A
secondary mirror (or
secondary) is the second deflecting or focusing mirror element in a
reflecting telescope. Light gathered by the
primary mirror is directed towards a
focal point typically past the location of the secondary. Secondary mirrors in the form of an optically flat
diagonal mirror are used to re-direct the light path in designs such as
Newtonian reflectors. They are also used to re-direct and extend the light path and modify the final image in designs such as
Cassegrain reflectors.