Cassegrain configuration(kass -ĕ-grayn) One of the configurations in which a large reflecting telescope can be used. A small secondary mirror with a convex hyperboloid shape intercepts the beam of radiation before it reaches the prime focus of the primary mirror (see illustration). The secondary reflects the beam back down the tube, through a central hole in the primary mirror, to a focal point just behind it. This is the Cassegrain focus. It is more accessible than the prime focus and is incorporated into many reflecting telescopes, notably the Schmidt telescope and the Maksutov telescope.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006