1.  collimator(n = noun.artifact) - a small telescope attached to a large telescope to use in setting the line of the larger one;
is a kind of scope, telescope
Derived form verb collimate₂

2.  collimator(n = noun.artifact) - 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;
is a kind of optical device
is a part of prism spectroscope, spectroscope

1.  A telescope arranged and used to determine errors of collimation, both vertical and horizontal. Nichol. [1913 Webster]

2.  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]

collimator, n.
1 a device for producing a parallel beam of rays or radiation.
2 a small fixed telescope used for adjusting the line of sight of an astronomical telescope, etc.