Reduction of the PSPC Gas-Flow RateThe HRI was moved into the focus on June 10 for the last time in AO-3. The PSPC gas-flow rate was reduced the same day. The PSPC had time until June 15th (the beginning of AO-4) to settle down at the new equilibrium.
Immediately after the June 15 switch-on, an extended radioactive source calibration was started, which was followed by one orbit of filter-wheel-closed background measurements. This procedure was repeated the next day. During the first week after the switch-on, the radioactive calibrations were repeated once per day. Background measurements are taken once per week for one orbit.
The PSPC performance at the reduced flow rate looks very good. The gain dropped slightly after the switchover. The original estimated PSPC gas lifetime was until the end of November 1993, i.e. 5.5 months from the beginning of AO-4. Now the flow rate has been reduced by about 35percent , from 3.8 to 2.5 ml/min. This flow rate leaves a gas safety margin of 2.4 (+/-1 ) months in AO-4 if operations continue nominally.