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Spatial Gain Variations

  

It was also found from the ground based flat-field measurements that the gain of the HRI varies spatially across the detector. Figure 5.17 is a contour map of the mean pulse height channel for the Cu-K (0.93 keV) flat field data averaged over a spatial scale of about 1'. The mean pulse height channel increases from 4 near the center of the detector to approximately 7 near the edge. However, within 5' of the optical axis, the gain only varies by 10%. The spatial variation in the gain is also similar at all photon energies. In Fig. 5.18 we show the computed average channel for HZ 43 and N132D plotted against the expected value at that position on the detector from the Cu gain map.

The RSDC at SAO has produced fits versions of the gain maps such as that shown in Fig. 5.17   (available via anonymous ftp: sao-ftp.harvard.edu and on legacy). These maps have a scale of 8''/pix and are thus useful (when rolled to match an observation) for determining the relative gains of features. Users should be mindful that

• The gain maps were made from ground calibrations before launch, and therefore do not represent accurate values for any given date during the mission (see the next section);
• the normal satellite's wobble will mean that any X-ray feature will actually experience the gains  from somewhat different areas of the detector; and
• observations taken over intervals of more than a few days will have a range of roll angles.