NEW: pixel-based empirical CTE correction software
UPDATE As of Feb 2016, fully-calibrated CTE-corrected images are available from MAST. Observers with data processed before Feb 2016 may re-request their data from the archive to receive CTE-corrected versions.
The pixel-based CTE correction, available as a stand-alone FORTRAN program, has now been incorporated into the CALWF3 pipeline for all UVIS full-frame images (calwf3 version 3.3). Controlled via a new header calibration switch (PCTECORR = PERFORM), and associated calibration table (PCTETAB) and CTE-corrected reference files (e.g. DRKCFILE), CALWF3 will by default produce two sets of products: the standard non-CTE-corrected (e.g. *_raw.fits, *_flt.fits, *_drz.fits) files as well as the new CTE-corrected results (*_flc.fits, *_drc.fits). Observers will be able to use the *_flc.fits and *_drc.fits data products in the same way as *_flt.fits and *_drz.fits files.
One new type of reference file has been added to CALWF3: a sink-pixel file (SNKCFILE), which allows for the flagging of sink pixels in the science data quality (DQ) extension; no change is made to science data pixels. A type of bad pixel, sink pixels register systematically low, presumably due to a large number of traps within the pixel and can generate trails very similar to CTE trails (2014-19 and 2014-22 ). The sinks and their trails are now flagged as part of the DQICORR correction step with DQ bit values of 1024 in both the CTE- and non-CTE-corrected data products.
For more details on the new version of calwf3, please see:
ISR 2016-02: The Updated Calibration Pipeline for WFC3/UVIS: A Cookbook to Calwf3 3.3
ISR 2016-01: The Updated Calibration Pipeline for WFC3/UVIS: A Reference Guide to Calwf3 3.3
UPDATE As of Nov 14, 2013, there is a new "parallelized" version of the software available that uses OpenMP to run on multiple threads simultaneously, speeding up execution time by a factor that is close to the number of cores available on the user's CPU. See the CTE Tools page for details. The parallel code works with subarrays.
UPDATE As of May 20, 2013, software for correcting subarray WFC3/UVIS CTE is available, also on 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 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 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).
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
CTE trends 2009-2015
The longterm behavior of the charge transfer efficiency (CTE) in WFC3/UVIS is monitored using observations of external star clusters. Flux loss due to CTE degradation is a function of the source’s distance from the amplifier, the source signal level, the background within the image, and the epoch of the observations. The worst-case flux losses occur in images with extremely low backgrounds. In such data, based on photometry within a 3-pixel radius aperture and losses measured across 2048 pixels, the flux losses in early 2015 for faint sources (500-2000 e-) can be as high as ~50+/-2%; losses for brighter sources (8000-32000 e-) are considerably less: ~5 +/-1%. Ensuring a modest amount of background can reduce the losses substantially: ~12e-/pix, added via post-flash, reduced the losses to ~15+/-1% and ~4+/-1% for faint and bright sources, respectively. Applying the empirical pixel-based CTE correction algorithm can also reduce flux losses: to ~10+/-1% and ~0+/-1% (unflashed images, no background) and to 3+/-1% and 0.5+/-1% (post-flashed), for the faint and bright sources, respectively. We find that the CTE correction appears to slightly over-correct (1-5%) bright sources in low image backgrounds and over-correct most sources in post-flashed images. We empirically fit the flux losses as a function of source flux, observation date, background level, and distance from the amplifier with a 2nd order polynomial and provide tabulated coefficients.ISR 2015-03: WFC3/UVIS Charge Transfer Efficiency 2009 - 2015