UVIS PHOTOMETRIC CALIBRATION

Motivated by the different properties of each of the WFC3/UVIS e2v CCD detectors, UVIS photometric calibrations are now determined independently for each CCD. This follows the standard practice established by previous generations of imaging instruments on HST. New inverse sensitivity values (PHOTFLAM, PHTFLAM1, PHTFLAM2) have been calculated for the 42 ‘full frame’ filters using observations of the HST white dwarf primary standard stars (G191B2B, GD153, GRW+70D5824, and GD71) and the G-type sun-like standard P330E obtained between 2009 and 2019.

New 2020 Solutions:

• Models: Models for the CALSPEC standard white dwarfs were improved and the Vega reference flux was increased (Bohlin et al. 2020). As a consequence, the standard white dwarf fluxes increase by ~2% for wavelengths in the range 0.15-0.4 micron and ~1.5% in the range 0.4-1 micron covered by the UVIS detector. 
• Inverse sensitivity:  Corrected the image header 'inverse sensitivity keywords' to account for changes of ~0.1-0.2% per year according to the filter.
• Chip sensitivity ratio: Improved the chip-sensitivity ratio (PHTRATIO) by up to 1%, in agreement with early dithered star cluster and dithered standard white dwarf observations.
• Encircled Energy: Improved the encircled energy (EE) correction by 1% in the ultraviolet filters and by 0.5% at wavelengths larger than 7,500 A, in close agreement with the 2009 EE values.
• New ISR and paper:  ISR 2021-04 describes how the new UVIS inverse sensitivities were derived, while Calamida et al. (2022) details the updated calibrations for both detectors.
• Jupyter notebooks: A Jupyter notebook (click here for preview) to correct UVIS data for time-dependent sensitivity is available here (60 MB .zip file). A notebook (click here for preview)that shows how to use stsynphot to calculate the latest UVIS and IR photometric zeropoints for any observation date is available here: download here (6 KB .zip)

2020 Solution

The new calibration provides the inverse sensitivity values that take into account the sensitivity change with time of the UVIS detectors (see Figure 1). The IMPHTTAB carries the values for UVIS1 and UVIS2 via the keywords PHOTFLAM, PHTFLAM1, and PHTFLAM2 at the epoch of the observation.

The UVIS1 and UVIS2 detectors have different quantum efficiencies in the ultra-violet (UV) regime (lλ < 4,000 Å): count rate ratios change as a function of the spectral type of the source. When calibrating photometry of stars cooler than Teff ~ 30,000 K in the UV filters (e.g. when observing open and globular clusters, resolved local group galaxies, Galactic stellar populations), color term transformations need to be applied to UVIS2 magnitudes. Sources of any spectral type observed on one detector only will not require any magnitude offset.

Figure 2 below shows the synthetic ST magnitude difference, UVIS1 – UVIS2, for a sample of CALSPEC stars of varying spectral type (the white dwarfs (WDs) include G191B2B, GD71 and GD153), as computed for three UV filters, F218W, F225W, F275W. Cool red sources such as the G-type star P330E measured on UVIS2 have a magnitude offset relative to UVIS1 up to ~0.08 mag when observed with the F225W filter. F-type stars such as HD160617 have a magnitude offset of ~0.04 mag.

Figure 3 shows the ST magnitude difference for the F225W and F275W filters for ω Cen Extreme Horizontal Branch (EHB), Horizontal Branch (HB), Main Sequence (MS), and Red-Giant Branch (RGB) stars measured on different detectors and amplifiers, A (UVIS1) and C (UVIS2). WFC3 observations of ω Cen validate the results of synthetic photometry: red stars (RGBs) observed on UVIS2 have a magnitude offset up to 0.08 mag relative to UVIS1.

Calamida et al. 2018 ( WFC3 ISR-2018-08) provides lookup tables with color term transformations to be applied to UVIS2 magnitudes when observing with three UV filters, F218W, F225W, and F275W. UVIS2 magnitudes are scaled to UVIS1 by the WFC3 calibration pipeline (cal_wf3) using UVIS2 to UVIS1 modified inverse sensitivity ratio, PHTRATIO (see UV Filter Modified Keywords for more background and for a table of relevant values). PHTRATIO is derived using photometry of the CALSPEC WDs and is valid for hot stars, Teff > 30,000 K. For cooler stars, when observing with UV filters, PHTRATIO is not equal to the ratio of the two detectors' count rates but changes with the stellar spectral type. Photometry for cooler stars measured on UVIS2 needs to be corrected by applying a magnitude offset according to their color, if available, or temperature or spectral type.

Before applying the offset to magnitudes measured on UVIS2, photometry must be calibrated using UVIS1 inverse sensitivities.

STMag (UVIS1) = -21.1 -2.5 x log(PHOTFLAM)

STMag (UVIS2) = -21.1 -2.5 x log(PHOTFLAM) + Delta (Mag)

where Delta (Mag) = Mag(UVIS1 – UVIS2) is listed in lookup tables in WFC3 ISR-2018-08. For more details and to consult the color transformation tables please see the referred ISR; for examples on how to derive color transformations between different filters of the detector, see this Jupyter notebook.