Instrument Science Report 2002-03:
Charge Transfer Efficiency for Very Faint Objects and a Reexamination of the Long-vs-Short Problem for the WFPC2
B. Whitmore, I. Heyer
May 06, 2002
An analysis of WFPC2 observations of Omega Cen and NGC 2419 leads
to the following results.
1. The correction formula developed by Whitmore, Heyer, and Casertano (1999; hereafter WHC99) does a reasonable job of correcting for CTE loss down to extremely low count levels (i.e., so faint you cannot actually see the object but only know of its existence from longer exposures). There is no sharp cutoff to the detection threshold for very faint stars.
2. A comparison of the WHC99 formula with the Dolphin (2000b; 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 overestimates, and the WHC99 formula underestimates, the correction by tens of percent. Our current recommendation is to use the D00 formula for CTE loss correction.
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. The most likely explanation is that this is due to an overestimate of the sky measurement in the short exposure, which is probably due to the presence of scattered light around bright stars and the subsequent improvement in CTE loss in these regions. No correction formula has been derived in the current study since the effect is dependent on the analysis parameters (aperture size) and probably also on the photometry package (PSF-fitting, aperture photometry, ...).
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.
5. The detection thresholds for typical broad band observations have been reduced by ~ 0.1 - 0.2 mag in the ~7 years since WFPC2 was launched. For worst-case observations (F336W) the effect is currently ~ 0.4 magnitudes.
The Complete Paper (PDF 3.47 MB) is available.