Autocollimator

Okay kiddo, an autocollimator is like a fancy ruler for measuring how straight things are. It has a tiny light inside that shines on a mirror and bounces back to the autocollimator. If the light beam goes straight back without bouncing off to the side, that means the thing you're measuring (like a piece of metal or a machine part) is straight! But if the beam bounces off to the side, that means it's not straight and needs some fixing up. So basically, an autocollimator helps people make sure things are perfectly straight and in the right place. Cool, huh?