1. A small telescope attached to a large telescope to use in setting the line of the larger one
   "The astronomer aligned the collimator before using the main telescope"
    
2. 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
   "The scientist used a collimator to create a parallel beam of light for the experiment"

Derived forms: collimators

Type of: optical device, scope [informal], telescope

Part of: prism spectroscope, spectroscope

Encyclopedia: Collimator