The first X-ray instruments for astronomical purposes were equipped with colli-mators and large area detectors rather than with imaging capabilities. First X-ray mirrors designed as Wolter optics were flown in 1963 on a sounding rocket for solar imaging. The first long term X-ray observations were done with two small Wolter telescopes for solar imaging operated on the space station Skylab from 1973 to 1975. The era of satellite-based Wolter telescopes begun in 1978 with HEAO-2, which was then renamed to Einstein Observatory. Currently, two major X-ray observatories with Wolter telescopes are operating: Chandra and XMM-Newton. They complement each other because Chandra has an unprecedented angular resolution of better than 1" whereas XMM-Newton by far has the largest collecting area. Since 2005 they are joined by Suzaku with five highly nested foil telescopes (Table 6.1). Missions with much larger collecting areas like Constellation-X and XEUS are planned for the second decade of the twenty-first century.
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