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WFC3 Photometric Calibration

News

Jun 19, 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 WFC3 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.

UVIS Photometric Calibration
UVIS Encircled Energy
IR Photometric Calibration
IR Encircled Energy
Photometric Systems
FAQ

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)

The WFC3 team recommends using the most up-to-date reference files for processing data. Users should make sure that the same version of the reference files and the processing pipeline are used to analyze their whole datasets by checking the image header diagnostic keywords: OPUS_VER, CSYS_VER, CAL_VER, and the reference file keywords: BPIXTAB, CCDTAB , OSCNTAB BIASFREE, FLSHFILE, CRREJTAB, SHADFILE, DARKFILE, PFLTFILE, IMPHTTAB, IDCTAB MDRIZTAB, D2IMFILE, NPOLFILE, PCTETAB, DRKCFILE, BIACFILE, SNKCFILE. This is particularly important for data taken at multiple epochs. For example, if the data are retrieved from MAST at different times (e.g. after the execution of each visit), it may be possible to observe systematic differences, due to changes in the reference files, or more generally, to the whole data processing flow. Either retrieve the whole dataset from MAST or reprocess the dataset off line with a self consistent configuration of calwf3 and reference files.


Frequently Asked Questions (FAQ)

How do I get the latest calibration?
(Easiest: Retrieve recalibrated data from MAST . Less Easy: download the reference files from CRDS and reprocess the raw files)
Why does the latest dataset look different from previous sets?
(On June 15, 2017, 18 bias files, 4 post-flash files, and 1 image photometry table for WFC3/UVIS were delivered to the Calibration Reference Data System (CRDS) and went live in the 2017.2 release of the MAST processing pipeline for WFC3. These files will be used for the processing all WFC3/UVIS data. The bias files replace the previously used biases with an updated set of images t a yearly cadence that are made through 2016. The post flash files replace the previous low and medium flash-current reference images with updated much more deeply-stacked images. The Image photometry table replaces the previous UVIS table with updated solutions for the infinite aperture. if the data are retrieved from MAST at different times (e.g. after the execution of each visit), it may be possible to observe systematic differences, due to changes in the reference files, or more generally, to the whole data processing flow. There are two options on how to proceed: retrieve the entire pertinent dataset for the scientific investigation from MAST or reprocess the data offline using self consistent configuration of calwf3 and reference files).
What are the uncertainties on the photometry?
(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)
How does the new calibration compare with prior versions?
(Links below provide delivery dates, reference file names, and links to supporting ISRs)
UVIS History
IR History
What aperture correction should I use?
(See EE tables for UVIS and IR .)
Where can I find the latest documentation?
(see ISR tables below)
ISR Title Topic
UVIS ISRs
ISR 2016-01 The Updated Calibration Pipeline for WFC3/UVIS: A Reference Guide to Calwf3 3.3

Overview of new chip-dependent calibration

ISR 2016-02 The Updated Calibration Pipeline for WFC3/UVIS: A Cookbook to Calwf3 3.3 Cookbook for manual reprocessing
ISR 2016-03 UVIS 2.0: Chip-dependent Inverse Sensitivity Values Chip-dependent Photometric calibration
ISR 2016-04 UVIS 2.0: Chip-Dependent Flats Flats no longer correct for chip QE offset
ISR 2016-05 UVIS 2.0: Ultraviolet Flats UV Flats correct for 3% temperature residuals
ISR 2016-06 UVIS 2.0: Encircled Energy, in preparation Filter-dependent values replace 2009 model
ISR 2016-07 Updated WFC3/UVIS Chip Dependent SYNPHOT/PYSYNPHOT Files Pysynphot files (Called by ETC)

ISR 2017-07

WFC3 Chip Dependent Photometry with the UV filters

Effect of  bandpass differences on UV photometry

ISR 2017-14

WFC3/UVIS Updated 2017 Chip-dependent Inverse Sensitivity Values

Improved in-flight solutions change by <1% from 2016
IR ISRs

ISR 2009-30

WFC3 SMOV Proposal 11451: The Photometric Performance and Calibration of WFC3/IR First In-flight photometric calibration
ISR 2009-37 WFC3 SMOV Programs 11437/9: IR On-orbit PSF Evaluation In-flight encircled energy
ISR 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

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.

 

 


Created 09/09/2009 MJD
Modified 03/06/2012 by CMP
Modified 06/07/2017 by HGK