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J. Biretta, S. Baggett, A. Riess, A. Schultz, S. Casertano, S. Gonzaga, I. Heyer, A. Koekemoer, J. Mack, M. McMaster, M. Wiggs (STScI)
We present an overview of Charge Transfer Efficiency (CTE) issues in the WFPC2 CCDs, including results of recent on-orbit tests, and advice on mitigating CTE effects. CTE causes targets far from the CCD readout amplifier to appear fainter than targets near the amplifier. For bright targets, the maximum effect is only a few percent, but for faint stellar targets in images with very low background, the CTE effects can reach fifty percent or more. Studies using hotpixels, cosmic rays, and residual images as probes of CTE have revealed at least four distinct components of CTE losses. The largest effect appears related to trapping and release of charge on timescales of hundreds of milliseconds during the readout process. This is manifest as tails on images which extend for dozens of pixels in the Y-direction (parallel register direction) on the CCDs, which have the effect of robbing counts from typical small apertures used for photometry. Extended targets also are subject to CTE effects. Recent work shows the effect for faint galaxies is roughly similar to that for a stellar target with the same total counts. There are also small effects on the shapes of faint galaxies. Long-term photometric monitoring shows CTE is steadily increasing with time, and there is also some evidence for an acceleration of the effect. Preflashing the CCDs can reduce CTE effects, but the added noise usually makes this unattractive. A noise-less preflash technique has been tested, but only appears to give a modest improvement. We discuss these and other recent results, suggest observational strategies for reducing the impact of CTE, and review photometric corrections which can be applied during data analysis.