Autocollimation

Under Autocollimation one understands an optical structure in which a collimated beam of light leaves an optical system and is reflected by a plane mirror back into the system.

One application is the autocollimator for the precise measurement of small angles and the Littrow spectrometer.

Autocollimation with the determination of the focal length of lenses or lens systems, is possible. In the case of a convergent lens while an object (or a hole aperture) is initially positioned approximately at the focal point of the system, and a plane mirror (in any ratio) on the other side of the lens. Now the object is moved until the image of the object, which is generated by the lens and mirror, sharp in the same plane as the object is located (see drawing).

In the case of a thick lens or lens system, the focal length can be determined from the ratio of the semi- diameter bundle between optics and plane mirror to the tangent of half the angular aperture. To this end, the divergent beam through an aperture must be limited so that its rays pass completely through the optics. The result can be the location of one of the two principal planes determined.

Autocollimation is also suitable for the measurement of aberrations not too great optics (large precision flat mirror does not exist ).