noun. optical device consisting of a tube containing a convex achromatic lens at one end and a slit at the other with the slit at the focus of the lens; light rays leave the slit as a parallel beam.
noun. a small telescope attached to a large telescope to use in setting the line of the larger one.
1. Noun, singular or mass Ghilani and Wolf claim that a key feature of a theodolite is its collimator -- a device that narrows a beam of light.