: the process of collimating an instrument (such as a telescope) having objective and cross hairs by directing it toward a plane mirror and adjusting the lens and cross hairs so that the latter coincide with their own reflected image.

What is the Autocollimation method?

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. ... Light from an origin point O is collimated (made parallel) by a high quality objective lens.

How do you measure focal length in Autocollimation?

By sliding the lens-mirror combination back and forth along the axis a sharp image of the bright spot is obtained on the white card. The distance between the card (object-image screen) and the lens is the focal length.