, but it is the basis for the other designs discussed here. The Maksutov camera, like the more common Schmidt camera, uses a spherical mirror to reflect light onto a curved film plane located inside the telescope tube. The curved focal plane eliminates the inherent field curvature. A full-aperture corrector lens is then placed at the center of curvature of the mirror. Placement at this location eliminates coma. The lens itself is used to correct the spherical aberration of the mirror. This is the same principle used in the Schmidt camera, but the Maksutov corrector lens uses only spherical surfaces, unlike the Schmidt’s aspheric shape. The overall system is very fast, usually f/1.5 to f/3. The only drawback to the Maksutov corrector versus the Schmidt is stronger chromatic aberration.
The basis of the Mak-Cass design is the classical Cassegrain telescope. This design requires two aspheric mirrors (parabolic and hyperbolic). To ease the manufacture of the mirrors, they can be left spherical and a Maksutov corrector lens used to eliminate the resulting spherical aberration. The interesting possibility exists to make the curvature of the rear surface of the corrector lens coincide with the curvature of the secondary mirror, allowing the secondary to simply be an aluminized spot on the center of the corrector. This is known as the Gregory design. This is a bit of a trade-off in optical quality, since in an optimal system the secondary curvature would be slightly different from the shape of the glass, but in small sizes the effect is negligible and the cost savings are usually worth it. The final system is an all-spherical design with no spherical aberration. A more sophisticated design with the secondary mirror separate from the corrector lens is more difficult to manufacture but yields considerably better images, especially off-axis.
Mak-Cass focal ratios can vary greatly depending on whether the telescope is intended for photographic or visual application. Fast designs range from around f/5.6 to f/8, while slower versions are commonly in the f/12 to f/15 range. Primary mirror focal ratios are usually f/2 to f/4, with secondary magnifications typically between 3x and 4x.
The combination of lens and mirrors eliminates spherical aberration. The Gregory Mak-Cass suffers from both coma and astigmatism, limiting the useful field of view. By adding the extra degree of freedom of a separate secondary mirror, most of the off-axis aberrations are eliminated. Because of the thick corrector lens, Maksutovs suffer from more chromatic aberration than equivalent Schmidt designs, but the amount is still quite small until the focal ratio gets very fast. As with most Cassegrains, field curvature is an issue. For photographic applications, a field-flattener may be used. As with most telescopes, distortion is negligible.
The idea behind the Maksutov-Newtonian is to take the basic Newtonian design and minimize coma. Fast Newtonians suffer from large amounts of coma. The Mak-Newt replaces the parabolic primary mirror on the standard Newtonian with an easier-to-make spherical version. A Maksutov corrector is then placed just ahead of the secondary mirror (supporting the secondary and eliminating the need for spider vanes). As in a Maksutov camera, placing the corrector at the center of curvature would eliminate coma entirely, but would result in an absurdly long optical tube. Typical focal ratios range from f/4 to f/6.
The Maksutov corrector eliminates spherical aberration and minimizes (but does not eliminate) coma. Coma is around a quarter of that in a comparable standard Newtonian, and about half that of the similar Schmidt-Newtonian design. The corrector lens does introduce a small amount of chromatic aberration. There is some astigmatism, but the effect is lost in the coma. Field curvature exists although it is typically less than in a comparably-sized Cassegrain. As with most telescopes, distortion is negligible.