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.
Nov 21, 2016: PHTFLAM2 for UV filters are set so that PHTRATIO equals the UV count-rate across chips for blue sources to facilitate astrodrizzle (IMPHTTAB=0bi2206ti_imp.fits). See ISR 2017-07 for details.
Feb 23, 2016: First UVIS chip-dependent solutions are available. Pipeline products use calwf3 v3.3+ to scale the UVIS2 pixel values by the inverse sensitivity ratio (PHTRATIO). The IMPHTTAB reference file (z7n21066i_imp.fits) writes PHOTFLAM values for a 10 pixel aperture. See ISR 2016-03 for details.
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.
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)
Frequently Asked Questions
On June 15, 2017, a new image photometry table (IMPHTTAB) for WFC3/UVIS was delivered to the Calibration Reference Data System (CRDS) as part of OPUS version 2017.2. The inverse sensitivity values reported in the image header (PHOTFLAM, PHTFLAM1, PHTFLAM2) revert back to the ‘infinite’ aperture and supercede the 2016 values which were reported for an 0.4" aperture.
For datasets retrieved from MAST at different times (e.g. after the execution of each visit), it may be possible to observe systematic differences between visits due to changes in processing. Users are advised to verify that the same versions of pipeline software and reference files were used to analyze their data by inspecting the image header diagnostic keywords. For software, these are OPUS_VER and CAL_VER and for reference files, BPIXTAB, CCDTAB, OSCNTAB, BIASFILE, FLSHFILE, DARKFILE, PFLTFILE, IMPHTTAB, MDRIZTAB, IDCTAB, D2IMFILE, NPOLFILE, PCTETAB, DRKCFILE, BIACFILE, SNKCFILE.
UVIS: the formal uncertainty on the UVIS photometric calibration relative to STIS is ~1.8% in the broad band and medium band filters, and ~5% for the narrow band and long-pass filters
IR: current estimates of the uncertainty of the IR photometry relative to STIS is 2-3% for the broad and medium band filter, and between ~5-7% in the narrow band filters
see ISR tables below
|2016-01||The Updated Calibration Pipeline for WFC3/UVIS: A Reference Guide to Calwf3 3.3||Overview of new chip-dependent calibration|
|2016-02||The Updated Calibration Pipeline for WFC3/UVIS: A Cookbook to Calwf3 3.3||Cookbook for manual reprocessing|
|2016-03||UVIS 2.0: Chip-dependent Inverse Sensitivity Values||Chip-dependent Photometric calibration|
|2016-04||UVIS 2.0: Chip-Dependent Flats||Flats no longer correct for chip QE offset|
|2016-05||UVIS 2.0: Ultraviolet Flats||UV Flats correct for 3% temperature residuals|
|2016-06||UVIS 2.0: Encircled Energy, in preparation||Filter-dependent values replace 2009 model|
|2016-07||Updated WFC3/UVIS Chip Dependent SYNPHOT/PYSYNPHOT Files||Pysynphot files (Called by ETC)|
|2017-07||WFC3 Chip Dependent Photometry with the UV filters||Effect of bandpass differences on UV photometry|
|2017-14||WFC3/UVIS Updated 2017 Chip-dependent Inverse Sensitivity Values||Improved in-flight solutions change by <1% from 2016|
|2009-30||WFC3 SMOV Proposal 11451: The Photometric Performance and Calibration of WFC3/IR||First In-flight photometric calibration|
|2009-37||WFC3 SMOV Programs 11437/9: IR On-orbit PSF Evaluation||In-flight encircled energy|
|2011-11||Sky Flats: Generating Improved WFC3 IR Flat-fields||In-flight corrections to the ground flats|
|none||2012 IR zeropoints available via website only||Revised in-flight photometric calibration|
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.