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CTE in the WFC3 UVIS

For a general discussion of observational and post-observational strategies for dealing with CTE losses on the WFC3/UVIS detector, see Section 6.9 of the WFC3 Instrument Handbook. Additional material is added to this CTE web page as it becomes available.

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

Full text available here

ABSTRACT:

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.

Normalized post-flash pattern in jpg and fits formats.

Ratio of post-flash illumination pattern in shutter blade A to shutter blade B jpg

Coefficients for the aperture photometry correction formula

The following text files contain coefficients to be used to correct flux loss due to CTE degradation in point-source photometry. Their columns are sorted by background level. Please refer to Equations 1-3 in ISR 2017-09 [link ISR please] for the flux correction formula and note that the expansion in Equation 2 was misprinted in pre-2017 instrument science reports. Observers should try to use coefficients derived from conditions that most closely match their data, with background levels being the most important consideration. If observers have pre-2016 data with faint sources, they may consider using coefficients from earlier epochs for those sources. However, observers are cautioned against applying the coefficients forward in time.

Note: The 2012 and 2015 CTE ISRs reported the coefficients in the incorrect order. The files here are accurate.

2016

* These were calculated using images corrected by the pixel-based CTE correction software. See ISR 2017-09

2015

2012

See ISR 2015-03

2011

See ISR 2012-09

CTE trends 2009-2016

The evolution of the charge transfer efficiency (CTE) of Hubble’s WFC3/UVIS detector has been monitored since 2009, using observations of star clusters NGC 104 and NGC 6791. We present here results from an updated analysis pipeline with an improved cross-identification of sources and a new capability to study CTE loss mitigation by use of flash levels 6 - 116 e-/pix. We measure flux loss due to CTE degradation as a function of source brightness, observation date, background level, and vertical distance from the readout amplifier with a 2nd degree polynomial, whose coefficients are provided to allow observers to estimate flux corrections for their point-source photometry. Current flux losses for images with the recommended minimum 12 e-/pix background are 5 - 15%, depending on source brightness. These losses are further reduced via use of the CTE-corrected images now available from the Mikulski Archive for Space Telescopes. Within its overall growth, there appears to be a flattening in the CTE degradation rate that may be correlated with the Solar cycle maximum. The data reported here span from October 2009 to August 2016 and were calibrated with calwf3 v3.3.

ISR 2017-09: WFC3/UVIS External CTE Monitor: 2016 Updates on Coefficients and Analysis Pipeline

Charge transfer efficiency and charge injection in the WFC3 UVIS detector

Charge Injection (CI) is not being offered to observers at this time. The use of post-flash is preferable to charge injection.

Relevant Documents:

ISR 2017-09: WFC3/UVIS External CTE Monitor: 2016 Updates on Coefficients and Analysis Pipeline

ISR 2016-17: WFC3/UVIS External CTE Monitor: Single-Chip CTE Measurements

ISR 2016-10: WFC3/UVIS EPER CTE Cycles Aug 2009 - Apr 2016

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

ISR 2015-03: WFC3/UVIS Charge Transfer Efficiency 2009 - 2015

ISR 2014-22: Flagging the Sink Pixels in WFC3/UVIS

ISR 2014-19: Sink Pixels and CTE in the WFC3/UVIS Detector

ISR 2014-02: The Impact of x-CTE in the WFC3/UVIS detector on Astrometry

ISR 2013-12: WFC3 Post-Flash Calibration

WFC3-ISR-2012-12: WFC3/UVIS Sky Backgrounds

AAS Poster: Fitting a Pixel-Based CTE Model to the WFC3/UVIS CCD Detector

WFC3-ISR-2012-09: WFC3 UVIS Charge Transfer Efficiency October 2009 to October 2011

WFC3-TIR-2012-01: WFC3/UVIS TV3 Post-flash Results

WFC3 UVIS CTE and Charge Injection: June 2011 Update for Cycle 19 Observers

WFC3-ISR-2011-06: WFC/UVIS-Cycle 17: CTE External Monitoring - NGC 6791

WFC3-ISR-2011-02: WFC3/UVIS Charge Injection Behavior

WFC3 UVIS CTE Whitepaper