Catadioptric telescopes use a combination of mirrors and lenses to fold the optics and form an image. There are two popular designs, the Schmidt-Cassegrain and the Maksutov-Cassegrain.
In the Schmidt-Cassegrain, light enters through a thin aspheric Schmidt correcting lens, then strikes the spherical primary mirror and is reflected back up the tube to be intercepted by a small secondary mirror. The mirror then reflects the light out the back of the instrument where the image is formed at the eyepiece.
ﾷ Most versatile type of telescope
ﾷ Best near focus capability of any type telescope
ﾷ First-rate for deep sky observing or astrophotography with fast film cameras or CCD’s
ﾷ Excellent for lunar, planetary and binary star observing plus terrestrial viewing and photography
ﾷ Closed tube design reduces image degrading air currents
ﾷ Compact and durable
ﾷ More expensive than reflectors of equal aperture
ﾷ Slight light loss due to secondary mirror obstruction compared to refractors
The Maksutov-Cassegrain telescope design has basically the same advantages and disadvantages as the Schmidt. It uses a thick meniscus-correcting lens with a strong curvature and a secondary mirror that is usually an aluminized spot on the corrector. The Maksutov secondary mirror is typically smaller than the Schmidt’s giving it slightly better resolution for planetary observing.
However, the Maksutov is heavier than the Schmidt and because of the thick correcting lens, it takes a long time to reach thermal stability at night in larger apertures. The Maksutov optical design typically is easier to make but requires more material for the corrector lens than the Schmidt Cassegrain.