coded-mask telescopeA telescope system that enables high-quality sky images to be made using nonfocusing optics; it is a development of the simple pinhole camera. The technique is particularly applicable at gamma-ray wavelengths and greatly improves the accuracy with which cosmic γ-ray sources can be located within every point in the telescope's field of view, typically 10° square. The system consists of a crosswordlike mask of γ-ray opaque lead or tungsten elements positioned over, but separated from, a flat γ-ray detector. The detector is capable of measuring the point at which a γ-ray photon from a cosmic source makes contact, as well as its energy. A patterned shadow of the mask will be cast on the detector plane by photons not reaching the detector. The location of the γ-ray source can be determined uniquely from the position of the shadow and the known mask-detector separation.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006