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The CsI coated MCP is sensitive to ultraviolet radiation as well as soft X-rays, and must be protected from the background generated by the geocoronal UV and hot stellar UV continuum. In addition, since the MCP's are operated at high negative electrical potential with respect to the spacecraft ground, positive ions in the satellite environment must be blocked from reaching the detector. These functions are served by two aluminized plastic windows (the UV/Ion Filter and an Electrostatic Shield). These materials were selected to insure a safe level of attenuation of both the UV and ion background sources, while still providing high transmission of X-rays in the ROSAT band (0.1 to 2.4 keV). The geocoronal background rate is estimated to be less than 0.1 counts s over the total field of view (or less than counts arcmin s ).The star Car (AO III, V=1.7 mag, d=26 pc) yields a count rate of counts s . This detection of Car by the HRI during the on-orbit calibration phase, is assumed to be due to far UV flux from the star. additional calibrations are being planned to confirm this result.
The filter transmission in the ROSAT X-ray band and in the UV band are plotted in Figures 4.11 and 4.12. The features around 0.27 keV and 1.7 keV are due to carbon and aluminum absorption edges.