The Cassegrain Reflecting Telescope

A Cassegrain telescope uses mirrors to reflect light from a primary concave mirror to a reflecting secondary convex mirror, then back through a hole in the centre of the primary mirror to an eyepiece where it is brought to a focus.

The Cassegrain has the advantage over a Newtonian by having a large f-number (ratio of focal length to primary mirror diameter), allows much magnification, and the design means the Cassegrain reflector is more compact, allowing a more powerful telescope in the same length of tube.