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The HRI uses 36 mm diameter MCPs. When they are assembled in their holding fixture the open aperture is 30 mm. The readout system has a working area of 27 mm 27 mm so that the net useful field of view is given by the intersection of the circular MCP and square readout. This is shown in Figure 4.9 . The CsI photocathode used in the HRI is evaporated in the pattern shown in Figure 4.9 , so that there are four small ``tabs'' of uncoated MCP area which are used to monitor the stability of the photocathode.