Autocollimation

Autocollimation is an optical setup where a collimated beam (of parallel light rays) leaves an optical system and is reflected back into the same system by a plane mirror.

It is used for measuring small tilting angles of the mirror, see autocollimator, or for testing the quality of the optical system or of a part of it. Large-aperture optics, however, are tested with a null corrector avoiding the production of a large plane mirror.