The variations in gain can be seen from ground-based and in-flight calibration observations. The change in gain across the detector surface is shown in Figure 17 of the HRI Calibration Report (David et al. 1995). This map was produced from flat-field observations from six different energies spanning 0.18 to 1.74 keV. The temporal gain variation is monitored from on-axis monthly observations of the supernova remnant N132D (Figure 1; http://hea-www.harvard.edu/rosat/hri_status.html ). To a first-order approximation, the mean pulse height channel decreases 0.5 channels/year. To compensate for this decline, the gain of the instrument was increased in June of 1994. This restored the pulse height distribution to the position in channel space as seen right after launch. Further changes in the High Voltage level were implemented in March and May 1997.
Figure 1: Temporal gain monitoring of the ROSAT HRI using monthly on-axis observations of N132D. The discontinuities in the graph show the high voltage changes in June of 1994, April of 1997 and December of 1997.