Autocollimation

Autocollimation is an optical return-path method: a source pattern is projected through a collimator, reflected back from an external surface, and examined through the same optical system. It is the process used by an autocollimator, rather than the instrument itself. A plane mirror normal to the outgoing collimated light returns the light along the reference direction.^[ref]

A typical apparatus uses an illuminated reticle near a focal plane, an objective and a beam splitter that separates the illumination and viewing paths. The outward projection and the return image are compared, making small changes in reflector orientation observable.^[ref]

For a small mirror tilt $\theta$, reflection changes the return direction by $2\theta$. In the ideal paraxial model, return-image displacement is $x\approx 2f\theta$, where $f$ is the effective focal length. An original example with $f=150\mathrm{mm}$ and $\theta =20\mu \mathrm{rad}$ gives $x\approx 6\mu \mathrm{m}$. This is displacement in the image plane, not a direct measurement of mirror translation.

The angular interpretation explains why simply moving an ideal plane mirror farther away does not multiply the focal-plane displacement by that distance. In practice, finite aperture, beam walk, alignment and diffraction limit the usable return. Measurement calibration and a stable reference remain necessary to distinguish a genuine angular change from movement of the optical system.