WFPC2 Technical Instrument Report 98-03:
WFPC2 Dark Current Evolution
-Baggett, Casertano, and Wiggs
The dark current level in WFPC2 has been slowly increasing over the instrument's life. The cold junction temperatures have also been slowly increasing, but the temperature increase can only account for a very small part of the increase in dark current. Most of the dark current increase is likely caused by radiation damage in the CCDs and/or increased luminescence in the CCD windows.