PIPELINE

Correction For Charge Transfer Efficiency (PCTECORR)

The charge transfer (CTE) of the UVIS detector has been declining over time as on-orbit radiation damage creates charge traps in the CCDs. Faint sources in particular can suffer large flux losses or even be lost entirely if observations are not planned and analyzed carefully. The CTE depends on the morphology of the source, the distribution of electrons in the field of view, and the population of charge traps in the detector column between the source and the transfer register. Further details of the current understanding of the state of the WFC3/UVIS charge transfer efficiency (CTE) are presented in Chapter 6 of the data handbook. The PCTECORR step aims to mitigate the flux loss incurred from CTE.

Sink Pixel Detection and Marking

Sink pixels are a type of image defect. These pixels contain a number of charge traps and under-report the number of electrons that were generated in them during an exposure. These pixels can have an impact on nearby upstread or downstream pixels, though they often only impact one or two pixels when the background is high, they can impact up to 10 pixels if the background is low.

Flagging of SINK pixels in the DQ extension of calibrated images is controlled with the DQICORR header keyword, happens after the bias correction has been performed, and is done in the amp-rotated CDAB full image format used and described in the CTE correction. When set to perform, the sink pixels are located and flagged with help from the SNKCFILE reference image. Given the reference image, the procedure for flagging the sink pixel in science data involves:

ISR 2014-19 has a detailed analysis on detection of the sink pixels, while the strategy for flagging them is discussed in ISR 2014-22.

The pipeline currently does no further analysis or correction on pixels which have been flagged as affected by sink pixels.

Photometry Keywords (PHOTCORR)

The PHOTCORR step is performed using tables of precomputed values instead of calls to SYNPHOT, it uses the reference table specified in the IMPHTTAB header keyword. Each DETECTOR uses a different table.  If you do not wish to use this feature, set the header keyword PHOTCORR to OMIT. However, if you intend to use the FLUXCORR step, then PHOTCORR must be set to PERFORM as well.

For calwf3 versions 3.3 and beyond, the value PHOTFNU is calculated specific for each UVIS chip, see the section on FLUXCORR for more information.

Flux normalization for UVIS1 and UVIS2 (FLUXCORR)

The FLUXCORR step was added in calwf3 v3.1.2 to scale the UVIS chips so that the flux correction over both chips is uniform. This requires new keywords which specify new PHOTFLAM values to use for each chip as well as a keyword to specify the scaling factor for the chips. New flatfields must be used and will replace the old flatfields in CDBS but the change will not be noticable to users. Users should be aware that flatfield images used in conjunction with v3.2.1 of the software should not be used with older versions as the data, and vice versa will be scaled incorrectly.

The new keywords include:

• PHTFLAM1: The FLAM for UVIS 1
• PHTFLAM2: The FLAM for UVIS 2
• PHTRATIO: The ratio: PHTFLAM2 / PHTFLAM1, which is calculated by calwf3 and is multiplied with UVIS2 (SCI,1 in the data file)

In order for FLUXCORR to work properly the value of PHOTCORR must also be set to perform since this populates the header of the data with the keywords FLUXCORR requires to compute the PHTRATIO.

This step is performed by default in the pipeline and the PHOTFLAM keyword will be valid for both chips after the correction has been applied.

A detailed description of the improved calwf3, Version 3.3, which is more generally referred to as the UVIS2.0 update is available in Chapter 3 of the WFC3 Data Handbook and in ISR 2016-01.