STIS Performance after Servicing Mission 4

Charles R. Proffitt, proffitt@stsci.edu, Paul Goudfrooij, Michael A. Wolfe, Daniel Lennon, Wei Zheng, Ralph C. Bohlin, and Alessandra Aloisi

The Space Telescope Imaging Spectrograph (STIS) had been originally installed in the Hubble Space Telescope in February 1997. On May 16, 2001, the primary side-one package of support electronics failed, but STIS was able to continue operations using the redundant side-two electronics until August 3, 2004, when an electrical malfunction in a power supply stopped STIS operations.  On May 17, 2009, during the fourth EVA of Servicing Mission 4, astronauts Michael Good and Mike Massimino undertook an eight-hour spacewalk and replaced the STIS circuit board containing a failed component. This repair restored the STIS side-two electronics. All three of the STIS detectors are now being used for science observations.

In most respects, after the 2009 repair STIS operates in much the same way as it did prior to the 2004 failure. Most changes are close to what was expected. The degradation of the STIS charge-coupled device (CCD) by radiation damage and the modest changes in optical throughput are consistent with extrapolations of previously observed trends. The biggest surprise is in the dark count rate for the near-ultraviolet (NUV), multi-anode, micro-channel array (MAMA) detector. That dark rate is several times larger than had been expected, and is only slowly decreasing back towards its expected range.