The act of making a beam of light parallel by a suitable arrangement of lenses or mirrors (a collimator), or of bringing the components of a system, particularly a telescope, into correct alignment. In high-energy astrophysics (X-ray and gamma-ray astronomy), collimation is often used to restrict the field of view of non-imaging instruments to improve the accuracy of source location and reduce source confusion; the collimator is then made of a material which is opaque to the radiation in question, such as glass or stainless steel.