Radiation damage effects on the CCD detector of the space telescope imaging spectrograph

The CCD detector in the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope is a backside-illuminated, 1024 x 1024 pixel device developed by Scientific Imaging Technologies for the STIS program. It supports a wide-ranging observing program of imaging and spectroscopy over the 200-1000 nm wavelength range at the high angular resolution of HST. In the three years since launch, radiation damage to the CCD (primarily due to high energy protons encountered in crossings of the South Atlantic Anomaly) has caused a degradation in the charge transfer efficiency (CTE) and an increase in the dark current. The growth in "hot pixel" count has been readily observable over the entire period since launch; the CTE degradation has become apparent more slowly, as it has evolved with time and as in-flight calibration test methods have improved. We report here on the calibration tests developed to measure the CTE performance in flight, on the results of those tests, on the monitoring of the CCD dark current, and on potential amelioration strategies for minimizing the scientific impact of the decline in STIS CCD performance.