The sensitivity of astronomical observations in the soft X-ray band has been increased dramatically by the use of imaging optics and position sensitive focal plane detectors. The tremendous sensitivity increase of imaging telescopes compared with detectors with mechanical collimators has two reasons. The effective collecting area of the telescope is determined by the sensitive area of the mirror system and not any more by the area of the detector. For ROSAT (ROentgen SATellit) [23] the sensitive detector area is a factor of ten smaller than the effective collecting area of the mirror. Because of the imaging capability of the instrument, the relevant background for the detection of a point source is limited to an angular resolution element of the mirror-detector system (compare NXBxRE in Tables 4.1 and 4.2). For these geometrical reasons, the background noise of an imaging system is several orders of magnitude less for the detection of a point source compared with a collimated detector with the same sensitive area. Three astronomy satellites with imaging X-ray telescopes had position sensitive proportional counters. The Einstein satellite [8] and ROSAT used multiwire proportional counters, while EXOSAT (European X-ray Observatory SATellite) [22] had a proportional counter with parallel plate geometry. JEM-X [11], the X- ray monitor aboard INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory) [25] has a position sensitive micro strip gas chamber in a coded aperture mask telescope.

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