a small telescope attached to a large telescope to use in setting the line of the larger one
source: WordNet 3.0
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
source: WordNet 3.0
A telescope arranged and used to determine errors of collimation, both vertical and horizontal.
