Instrument Science Report 00-03:
Update on Charge Trapping and CTE Residual Images in WFPC2
S. Baggett, J. Biretta, and J.C. Hsu
Sept. 18, 2000
We provide an update on the status of charge trapping in the WFPC2 CCDs, as measured from the strength of residual images in dark calibration frames taken after external science images. The new images support the original finding that the amount of charge trapped appears correlated with the maximum intensity clocked through the pixel during readout (Biretta & Mutchler, 1998). The charge in the residual image is found to be given roughly by ln(residual)~0.36ln(Imax), where Imax is the geometric mean of the maximum pixels in each residual image column. Furthermore, the amount of charge seen in the residual images appears to have been stable over the six years. This stability is in marked contrast to the evolving CTI (charge transfer inefficiency) found via photometry of external stellar images (e.g., Whitmore et al., 1999) and analysis of cosmic ray tails in dark frames (Riess et al., 1999). Finally, there is evidence that these residuals can be relatively long-lived: some residuals appear in darks started more than 20 minutes after the external image was read out.