WFPC2 Technical Instrument Report 99-02:

Preliminary Results of the Noiseless Preflash Test

-Schultz, Heyer, and Biretta

We report preliminary results from analysis of the WFPC2 noiseless preflash test data (program ID: 8450). The short 16 sec. exposures were analyzed for this report. (The long exposure observations of 80 sec. and 400 sec. were lost due to guide star reacquisition failure, HOPR 587 was filed against 8450, and a repeat has been approved.) The calibration lamp was used to preflash the wide field CCDs to about 2000 counts, and the preflash was read out before the start of the science observations. In principle, this might fill the CCD traps while introducing no noise in the science image (i.e. noiseless). Stars within the globular cluster Omega Cen (HD116790, NGC5139) were positioned in the WF4, WF3, and WF2 apertures. Photometry of faint star images showed on average a 3.0+/-0.9% enhancement in the stellar counts (at Y=800) in the preflash exposures. This is consistent with the noiseless preflash giving only a partial reduction in CTE. The majority of the CTE effect must be due to traps which release their charge on timescales of less than two minutes.

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