October 15, 2020

About This Article

1. Updated WFC3 Flux Calibration

A new set of UVIS and IR inverse sensitivities (zeropoints) incorporate improvements to the HST CALSPEC models (Bohlin et al. 2020) as well as an increase in the Vega reference flux. 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 - 1.6 micron covered by both detectors.

The UVIS calibration also includes new corrections for temporal changes in the detector sensitivity derived from over 10 years of monitoring data (Calamida et al. 2020). This new method:

  • Corrects the image header 'inverse sensitivity' keyword (PHOTFLAM) to account for changes of ~ 0.1 - 0.2% per year according to the filter (see Fig. 1);
  • Improves the chip-sensitivity ratio (PHTRATIO) in FLT/FLC data by up to 1%, in agreement with early dithered star cluster and standard white dwarf observations;
  • Improves 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.

The IR inverse sensitivities change primarily due to the new models, and they incorporate new flat fields in the calibration of the flux standards (Bajaj et al. 2020). The updated 'pixel to pixel' flats correct for spatial sensitivity residuals up to 0.5% in the center of the detector and up to 2% at the edges (Mack et al. 2020).  These flats were computed by stacking deep exposures acquired over 10 years, illuminating the detector with high signal-to-noise images of the sky background. A new set of 'delta' flats (available for six filters: F098M, F105W, F110W, F125W, F140W, and F160W) correct for low-sensitivity artifacts known as 'blobs', as new blobs appear over time. (Olszewski et al. 2020).

The new inverse sensitivities are available on the WFC3 Photometric Calibration webpage. A Jupyter notebook (available here) shows users how to work with the new time-dependent UVIS solutions. 

For both detectors, all image header photometry keywords will be updated with new values, among them the 'inverse sensitivity' PHOTFLAM, PHTFLAM* and PHTRATIO. Please note that the pixel values in the UVIS1 science arrays will not change; UVIS2 pixel values may change by up to 1% due to new values of the chip-sensitivity ratio. This ratio is used by calwf3 to flux correct UVIS2 such that the count rates match UVIS1 (for details see Section 3.12 of the Data Handbook).  WFC3 IR pixel values will change slightly due to the flat field updates.


Fig. 1:  Relative aperture photometry performed at 10 pixels on FLC images collected in the F814W filter for the four CALSPEC white dwarfs, namely GRW70 (filled circle), GD153 (diamond), GD71 (square), G191B2B (triangle), and the G-type star P330E (horizontal triangle) versus MJD. Photometry performed on scanned images for GRW70 (blue) and GD153 (green) is also shown. The solid line is the fit to the photometry of all the stars, a sensitivity change of -0.11% per year.

bigger plot

Fig. 2: Difference in the new and old zeropoints (ZP, 2020-2017 for UVIS2, 2020-2012 for IR) in the ST photometric system for the wide- and medium-band filters over the entire WFC3 wavelength range.


Fig. 3: Ratio of 'Observed to Synthetic' photometry for UVIS2 and IR wide- and medium-band filters for the four CALSPEC white dwarfs, namely GRW70 (black filled circle), GD153 (red diamond), GD71 (cyan square), G191B2B (magenta triangle), and the G-type star P330E (green triangle). The RMS scatter around 1.0 is 0.5% for the UVIS2 detector and 0.6% for the IR.



  • Bajaj et al. 2020, in prep. "Updated WFC3 IR Zeropoints
  • Bohlin et al. 2020AJ....160...21B
  • Calamida et al. 2020, in prep. "New time-dependent WFC3-UVIS Inverse Sensitivities"
  • Mack et al. 2020, in prep. "WFC3/IR Filter-Dependent Sky Flats"
  • Olszewksi & Mack 2020, in prep. "WFC3/IR Blob Flats"


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