The main executable which processes data taken with either the UVIS or IR detectors of the WFC3 instrument onboard the Hubble Space Telescope is called
The code is organized into subroutines that are called by the
calwf3 executable. The subroutines may be called independently if users desire specialized processing for their dataset (for more details, please see the
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 (see the WFC3 CTE webpage for more details). 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 upstream 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:
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.
calwf3 versions 3.3 and beyond, the value PHOTFNU is calculated specific for each UVIS chip, see the below section on FLUXCORR for more information. Users may also want to adjust the photometric calibration of WFC3/UVIS photometry in UV filters to improve precision, as explained in the UV Filter Modified Keywords webpage.
Full-Well Saturation Flagging
Full-well saturation occurs when accumulating charge from a central pixel begins to spill into neighboring pixels. ISR 2010-10 demonstrated a ~10% variation in the full-well depths across both WFC3/UVIS CCDs. Saturated pixels are flagged in the DQ extension of calibrated images when the BLEVCORR and BIASCORR header keywords are set to PERFORM. A 2D saturation map reference file (SATUFILE) containing pixel-to-pixel saturation thresholds is applied to the image science data to identify saturated pixels. This first version of the SATUFILE effectively flags the same saturated pixels as older versions of
calwf3, which applied a single scalar value to the entire image. An updated version of this reference file which will enable improved pixel flagging will be possible in the future. ISR 2023-08 has a detailed analysis on the implementation of the SATUFILE keyword and reference file in
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 flat fields must be used and will replace the old flat fields in CDBS but the change will not be noticable to users. Users should be aware that flat field 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
calwf3and 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.