Several calibration programs are executed every cycle to monitor the behavior of the STIS detectors.

For the CCD, the housing temperature, read noise and hot pixel annealing are monitored on a monthly basis.

For the MAMAs, dark rate observations are taken every week.

The quality of the initial pointing of HST is monitored to search for a degradation in the STIS to FGS alignment. All target acquisitions are examined to determine the slew required to center the target in the STIS slit. These slews represent the error in the target coordinates, the error in the guide star positions, and the error in the STIS-to-FGS alignment.

The Fine Guidance Sensors undergo alignment shifts which are generally largest following installation, then decline with time. FGS realignments were performed in Dec 1997 for FGS3, in October 1998 and October 2002 for FGS1r, and in March 2000 and October 2002 for FGS3r. The plots below show the total errors in initial STIS pointings for monthly or bi-monthly periods, sorted according to the FGS which acquired the dominant guidestar.