It was found from the ground based flat-field measurements that the gain of the HRI varies spatially across the detector. Figure 19 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 similar at all photon energies. In Figure 20 we show the computed average channel for HZ43 and AR Lac 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 Figure 19; available via anonftp: sao-ftp.harvard.edu (cd pub/rosat/calfiles) 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 (a) 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); (b) 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 (c) observations taken over intervals of more than a few days will often have a range of roll angles (Section 2.5.3).