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Hubble Space Telescope
UVIS Photometric Calibration

Table of Contents


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 and GD71) obtained between 2009 and 2015.

Changes from 2012

UVIS Inverse Sensitivity (Zeropoint) Tables

2017 solutions:

The inverse sensitivity values written to the PHOTFLAM image header keyword are the CORRECT values for flux calibration for all except the UV filters observed on UVIS2.   The IMPHTTAB carries the values for UVIS1 via the keywords PHOTFLAM and PHTFLAM1, and these are identical with the exception of the UV filters.  Inverse sensitivity values for UVIS2 are written to the keyword PHTFLAM2 but should not be used for flux calibration with calibrated data products retrieved from MAST, since calwf3 by default will scale the UVIS2 chip by the chip sensitivity ratio to match UVIS1.

2017 Photometry Values for UV filters link here

For UV filters with UVIS2 (F218W, F225W, F275W and F200LP), the PHTFLAM1 values should be used for flux calibration. These values have been adjusted so that the chip sensitivity ratio PHTRATIO (PHTFLAM2/PHTFLAM1) used to scale UVIS2 data,  matches the count rate ratio for sources similar in color to the white dwarf standards.

Note:  For precise photometry with the UV filters, users may wish to 'back out' the chip-sensitivity ratio scaling and apply separate zeropoints for each chip. For details, see ISR 2017-07.


Color Terms

Due to bandpass differences in the UV, the sensitivity ratio,  phtflam2/phtflam1, differs by up to ~5% percent from the ratio of the counts in UVIS1 to the counts in UVIS2, depending on the filter.  For the UV filters (F200LP, F218W, F225W and F275W), the UVIS2 images is scaled to the UVIS1 image based on count rate ratio of the hot, white dwarf standard stars in order to facilitate pipeline drizzling and maintain the relative photometry in counts   For red sources, the difference can be larger.  See also ISR 2017-07 for more details.

Pysynphot may be used along with the synthetic photometry tables to estimate the color terms of spectral type, or object color.   For more detail on computing color terms, see ISR 2014-16.

Vega Magnitudes

The 2017 VEGAMAG values are now computed using the CALSPEC STIS spectrum for Vega  (alpha_lyr_stis_008.fits) and supercede  the 2016 values which used the  CALSPEC Vega model (alpha_lyr_mod_002.fits).  Significant differences in between the model and STIS spectrum zeropoints are found in the  UV, up to 0.1 mag,  shortward of ~4000 Angstroms, since the model is less accurate in this region of the electromagnetic spectrum, where the physics of Vega is less well-understood.

Prior Calibration

Apr 30, 2009

Jun 11, 2009

Jul  16, 2009

Ground Flats



Thermal vacuum data, most filters

UV filters





Nov 18, 2009

In-flight Zeropoints (infinite)


Polynomial correction with wavelength

GD153, GRW+70d5824


10-20% higher sensitivity than ground test data


Click Here

Nov 10, 2009

In-flight EE


Based on deep observation in the F275W and F625W filters.

Other filters from optical models



Dec 14, 2011

In-flight Flats



Flare removal

L-flat from Omega Centauri

3-5% improvement


Mar 6, 2012

Revised Zeropoints

(infinite and 10 pix)


Filter-dependent correction

GD153, GD71, G191B2B, P330E

Master DRZ frames, per filter

Up to 5% filter correction

Available via webpage only: 

Infinite aperture
R=10 pixels

Aug 29, 2012

In-flight Flats (binned)



Dec 2011 solutions for 2x2 and 3x3 modes



Nov 15, 2013

First IMPHTTAB for UVIS (infinite)



HSTCAL replaces calls to STSDAS/synphot in calwf3 to populate PHOT keywords


HSTCAL Webpage

Feb 23, 2016

Chip-dependent calibration



Description of the new methodology




Feb 23, 2016

        Zeropoints (10 pix) 




First chip-dependent lookup table from 'master' DRZ frames, per chip per filter

GD153, GD71, G191B2B

3-4% change from 2012


Click here

Feb 23, 2016


        UV Flats



L-flat from CTE-corrected Omega Cen


1% for most filters

Up to 3% for UV filters



Apr 20, 2016

        PySynphot files




Nov 21, 2016

        UV zeropoints (10 pix)



Equalize UV count-rate across chips for blue sources

2% in F225W for

1% in F218W, F275W


Jun 12, 2017

        Zeropoints (infinite)



Better polynomial fits to data and updated models, matches April 2016 synphot delivery

~10% due to change in standard aperture