Catadioptiric (meaning ‘combining two optics’) telescopes combine mirrors and lenses in the main optical assembly, allowing the designers to create optical systems with fast f-ratios and very little coma or astigmatism.
Catadioptric designs have the following advantages:
- When used in a prime focus configuration they can have very wide coma free field of view (e.g. Schmidt camera).
- When used in a Cassegrain configuration it results in a long focal length instrument that is “folded” into a much smaller package.
- They employ spherical surfaces that are easier to manufacture.
- Catadioptric designs are low maintenance and rugged since some or all of their elements are fixed in alignment (collimation).
- Combining a moving primary mirror with a cassegrain configuration allow for large movements in the focal plane to accommodate cameras and CCDs.
- The corrector plates seal the tube assembly from dust and dirt. They also block air currents from the interior of the tube, thereby increasing image stability.
Catadioptric designs have the following disadvantages:
- The secondary structure blocks a portion of the light entering the tube.
- The secondary obstruction causes some image degradation due to introduced diffraction effects .
The two most common modern catadioptric configurations in telescopes are the Schmidt-Cassegrain and Maksutov-Cassegrain. Both of these designs are derivatives of the Schmidt Camera design.
A Schmidt camera, also referred to as the Schmidt telescope, is an astronomical camera designed to provide wide fields of view with limited aberrations. Other similar designs are the Wright Camera and Lurie-Houghton telescope.
The Schmidt camera was invented by Estonian optician Bernhard Schmidt in 1930 . Its optical components are an easy-to-make spherical primary mirror, and an aspherical correcting lens, known as a corrector plate, which is located at the center of curvature of the primary mirror. The film or other detector is placed inside the camera, at the prime focus. The design is noted for allowing very fast focal ratios, while controlling coma and astigmatism.
Schmidt cameras have very strongly curved focal planes, thus requiring that the film, plate, or other detector be correspondingly curved. In some cases the detector is made curved; in others flat media is mechanically conformed to the shape of the focal plane through the use of retaining clips or bolts, or by the application of a vacuum.
The Schmidt Camera was operated by placing the detector, film or plate in the holder, and then uncovering the objective aperture for a given amount of time to expose the detector, film or plate.
The Schmidt-Cassegrain is a design based on the Schmidt camera. An early Schmidt-Cassegrain was patented in 1946 by artist/architect/physicist Roger Hayward. As in the Schmidt camera this design uses a spherical primary mirror and a Schmidt corrector plate to correct for spherical aberration. From the Cassegrain, it inherits the convex secondary mirror, perforated primary mirror, and a final focal plane located behind the primary. Some designs include additional optical elements (such as field flatteners) near the focal plane.
The Maksutov-Cassegrain was derived from the Maksutov-Bouwers telescope, which was invented independently in 1940 (Bouwers) and 1941 (Maksutov). Both of these designs were modifications of the Schmidt Camera design, and did not use a secondary mirror. Instead the media (film, plate, etc) was placed at the prime focus. However, 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.
In 1957 John Gregory, a designer for Perkin-Elmer, developed a new meniscus design known as the Maksutov-Cassegrain, based on Maksutov’s idea. Gregory later published his design for two f/15 and f/23 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 (called either a “Gregory-Maksutov” or “Spot-Maksutov”) that may use all spherical surfaces and has, as the 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