Types of telescopes

Newtonians are a type of reflecting telescope. A concave parabolic primary mirror at the back of the telescope collects and focuses incoming light onto a flat secondary (diagonal) mirror that in turn reflects the image out of an opening at the side of the front end of the main tube and into the eyepiece. For large f/ratio Newtonians, the primary mirror often has a spherical shape.

There is a big lens on one end and a small lens or eyepiece on the other end. The light comes in through the big lens and travels the length of the tube down to the little lens where you look in.

A telescope that uses a combination of mirrors and lenses to increase the effective focal length of the telescope while allowing it to be folded into a more convenient and compact size. The use of a full-aperture correcting lens in these scopes virtually eliminates spherical aberration, chromatic aberration, and coma. The word catadioptric is derived by combining the term for an optical system that forms images by using mirrors (catoptric) with the one for a system that uses lenses (dioptric). The most popular catadioptric designs are the Schmidt-Cassegrain and Maksutov-Cassegrain.