NEW: Pixel-based Empirical CTE Correction Software Now Available
An alpha version of the software for correcting WFC3/UVIS CTE is now available at the CTE Tools page
The Efficacy of Post-Flashing for Mitigating CTE-Losses in WFC3/UVIS Images
AUTHORS: Jay Anderson, John MacKenty, Sylvia Baggett, and Kai Noeske
We report on a set of calibration observations taken recently to demonstrate the effectiveness of post-flashing on preserving signal from CTE losses. A study of warm pixels (WPs) in dark exposures has shown that CTE losses in UVIS are pathological for charge packets smaller than 10 to 12 electrons (Anderson 2012), but when packets are larger than this, the losses become less severe. This suggests that if we could introduce a background of only about 12 electrons, we might be able to markedly improve charge transfer efficiency. For this reason, the Institute and Goddard have fast-tracked a procedure for adding post-flash flux to images. The procedure was validated using internal observations in May 2012. More recently, external observations have been acquired to demonstrate the effectiveness of the mitigation on real science data. This report summarizes our findings and shows that by adding only 12 electrons background to an image, users can reduce losses for low S/N sources from the pathological level of more than 90% to a moderate level of about 15%. Adding more electrons than this does not result in additional improvement, so 12 electrons should be seen as a “sweet spot” for the UVIS detector.
A recent White Paper (MacKenty & Smith 2012) provides a variety of ways to address CTE issues in WFC3/UVIS and ACS imaging. The results presented here reinforce the general findings of that paper, adding specifics about the mitigation achieved as a function of the background in the image (see Baggett & Anderson 2012). Cycle 19 and Cycle 20 WFC3/UVIS users should take note of these results in planning (or re-planning) their Phase-2 submissions.
As discussed in the references above, the post-flash illumination pattern of WFC3 varies by about +/-20% across the full field of view. The flux is lowest in the lower left and upper left corners (C and A amps, respectively) and highest on the right side (B and D amps). The pdf below shows the normalized pattern for shutter blade A; differences between the shutter blades are less than a few percent.
Ratio of post-flash illumination pattern in shutter blade A to shutter blade B jpg
Charge Transfer Efficiency and Post-flash Illumination in the WFC3 UVIS Detector - Available for Cycles 20 and 21
Post-flash, available for use in Cycle 20 and beyond, will be preferable to the Charge Injection mode described below. A white paper, with instructions for determining whether post-flash is necessary for WFC3/UVIS images and if so, how to choose the optimum flash level, is available: WFC3 UVIS CTE with Post-Flash White Paper.
CTE Trends in WFC3 – UVIS Charge Transfer Efficiency October 2009 to October 2011
AUTHORS: K. Noeske, S. Baggett, H. Bushouse, L. Petro,R. Gilliland, V. Khozurina-Platais
Using observations of calibration fields in the star clusters NGC6791 and NGC104/47 Tuc, we have investigated the evolution of the charge transfer efficiency (CTE) of the WFC3/UVIS detectors over a 2-year period starting 5 months after the WFC3 installation, ranging from October 2009 to October 2011. We find a strong evolution of the CTE, amounting to more than 0.1 mag CTE loss per year for stars with a total flux of 1000 electrons, at large distances from the readout amplifiers, in short-exposure zero sky background images. These maximum possible CTE losses for such stars have reached 0.3 mag (24%) as of October 2011 in 3-pixel radius apertures. For even fainter stars around 300 electrons, these losses can reach 50%.
The CTE degradation and associated losses decrease for increasing source fluxes, and for higher sky background levels which partially fill the detector charge traps during readout. Already a low background of 2-3 electrons per pixel significantly lowers CTE losses, while a 20-30 electron per pixel background in longer-exposure broadband images brings CTE losses to few percent for sources ranging from few 100 to 10s of 1000s of electrons. For faint sources on near-zero sky backgrounds, we also see significantly lower CTE losses in longer (approx. 350 seconds) exposures than in shorter ones (30-60 seconds). This most likely reflects that the larger total charge collected in longer images has a similar, yet non-uniformly distributed trap-filling effect as a higher sky background.
We present an empirical polynomial model that corrects CTE losses for point source aperture photometry as a function of observation date, source flux, and source distance on the detector from the readout amplifiers. The model coefficients are given for low and high backgrounds and long and short exposures separately, while we collect further calibration data to include a smooth parametrization of the latter parameters.
Charge Transfer Efficiency and Charge Injection in the WFC3 UVIS DetectorCharge Injection (CI) is not being offered to observers at this time.
WFC3-ISR-2012-12: WFC3/UVIS Sky Backgrounds
S. Baggett & J. Anderson 28 Jun 2012
AAS Poster: Fitting a Pixel-Based CTE Model to the WFC3/UVIS CCD Detector
Anderson et al. 11 Jun 2012
WFC3-ISR-2012-09: WFC3 UVIS Charge Transfer Efficiency October 2009 to October 2011
Noeske, K., Baggett, S., Bushouse, H., Petro, L., Gilliland, R., Khozurina-Platais, V. 08 Jun 2012
WFC3-TIR-2012-01: WFC3/UVIS TV3 Post-flash Results
S. Baggett & T. Wheeler 29 Mar 2012
WFC3 UVIS CTE and Charge Injection: June 2011 Update for Cycle 19 Observers
Noeske et al. 29 Jun 2011
WFC3-ISR-2011-06: WFC/UVIS-Cycle 17: CTE External Monitoring - NGC 6791
V. Khozurina-Platais, R. Gilliland, S. Baggett 03 Feb 2011
WFC3-ISR-2011-02: WFC3/UVIS Charge Injection Behavior
Results of an Initial Test. H. Bushouse et al. 07 Jan 2011
WFC3 UVIS CTE Whitepaper
Baggett et al. 07 Jan 2011