Diagram of the ROSAT X-ray Telescope (the Wide Field Camera is not shown). X-rays would enter from the left and were focussed by the Wolter Type 1 mirror. The ROSAT mirror consisted of four nested parabola/hyperbola pairs. The double reflection is required for focusing in grazing-incidence optics of this type. The incident X-rays were focussed on the instruments carried by the focal plane assembly.
There were three scientific instruments, two Position Sensitive Proportional Counters (PSPC, redundant) and the High Resolution Imager (HRI, shown at the focal position). The star sensor is used for target aquisition and aspect determination. The Sun sensor is used for target aquisition as well.
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