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Wide Field Camera 3 Instrument Handbookfor Cycle 23 > 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 detectors, we have made preparations to perform photometric calibration independently for each of the CCD detectors (chips) in the WFC3/UVIS channel. Flatfields for each filter+detector have been created, normalized to the median value of each detector. New zeropoints have been calculated for each filter and detector. The UVIS1 and UVIS2 zeropoints differ by up to ~30% in the UV, with UVIS 2 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 IMPHTAB 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 IMPHTAB table
The synphot/pysynphot tables will be updated accordingly in CRDS.
We anticipate ingestion into the archive pipeline sometime in the 2nd quarter of 2015, but in mid December, 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 Handbookfor Cycle 23 > Chapter 6: UVIS Imaging with WFC3 > 6.10 Photometric Calibration

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