WFPC2 Technical Instrument Report 98-01:
Time Dependence of the Charge Transfer Efficiency on the WFPC2
New evidence is presented that the Charge Transfer Efficiency (CTE) problem has increased with time for the WFPC2. Using observations of the globular cluster Omega Cen in the F814W filter over a baseline of just over 3 years shows an increase in CTE loss for faint stars (20 - 50 DN at a gain of 15 within an aperture with a 2 pixel radius) at the top of the chip from 3 +/- 3% to 22 +/- 3%. The change with time is smaller for brighter stars, and for stars brighter than about 200 DN (equivalent to 400 DN for gain = 7) there is no significant change with time. These results are based on very short (14 second) exposures. In general, typical WFPC2 exposures are much longer than these short calibration images, resulting in higher background which significantly reduces the CTE loss and minimizes the CTE problem for most science observations.