WFPC2 Instrument Science Report 97-05:
Charge Trapping and CTE Residual Images in the WFPC2 CCDs
John Biretta and Max Mutchler
May 20, 1998
We have studied the charge trapping problem in the WFPC2 CCDs by examining residual images in dark calibration exposures taken at various intervals after non-saturated planetary exposures. Charge is trapped during read-out of the planet image, followed by a slow release of charge during the dark frame, hence resulting in a residual image. Our results indicate the dissipation timescale for charge caught in the traps is approximately 16 (+10 -5) minutes, and that the amount of trapped charge in a pixel appears to correlate with the maximum intensity clocked through it during read-out of the previous exposure. Our model predicts a photometric ramp similar to that observed on bright stars, suggesting that charge trapping may account for at least some of the observed photometric anomalies. We propose a "noiseless" preflash scheme which might significantly reduce the photometric effects of CTE.