Charge Transfer Efficiency for Very Faint Objects and a Reexamination of the Long-vs.-Short Problem for the WFPC2

An analysis of WFPC2 observations of Omega Cen and NGC 2419leads to the following results. 1. The correction formula developed by Whitmore, Heyer, and Casertano (1999; here- after WHC99) does a reasonable job of correcting for CTE loss down to extremely low count levels. There is no sharp cutoff to the detection threshold for very faint stars. 2. A comparison of the WHC99 formula with the Dolphin (2000; hereafter D00) formula shows reasonable agreement for bright and moderately bright stars, with the D00 formula giving better results. However, at very faint levels, the D00 formula overes- timates, and the WHC99 formula underestimates, the correction by tens of percent. Note: Our current recommendation is to use the new Dolphin 2002 (hereafter D02) formula for CTE loss correction, which is an improvement on the D00 formula. 3. A reexamination of the long-vs-short nonlinearity shows that the effect is very small (a few percent) or nonexistent for uncrowded fields, with less than ∼1000 stars per chip. However, for crowded fields, with ∼10,000 stars per chip, apparent nonlinearities of tens of percent are possible. We believe this is due to difficulties in measuring the sky values for the short exposures. 4. Preflashing may be a useful method of reducing the effects of CTE loss for certain observations (moderately bright objects on very faint backgrounds), but the effects of added noise and longer overheads limit its effectiveness.