WFC3 Instrument Handbook for Cycle 24
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Wide Field Camera 3 Instrument Handbook for Cycle 24 > Chapter 6: UVIS Imaging with WFC3 > 6.10 Photometric Calibration

6.10
Motivated by the very different quantum efficiency of each of the WFC3/UVIS e2v chips, we have made preparations to perform photometric calibration independently for each of the CCD chips in the WFC3/UVIS channel. This will be implemented in CALWF3 version 3.3 in the pipeline in 2016, possibly in the first quarter. Pixel-based CTE correction will also be included in this version of CALWF3. (See the article in the WFC3 STAN issue 22 for details.)
Flat fields for each filter+chip have been created, normalized to the median value of each chip. New zeropoints have been calculated for each filter and chip. The UVIS1 and UVIS2 zeropoints differ by up to ~30% in the UV, with UVIS2 being more sensitive. (See Figure 5.2.) Between 3500 and 7000 Angstroms, the difference in zeropoints is ~1%, and for the reddest filters the difference is ~2-3%, with UVIS2 being more sensitive.
Three new keywords will be added to the image headers:
The original inverse sensitivity keyword, PHOTFLAM, will be the same as PHTFLAM1.
CALWF3 will be modified to process each CCD independently. Two new calibration switches will be implemented:
A new IMPHTTAB has been created, which has 5 extensions compared to the “One Chip” IMPHTTAB which has only three. Extensions 4 and 5 contain the observing mode and zeropoint values for chip 1 and chip 2, respectively. The extensions are described in Table 6.11.
Table 6.11: Structure of new IMPHTTAB table
The synphot/pysynphot tables will be updated accordingly in CRDS.
We anticipate ingestion into the archive pipeline in 2016, possibly in the first quarter, but for those users who wish to reduce and calibrate their own image data, we will make available the new flats, imphttabs, and calwf3 through the WFC3 webpages at:
http://www.stsci.edu:8072/hst/wfc3/analysis/uvis_2_chip

Wide Field Camera 3 Instrument Handbook for Cycle 24 > Chapter 6: UVIS Imaging with WFC3 > 6.10 Photometric Calibration

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