Collimators are optical lens assemblies that take divergent or convergent incoming light rays and produces parallel light output. There are many types of collimators. Examples include laser collimators, x-ray collimators, and infrared radiometer collimators. Laser collimators use the principle of auto-collimation. From the center of the focuser, laser collimators send a beam to the diagonal, on to the primary, back to the diagonal then back to the focuser. When collimated, the beam hits the center of both mirrors and returns directly to its source. X-ray collimators are devices used to filter a stream of rays so that only those traveling parallel to each other in a certain direction are allowed through. Infrared radiometer collimators are the devices that filter infrared rays. Other collimators are commonly available.

There are several ways in which collimators function. Collimator function is based on spot size, insertion loss, and working distance. Collimation refers to the adjustment of the position and orientation of optical elements within a telescope and other optical instrument to achieve optimum performance. Barlowed collimation is a technique developed for adjusting the angular alignment of the Newtonian primary mirror. Barlowed collimation projects a shadow of the mark on the center of the primary up to the face of the collimator. The primary is adjusted by centering the mark's shadow on the face of the collimator. Barlowed collimation is a type of laser collimation. Proper collimators for telescopes are extremely important for high quality images. When collimated, the image formed on the axis of the primary mirror is in the center of the focuser. Laser collimators help to achieve this effect quickly by helping in adjusting the angles of the secondary then the primary mirror. Proper collimation of a laser source with long enough coherence length can be verified with a shearing interferometer. Collimation process, when performed with accurate tools and correct technique, enables optimum performance of a telescope. Collimators are specified according to size and pitch. Collimators are built from tungsten, stainless steel and ceramics. Collimators are designed and manufactured according to various industry specifications.

Collimators are used in many applications. Examples include in the medical industry for high resolution cameras that are utilized in imaging the brain and other organs, military industry, and nuclear power stations for monitoring radioactivity. Collimators should adhere to Digital Imaging and Communications in Medicine (DICOM) standards according to their applications in the medical field.