The utilization of X-ray mirrors for extra-solar X-ray astronomy had to await the development of electronic detectors with both high quantum efficiency and the ability to determine the location of the arrival of an X-ray photon in two dimensions. The first such detectors were the Imaging Proportional Counter and the microchannel plate detector. Subsequently, more sensitive detectors including CCD spectrometers and imaging gas scintillation proportional counters have been employed.
As was true with the solar observations, the initial observations employing X-ray imaging systems utilized sounding rockets. The first successful X-ray image of an extra-solar object was obtained by Paul Gorenstein of the Smithsonian Astrophysical Observatory and collaborators in 1975. They used a Kirkpatrick-Baez mirror coupled with an imaging proportional counter to obtain an image of the Virgo cluster of galaxies (and subsequently two other clusters). The inaugural use of Wolter optics for extra-solar astronomy was performed by Saul Rappaport of MIT and his collaborators in 1977. Over the course of two rocket flights, they obtained the first true images of supernova remnants. Figure 1 shows their image of the Cygnus Loop supernova remnant.
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