|WFC3 Instrument Handbook for Cycle 26|
The two WFC3/UVIS CCDs have very different quantum efficiencies, particularly in the UV where the difference in sensitivity can be as large as 30% around 2200 Å. UVIS2 is more sensitive in the UV; UVIS1 and UVIS2 are about equally sensitive at wavelengths longer than ~4500 Å. See Figure 5.2. The photometric calibration is now carried out independently for each CCD. Calwf3 will scale UVIS2 to UVIS1 by default, so that users will still only need to keep track of a single set of zeropoints. Users can reprocess their data to maintain calibration independence between the two chips. See http://www.stsci.edu/hst/wfc3/analysis/uvis_2_chip.Improved flat fields for each filter+chip have been created to support new the chip-dependent calibration (WFC3 ISR 2016-04). See Section 5.4.3.New values of the inverse sensitivity (often referred to as zeropoints) for each filter and detector combination were calculated snd can be found in Table 9 (UVIS1) and Table 10(UVIS2) in Appendix A of WFC3 ISR 2016-03 and via:Users should be aware that for data retrieved before 23 February 2016 only ONE value, PHOTFLAM, was used. Re-retrieving data from MAST or reprocessing images with the latest version of calwf3 may be necessary. New flats, imphttabs, and calwf3 version 3.3 are available through the WFC3 webpages at:On 23 February 2016, the WFC3/UVIS processing pipeline, calwf3, was modified to perform photometric calibration independently for each of the CCDs. Versions 3.3 and later of calwf3 incorporate chip dependent photometry. Pixel-based CTE correction is also included with version 3.3 of calwf3. (See the article in the WFC3 STAN issue 22 for details. Pixel-based CTE correction for subarrays was added in version 3.4 of calwf3, activated in MAST in October 2016.)(An incorrect description of PHTRATIO in the header as PHTFLAM1/PHTFLAM2 is being fixed.) The inverse sensitivity keyword, PHOTFLAM was set to have the same value as PHTFLAM1 for backward compatibility with any existing user software. PHTRATIO is used by CALWF3 to scale UVIS2 to UVIS1.calwf3 was modified to process each CCD independently. It uses the calibration switches PHOTCORR and the newly implemented FLUXCORR, whose default values are set to PERFORM.
• PHOTCORR: When this switch is set to “PERFORM”, calwf3 calculates PHTRATIO and populates the photometry keyword values, including the values of the new chip-dependent keywords PHTFLAM1 and PHTFLAM2, in the header.
• FLUXCORR: When this switch is set to “PERFORM”, calwf3 scales the flux on UVIS2 (CHIP 2) to UVIS1 such that the UVIS2 science array is multiplied by PHTRATIO. This means that only a single set of zeropoints is required for UVIS photometry.The photometry table, IMPHTTAB, has been modified to have five extensions. The first three have the same data values as in the old IMPHTTAB (pre 2016), namely, PHOTFLAM, PHOTPLAM and PHOTBW for each observing mode. For historical reasons, EXT 1, 4 and 5 list both uvis1 and uvis2 observing modes (e.g. wfc3, uvis1, f606w), but with these differences. Values in PHOTFLAM are identical to the UVIS1 values, regardless of observing mode, i.e. the PHOTFLAM value for wfc3,uvis2,f606w is the same as of wfc3,uvis1, f606w. In EXT4 only the values for uvis1 modes are provided, BUT values for uvis2 observing modes are set to zero. In EXT5, the values for uvis1 modes are set to zero, and the inverse sensitivity values for UVIS2 are provided. The extensions are described in Table 6.11.Starting on 23 Feb 2016, the values for the inverse sensitivity were calculated for a circular aperture with radius r=10 pixels, corresponding to r=0.3962 arcsec in the ‘native’ resolution of the UVIS channel. This was a departure from the approach implemented at the time WFC3 was installed in 2009. Based on feedback from the WFC3 user community and for consistency with other HST instruments, the photometric keyword values populated in the image header have reverted to the infinite aperture, starting on 15 Jun 2017. (See WFC3 ISR 2017-11.) A comparison of the old and new values of the inverse sensitivity for the 42 full frame UVIS filters is presented in WFC3 ISR 2017-14.The precision of the derived inverse sensitivity values for the bright standard stars is better than 1.5% (WFC3 ISR 2016-03). Accuracy of the photometry is estimated to be, on average, approximately 2-3%. See WFC3 ISR 2018-02 for a comparison of photometry for ACS/WFC and WFC3/UVIS wide filters with similar passbands.