- a small telescope with cross hairs at its focus, fixed to another telescope, surveying instrument, etc. for adjusting the line of sight
- a device that causes a beam of radiation, as light, to move as parallel rays
See collimator in American Heritage Dictionary 4
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.
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