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. 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. One special application is to determine the focal length of a diverging lens: A light source is placed at twice the focal length of a converging lens on one side and a screen at the same distance on the other side so that the image of the light source is the sharpest possible. When this is achieved, the screen is replaced with a mirror and the diverging lens is inserted between the converging lens and the mirror at such a distance to the mirror that the light returning through the diverging and converging lenses produces a sharp image on top of the luminous object. This is the case when the beam hitting the mirror is collimated. The distance found is the (negative) focal length of the diverging lens.