STIS Calibration Status

Last year’s failure of the STIS Side-1 electronics temporarily suspended use of the instrument. The Side-1 electronics are not repairable, but operations were resumed in August of 2001 using the redundant Side-2 electronics. STIS was fully returned to operation, with only minimal impacts on scientific performance. MAMA detector performance continues to be very good, with sensitivity changes of 1 to 2 percent per year. Although the detailed relation between the NUV MAMA detector temperature and dark current has changed, typical NUV dark current levels are similar to those in previous cycles. The FUV dark current varies irregularly, and it is now usually significantly higher than it had been during the first two years of STIS operations. The effects of radiation damage on the STIS CCD detector continue to follow previous trends, with declining charge transfer efficiency, increasing dark current, and increasing numbers of hot pixels. We also review the use and calibration of the E1 aperture positions which can be used to ameliorate CTE effects. 1. Side 1 Electronics Failure A fuse on the main STIS power bus blew on May 16, 2001, safing the STIS instrument. A diagnostic test that was intended to repower the primary Side-1 electronics in stages by using an alternate power bus, resulted in another blown fuse as soon as the first STIS internal relay was closed. After review of the available telemetry and detailed engineering analyses, the failure review board (Davis et al. 2001) identified a number of possible causes, but concluded that the most likely cause was a shorted tantalum capacitor. There is essentially no chance that this type of capacitor could be melted open once shorted, and on-orbit repair appears to be impractically complex. It was concluded that no portion of the Side-1 electronics can be recovered. Fortunately, STIS has a redundant set of electronics (Side-2), which was successfully used to reactivate STIS in early July 2001. The MAMA detectors and most instrument mechanisms perform much as they did on Side 1. However, because the Side-2 electronics lack a functioning temperature sensor for the STIS CCD detector, the CCD can no longer be operated at constant temperature. Instead, the thermo-electric cooler is operated at constant current, and while the mean detector temperature is actually lower than the −83C Side-1 set point, both the CCD temperature and dark current vary significantly (Brown 2001a). The STIS CCD also suffers from a ≈1 e−/pixel increase in read noise due to electronic pickup from the Side-2 electronics. This noise can under some circumstances be Science Programs, Computer Sciences Corporation Catholic University of America Institute for Astrophysics and Computational Science