The standard way of imaging in astronomy is by making use of refraction and reflection in employing lenses and mirrors. In the X-ray range, traditional glass lenses cannot be used because the X-rays are absorbed. Reflection under small incidence angles is possible at low (<20keV) X-ray energies and imaging is realized today through Wolter telescopes, which achieve imaging through double reflection on parabolic and hyperbolic surfaces. Developments are underway to extend the useful energy range to several tens of kiloelectronvolt by multilayer coated reflecting surfaces. The first detector systems for X-ray astronomy were just flat X-ray sensitive detectors (gas proportional counters or anorganic scintillation counters). They could hardly be called telescopes. In placing a mechanical, X-ray absorbing collimator in front of a flat X-ray detector (e.g., a "slat collimator" consisting of parallel metallic plates) the field of view (FOV) is restricted and an indirect imaging capability is achieved: when such a system scans the sky (in the direction perpendicular to the orientation of the plates) the flux of any existing X-ray source will be modulated according to the triangular collimator response function. This is the basic principle of imaging through aperture modulation. Aperture modulation telescopes ("shadow cameras") for high energy X-ray and gamma-ray astronomy make use of temporal and/or spatial aperture modulation.
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