An optical technique where a light source is reflected back onto itself using a mirror or reflective surface to precisely measure angles or alignment.
From auto- (self) + collimation (from Latin collineare, to align in a straight line). The term emerged in 19th-century optics when scientists needed ways to verify instrument precision without external references.
This is the optical equivalent of yelling into a canyon and listening to your echo—telescopes and surveying instruments use autocollimation to essentially check themselves, making it one of the most elegant self-verifying measurement techniques ever invented.
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