Maksutov telescope

Dmitri Maksutov may have been working with the idea of pairing a spherical primary mirror in conjunction with a negative meniscus lens as far back as 1936.

Maksutov's design notes from 1941 explored the possibility of a 'folded' Cassegrain-type construction with a secondary silvered "spot" on the convex side of the meniscus facing the primary mirror.

Most Maksutovs manufactured today are this type of 'Cassegrain' design (called either a "Gregory–Maksutov"[13] or "Spot-Maksutov") that use all-spherical surfaces and have, as secondary, a small aluminized spot on the inner face of the corrector.

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.

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)[16] 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.

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.

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.

[21] 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).