## News

Jun 4, 2019: IR Photometric Repeatability (see ISR 2019-07)

Jan 25, 2019: Calibration of the WFC3-IR Count-rate Nonlinearity, Sub-percent Accuracy for a Factor of a Million in Flux (see ISR 2019-01, also see STAN)

Nov 8, 2018: Temporal and Spatial Variations in UVIS Photometry (see ISR 2018-16)

Jun 29, 2018: Color term transformations for UV filters (see ISR 2018-08, also see STAN)

Jun 15, 2017: An improved UVIS photometric calibration is now live in MAST . The image photometry reference table (IMPHTTAB=1681905hi_imp.fits) reverts back to the pre-2016 convention, where the PHOTFLAM values correspond to the infinite aperture. These new solutions and the chip-dependent throughput tables available via psynphot  described in ISR 2016-07 are concordant.  For more details, see ISR 2017-14

## Current Photometric Calibration

WFC3/UVIS photometry is determined independently for each CCD based on data obtained between July 2009 and August 2015 for the three HST primary white dwarf standard stars, GD71, GD153 and G191B2B.  Photometric quantities are computed using chip-dependent flat fields and filter-based encircled energy values for each filter+CCD.

Prior to February 2016, WFC3/UVIS photometric calibrations were based on a ‘monolithic field view’, following the ACS model.   Flat fields were normalized to a single 100x100 pixel region on UVIS1. Encircled energy values for each filter were interpolated from the updated in-flight model (ISR 2009-38), and the inverse sensitivity values were computed by averaging results from the white dwarfs and the G-type standard stars.

WFC3/IR photometry has not changed since 2012.

### Errors:

Current estimates of the photometric uncertainties are:
UVIS: ~1% broad, 2% medium, 5-10% narrow, LP (1.3% statistical, 1.3% systematic)
IR: ~2% broad, 5-10% narrow (2% statistical, 2% systematic)

UVIS Photometric Calibration

UVIS Encircled Energy

IR Photometric Calibration

IR Encircled Energy

Easiest: Retrieve the data from MAST to pick up the latest improvements. Less Easy: download the reference files from CRDS and reprocess the RAW files offline with a self consistent version of calwf3 and reference files.

## Photometric Systems

The STmag and ABmag systems define an equivalent flux density for a source, corresponding to the flux density of a source of predefined spectral shape that would produce the observed count rate, and convert this equivalent flux to a magnitude. The conversion is chosen so that the magnitude in V corresponds roughly to that in the Johnson system.

In the STmag system, the flux density is expressed per unit wavelength, and the reference spectrum is flat in Fλ.  An object with Fλ = 3.63 x 10-9 erg cm-2 s-1 Å-1 will have STmag=0 in every filter, and its zero point is 21.10.

STmag = -2.5 log Fλ -21.10

In the ABmag system, the flux density is expressed per unit frequency, and the reference spectrum is flat in Fν.  Its zero point is 48.6.

ABmag = -2.5 log Fν - 48.6

ABmag = STmag - 5 log (PHOTPLAM) + 18.6921

where Fν is expressed in erg cm-2 s-1 Hz-1, and Fλ in erg cm-2 s-1 Å-1. An object with Fν = 3.63 x 10-20 erg cm-2 s-1 Hz-1 will have magnitude AB =0 in every filter.

Formally, the VEGAmag system is defined such that  Vega (Alpha Lyra) by definition has magnitude 0 at all wavelengths. The magnitude of a star with flux F relative to Vega is

mvega= -2.5 log10 (F/Fvega)


where Fvega is the absolute CALSPEC flux of Vega; for photometry the fluxes must be averaged over the band pass. See Bohlin 2014 (AJ, 147,127, "Hubble Space Telescope CALSPEC Flux Standards: Sirius and Vega") for the equations that define the average flux.

LAST UPDATED: 10/09/2019