an optical device used to produce beams of parallel rays. A collimator consists of an objective lens or concave mirror in whose focal plane the illuminated object is placed. The opening of an opaque stop, such as a narrow slit of constant or adjustable width, is most commonly used for this purpose. The relative positions of the lens and the object are set by attaching them inside the body of the instrument, which is usually tubular. The blackened inner walls of the instrument’s body absorb rays whose direction does not coincide with the direction desired.
The parallel nature of the beam emerging from a collimator is approximate. Rays emanating from one point of the object cannot be exactly mutually parallel because of diffraction and aberrations of the lens. The finiteness of the dimensions of the object results in the spreading of beams coming from various points on the object. The focal distance, the focal aperture, and the quality of correction of aberrations of the lens, as well as the shape and dimensions of the object, are selected according to the purpose of the collimator and the conditions of its use.
Collimators are used in astronomy to align large measuring instruments and determine their collimation error, in spectral instruments to produce light beams that are directed into a dispersing system, and in various measuring, testing, and opticomechanical instruments used for alignment. They are part of autocollimating devices.