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.
Light from an origin point O is collimated (made parallel) by a high quality objective lens.
The image of the target graticule is always seen in focus and at constant magnification in the eyepiece, regardless of the distance between the Autocollimator and the reflecting surface.
However, at long working distances only a portion of the reflected target graticule may appear in the eyepiece, owing to the failure of obliquely returning rays to enter the autocollimator.