= Autocollimation
\b[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.<Autocollimator Principles of Operation by Micro-Radian Instruments>[Principles of Operation]{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.<Measurements with autocollimators by TRIOPTICS>[Opening shared-path collimator/telescope description]{ref}
For a small mirror tilt $\theta$, reflection changes the return direction by $2\theta$. In the ideal paraxial model, return-image displacement is $x\approx2f\theta$, where $f$ is the effective <focal length>. An original example with $f=150\,\mathrm{mm}$ and $\theta=20\,\mathrm{\mu rad}$ gives $x\approx6\,\mathrm{\mu m}$. 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.
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