STIS Post-Safing Updates
On October 5, 2018, the Hubble Space Telescope (HST) experienced a failure of Gyro-2, and normal HST operations were suspended. Gyro-2 had been experiencing elevated jitter rates and increased failed acquisitions for the past year. After the failure, the team at NASA Goddard powered on Gyro-3 (the last of the backup gyros). Upon start-up, however, it was found that the Gyro-3 rate bias was significantly higher than normal and higher, even, than operational levels. Because it was not performing at the required level for operations, Gyro-3 was reset and exercised in an attempt to restore it to its operational limits. HST remained in a suspended state until October 27, 2018 when the Gyro-3 bias drift rate was finally brought within operational levels, though still larger than expected (see Figure 1). The telescope jitter improved from before Gyro-2 failed, and over the past 4 months, the Gyro-3 bias drift rate has slowly decreased. All three of the gyros now in use are enhanced models, with a longer expected life than the three that have failed since SM4.
Figure 1: The RMS jitter in the STIS MAMA and CCD since SM4. The y-axes show the RMS jitter in the V2 (top panel) and V3 (bottom panel) frames. The purple points show that, although the jitter after switching to Gyro-3 is high, it’s still an improvement from the previous configuration.
After the almost month-long suspension, the STIS team monitored the first science and calibration data taken for all detectors very closely. While the CCD bias and the dark levels for both the CCD and the FUV-MAMA were nominal and performing as expected, the NUV-MAMA detector dark rate was initially ~2 times higher than normal, and took about one month to settle back into the normal regime. Since then, the NUV-MAMA dark rate has remained at this nominal level (see Figure 2). When STIS was last not in operation for an extended period of time in 2010, the elevated dark current took a few months to return back to normal levels, so this behavior was not unexpected.
Figure 2: The NUV-MAMA count rate from July 1, 2018 through the present. The greyed-out section is when HST was in safe mode.
While all of the STIS detectors have returned back to nominal values, we are still seeing some effects from the new Gyro-3. Although the Gyro-3 bias is continuously decreasing, it is still high, and can occasionally cause the loss of science observations. Users should always check their data promptly to make sure their data have not been adversely affected by any unforeseen instrumental effects.