A catadioptric telescope, also known as a ‘compound’ telescope, uses a combination of mirrors and lenses to increase the scope’s focal range and power while keeping weight, length and bulkiness to a minimum.
The most popular type of catadioptric telescope is the Schmidt-Cassegrain, used by amateur astronomers the world over. Based on Bernhard Schmidt’s original camera lens design, scopes with this configuration feature a corrector plate as the first optical element, which is then figured by placing a vacuum on the internal side, and grinding the lenses and mirrors to the exact specification and angle required to correct the spherical aberration caused by the primary mirror. Many variations of the Schmidt-Cassegrain configuration exist, but in general they can be divided into two principal design forms: compact and non-compact.
In the compact form, the corrector plate is located at or near the focus of the primary mirror. In the non-compact version, however, the corrector plate is moved back towards the center of the primary mirror. This has the effect of keeping the corrector at the center of curvature of the primary mirror.
Whichever arrangement is used, the layout of lenses and mirrors in a Schmidt-Cassegrain telescope allows for a considerable focal length to be combined with a man-portable, robust and affordable telescope design, all of which contribute to their popularity.
The Schmidt-Cassegrain’s ease of manufacture makes them very popular with budget or entry-level telescope manufacturers, making them readily available for amateurs looking to progress from, say, a Maksutov-Cassegrain. Schmidt-Cassegrain telescopes are also particularly useful for astronomical photography; their low weight makes them ideal for mounting on motorized tracking tripods, and as a result many Schmidt-Cassegrain mountings feature electronic tracking and drive controls to help amateurs take advantage of this ability. Disadvantages to the Schmidt-Cassegrain configuration include the inherent difficulty in repairing a vacuum-based telescope and a slight loss of focus and image sharpness when compared to refractor telescopes.
The other major style of catadioptric telescope – also popular among amateurs – is the Maksutov-Cassegrain configuration. Built around a full-diameter meniscus lens corrector plate and spherical primary and secondary mirrors, these telescopes are great for planetary imaging due to their high focus ratios. However, Maksutov-Cassegrains suffer when used for wide-field astrophotography.
The Maksutov-Cassegrain is a solid, rugged telescope configuration, and its fixed parts make it very reliable. Since the 1970s advances in manufacturing have made Maksutov telescopes increasingly affordable, and many new entrants in to the astronomy hobby cut their teeth on Maksutov telescopes.