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High Voltage (HV) checks are done by checking the HV ON/OFF flag, and checking whether the HV value itself is nominal. These pieces of information are stored in housekeeping frames in a rather obscure way.
Due to some misinformation, SASS was checking the wrong HV field. Consequently the HV value indeed remained unchecked and data intervals appeared in the accepted time intervals where the HV was turned on, but the HV had not yet reached its nominal value, as the gain of the PSPC is a strong function of HV, this lead to wrong pulse height information for the affected times. Typically the time between HV getting turned ON and the HV\ reaching its nominal value is about 18s. This effect thus will be of no great impact for most observations, but it will affect observations with HV ON and OFF occurrences and short observation intervals. This effect may be seen in the quality file and in the housekeeping plots. Every time when HV is switched ON a time span of 20s should be cut out of the accepted times.
This bug has been fixed from SASS release SASS5_7_7 onwards (1992 June 15). From this date on the HV value shows a behaviour synchronized with the ``HV ok/not ok.'' status flag. Note that due to some peculiarity in the housekeeping frame notation the HV OFF value appears at slightly more than 2800V.
Also note that in some SASS hardcopy the HV monitor strip plot shows considerable jitter. This is the product of the HV monitor, and not an indication of true variation in HV across the observation. The monitor is only good as a crude indicator of HV.