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The pulse height distribution for celestial sources and the UV\ calibration lamp have displayed a systematic change during the first 4 years of the mission: The peak and mean pulse height channel have shifted to lower values. Figure 5.19 shows the pulse height distribution of three HRI observations of N132D. Notice that the peak (and mean) in the pulse height distribution had decreased by 2 channels before the high voltage increase, a change comparable to the difference between the response of the HRI to the Al-K line (1.49 keV) and B-K line (0.18 keV) measured during the ground based tests.
We have used the in-orbit bright earth data to monitor the gain map, and determine whether the spatial structure observed during ground-based tests has varied during the mission. By composing gain maps of the bright earth data we find that the decrease in the HRI gain evident in Fig. 5.19 is uniform across the detector.