Friday 1 June 2012 10:30am EDT
During the last week Chandra's observing schedule was interrupted by a spacecraft safemode event that occurred at 11:33pm EDT on May 28. The safemode transition was identified at the first contact after the event, which started at 8:00am EDT on May 29. The safemode transition was nominal and all systems functioned as expected.
The cause of the safemode was a trip of the sun-position monitor. Telemetry showed that the fine-sun-sensor (FSS) began to produce erratic data as the spacecraft pitch-angle neared the edge of the FSS field-of-view (FOV). Analysis of the FSS data and the monitor logic indicates that both a region check and a comparison check with the coarse-sun-sensor data failed, which led to the trip. The observed behavior is consistent with expected FSS operation near the boundary of the FOV. There were no hardware failures. The possibility of a slight variation in the FSS FOV, which is being investigated and mission planning processes have been updated to increase the margin from the edge of the FOV. Since the safemode was not initiated by a hardware fault, the Chandra Operations team completed the safemode recovery procedure to the prime hardware at 9:30am EDT on May 30. The team completed the clean-up activities, transition to attitude control by pointing at stars, a fine attitude update required to resume science operations following the safemode, and a maneuver to an attitude to prepare for observing at 11:20pm on May 30. A new mission schedule and loads were approved and began operating at 3:50pm on May 31 with 193.0ks of science loss. The new loads included an observation of NGC 4088 ULX 1, which was accepted as a Director's Discretionary Time TOO on May 25. Scheduled observations of PLCKG266.6-27.3, ALPHA CEN, G21.5-09, Ruprecht 147, GJ 191, and G313.87-17.10 were impacted by the safemode and will be rescheduled.
A flight software patch was uplinked on May 30 to modify the RadMon process to use a flag generated by ACIS as a new channel used to monitor for high radiation. The step was taken to counter degradation in the EPHIN radiation detector due to increasing temperature trends through the mission. Real-time procedures were executed on may 31 to dump OBC-A memory as a follow-up to this patch and patching done during the safemode recovery. The dump will be used to update the baseline memory image maintained on the ground.
Of note this week was the release on May 30 of CALDB 4.4.10. This update includes ACIS time-dependent gain corrections for the period from February 1, 2012 through April 30, 2012 and an upgraded ACIS Contamination model file. For details see: http://cxc.harvard.edu/caldb/downloads/Release_notes/CALDB_v4.4.10.html
The schedule of targets for the next week is shown below and includes an observation of 4U 1630-47, which was accepted as a Director's Discretionary Time TOO on May 24, an observation of MAXI J0556-332, which was accepted as a Target of Opportunity on May 8, an observation of RXJ1622.7-2325_P1 coordinated with OVRO/CARMA, and an observation of M31* coordinated with HST and the EVLA.
All spacecraft subsystems continued to support nominal operations.
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