New CTE Correction Formula Available from Dolphin (2000)

-B. Whitmore

There is a paper that may be of interest to many WFPC2 users. The paper, authored by Andrew Dolphin, is titled "The Charge-Transfer Efficiency and Calibration of WFPC2" (Dolphin, 2000, PASP, 112, 1397). The paper compares WFPC2 observations with ground based observations of Omega Centauri and NGC 2419, using a baseline through March 2000, roughly a year longer than available for a similar study by Whitmore, Heyer, and Casertano (1999, PASP, 111, 1559). In general, Dolphin finds good agreement with the Whitmore, Heyer, and Casertano results, and the longer baseline and more extensive data set used by Dolphin result in less scatter in the residuals. In particular, Dolphin finds similar corrections to within a few hundreths of a magnitude in all cases except for recent (1998 and later) data with low counts. In these case the Dolphin corrections are larger than the Whitmore et al. corrections.

A preliminary comparison of the two formulae using August 2000 observations of Omega Cen confirms that the Dolphin formulae results in smaller residuals over most of the range of typical observations. Besides the tendency for the Whitmore et al. formula to underestimate the required correction for faint stars (i.e., in the range 100 - 500 electrons), as reported by Dolphin, we also find that the Whitmore et al. formula overestimates the required correction for very bright stars (i.e., brighter than 15,000 electrons) by a few percent. However, we also found that for extremely faint stars (i.e., 20 - 50 electrons), the Dolphin formulae overestimates the required correction by tens of percent, presumably because he was not able to constrain this part of the parameters space since he used ground-based observations for the comparison, which limits the sample to brighter stars. Hence, at present, the best compromise is probably to use the Dolphin formula for stars brighter than 100 electrons and the Whitmore, Heyer, Casertano formula for fainter stars.

Figure 1 shows the ratio of counts from a 14 second exposure of Omega Cen in August, 2000 to the counts in a 100 second exposure, vs. the Y position on the chip (WF2 in this case with gain=15). The raw values (filled circles) fall below a ratio of 0.14 due to CTE loss (i.e., the count rate and background are both lower for the short exposure, leading to more CTE loss). The different panels are for different target brightness, as described by the labels. The filled squares show the values corrected using the Whitmore, Heyer, and Casertano formula while the filled triangles show the values corrected using the Dolphin formula. Note that the Dolphin formula is somewhat better for the top 4 panels, but is much worse for the faintest stars in the bottom panel. An ISR is currently being written on this comparison and should be out shortly.

Some comments on preflashing. CTE loss can be reduced by increasing the background, hence filling some of the traps before the target reaches them. One can artificially enhance the background by adding a preflash. The problem with this approach is that this also adds noise. Figure 2 shows a calculation based on the Whitmore, Heyer, Casertano (1999) correction formula, assuming a very low background for the raw image (i.e., 0.1 electron, appropriate for a very short exposure, a narrow-band exposure, or an exposure in the UV) versus an exposure which has been preflashed with 25 electrons. The ratio of the S/N for the preflashed image versus the raw image is plotted vs the Log of the target brightness. The S/N estimates include the uncertainties in the CTE corrections. The three curves show the effects for a star near the bottom of the chip (i.e., X = 400, Y = 100, where the preflash is never an advantage since CTE loss is low and the preflash adds noise), near the center of the chip, and near the top of the chip (where the preflash is an advantage for the brighter targets). For more typical cases where the background is already sizeable, the gains due to a preflash are even smaller. We also note that preflashing may significantly increase the overhead time for an exposure.