'Collimator' definitions:

Definition of 'collimator'

(from WordNet)
noun
A small telescope attached to a large telescope to use in setting the line of the larger one
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

Definition of 'Collimator'

From: GCIDE
  • Collimator \Col"li*ma`tor\, n.
  • 1. (Astron.) A telescope arranged and used to determine errors of collimation, both vertical and horizontal. --Nichol. [1913 Webster]
  • 2. (Optics) A tube having a convex lens at one end and at the other a small opening or slit which is at the principal focus of the lens, used for producing a beam of parallel rays; also, a lens so used. [1913 Webster]