collimator

Collimate means to align and a collimator is a device that takes light traveling in different directions and makes it all travel in parallel. (The term is also used for a much different instrument that does a similar process for something other than light, e.g., particles.) Collimated light is light traveling in parallel beams rather than spreading or converging, an example being the output of a laser. Such collimation is imperfect and can only be approximated, much as optical images always have limits on their accuracy. Light from distant objects is close to being collimated and the term collimator is used for devices achieving such near-collimation to make objects appear distant, such as in a flight simulator. Light from astronomical point sources, such as stars, is closer to true collimation than the output of any collimator.

Collimating light that is diverging from a point can be carried out with a concave mirror (or convex lens) if the curve is just less than enough to focus the light to another point, i.e., at the border between focusing and merely reducing the divergence. A decollimator does the opposite, takes the parallel beams and, e.g., focuses them to a point.

Collimating is used in astronomy optics, such as spectroscopes, which may collimate after the initial dispersal (after the first prism or grating) so the separated wavelength's are "traveling in parallel", to allow physical placement of an additional dispersing device further along the optical path, i.e., to assist in the physical design of the instrument.

The word collimate is also commonly used in astronomy in another sense, for the process of aligning the optics in a telescope, e.g., aligning the mirrors and/or lenses so as to produce the best possible image.