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Also in the focal plane of the XMA is the High Resolution Imager. The HRI is a two-dimensional position sensitive detector based on microchannel plates (MCPs) that detects single X-ray photons and determines their position and time of arrival.
The ROSAT HRI is practically identical to that of the Einstein Observatory HRI. The only significant change that has implications for the performance of the detector has been the substitution of cesium iodide for magnesium fluoride as the detector photocathode. This has resulted in a considerable increase in the quantum efficiency of the HRI. As in the case of the Einstein instrument the ROSAT HRI has no intrinsic energy resolution; its band pass (in conjunction with the XMA) is defined by the UV/ion filter at low energies, and by the decrease in mirror reflectivity at high energies.