The STIS CCD is run at a constant current, which results in temperature variations of the detector. Fluctuations in temperature are known to affect the dark current. To regularly monitor the STIS CCD Dark Rate, 14 long (1100s) dark exposures and 28 short (60s) dark exposures are taken weekly.
For more information on the STIS CCD Dark Rate, see ISR 2012-02: Post SM4 behavior of STIS CCD Darks.
CCD Dark Rate