WFPC2 Instrument Science Report 97-05:

Charge Trapping and CTE Residual Images in the WFPC2 CCDs

John Biretta and Max Mutchler

May 20, 1998

Abstract:

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.

The complete paper is available, PDF (0.2 MB) and Post Script (1.6 MB).