1. A device capable of collimating radiation, as a long narrow tube in which strongly absorbing or reflecting walls permit only radiation traveling parallel to the tube axis to traverse the entire length.

1. an optical device that generates a parallel beam of light. often used to compensate for laser beam divergence.
2. A similar device that produces a parallel beam of particles such as neutrons.
3. A small telescope attached to a larger one, used to point it in the correct general direction

1. A telescope arranged and used to determine errors of collimation, both vertical and horizontal.
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.

1. A fixed telescope with a system of wires at its focus, and so arranged that another telescope can readily be brought into collimation with it, when an observer at the eyepiece of the latter can look into the objective of the former and see the cross-wires or slit in its focal plane. the intersection of the wires of the collimator is used as a standard point of reference.
2. the receiving telescope of a spectroscope, consisting of a slit through which the light enters, and a tube with a lens at its extremity which causes the rays to fall upon the prism or grating in parallel lines.

1. a small telescope attached to a large telescope to use in setting the line of the larger one
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