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The 36mm-diameter HRI MCPs have a holding-fixture open aperture diameter of 30mm, and the readout system has a working area of 27mm x 27mm. Hence the net useful field of view is given by the intersection of the circular MCP and square readout, as shown in Figure 4.9 of Appendix F. 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.