A pure cassegrain telescope uses a parabolic primary mirror, just like a Newtonian telescope, but instead of a flat diagonal mirror, the cassegrain has a hyperbolic convex mirror that reflects the light back through a hole in the primary mirror. Cassegrains typical have a very long focal length, which results in high magnification. Cassegrains are popular with some planetary observers.
These telescopes have a hole in the main mirror. After the light has struck the main mirror it hits a second mirror, towards the aperture. Then the light goes through the hole in the main mirror and then into the eyepiece. The Cassegrain set-up can be combined with a Schmidt lens to make a Schmidt-Cassegrain. See also Newtonian telescope.
In its classical form, a compound reflecting telescope employing a parabolic concave primary mirror and a small hyperbolic convex secondary mirror to form images. Variations of the Cassegrain design include the Maksutov-Cassegrain and the Schmidt-Cassegrain.