A catadioptric telescope is a type of telescope that combines elements of both refractors and reflectors using both mirrors and lenses to gather light and bring the observed object into focus.  As with all telescope types, it can gather more light than the eye can unassisted and will allow greater detail to be observed. 

The main or primary mirror of a catadioptric telescope has a concave curved surface which is accurately matched to a corrector lens on the front aperture of the telescope tube.  The paired corrector lens and primary mirror virtually eliminate all of the errors associated with both refractors and reflectors giving a flat coma free field of view. 

The light enters the optical tube through the corrector lens and is reflected from the primary to a curved secondary mounted on the inside of the corrector lens, and brought to focus in the eyepiece through a hole in the centre of the primary.  The folded light path means that long focal lengths can be accommodated in a short optical tube.

The main types of catadioptric are listed below.

Schmidt cassegrain

Schmidt cassegrain telescopes use a primary with a spherical figure and a convex secondary mounted behind the corrector lens.  The corrector lens corrects for the spherical aberration caused by primary.  Focusing is achieved by moving the primary mirror up and down in the optical tube.

Maksutov cassegrain

Maksutov cassegrain telescopes use a primary with a spherical figure and a curved meniscus corrector lens.  The secondary consists of a reflective aluminised spot in the centre of the corrector lens.  Focusing is achieved by moving the primary mirror up and down in the optical tube.

Maksutov newtonian

Maksutov newtonian telescopes use the standard newtonian configuration with a maksutov corrector lens to virtually eliminate coma and spherical aberration. 

Klevtsov cassegrain

Klevtsov cassegrain telescopes use a primary with a spherical figure and a secondary with a spherical meniscus figure that is aluminised on the rear rather than the front.  There is another meniscus corrector lens in front of the secondary.