## Overview

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

• Flat fields: Improved low-frequency in-flight corrections to the ground flats were computed in 2016 using CTE-corrected observations of Omega Centauri. The flats are normalized to the median value of each detector and no longer correct for sensitivity differences between CCDs. The photometric repeatability of white dwarf standards stepped across the field of view is better than 1% r.m.s. and 3% peak-to-peak over the full wavelength range of the detector (ISR 2016-04 and 2016-05).
• Inverse Sensitivity: The current chip-dependent inverse sensitivity values are systematically ~3.7% brighter than the 2012 solutions. Using the default AstroDrizzle configuration parameters when combining images with different signal to noise and telescope orientation on the sky can result in ‘clipping’ of pixel values in the PSF wings. This effect was discovered in the 2012 solutions and was corrected in the 2016 implementation.

## UVIS Inverse Sensitivity (Zeropoint) Tables

### 2017 Solution

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.

### 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.

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.

2017 Photometry Values for UltraViolet filters

Prior Calibration

## 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 are scaled to the UVIS1 images based on the count rate ratio of the hot white dwarf standard stars in order to facilitate the pipeline drizzling process and maintain the relative photometry in counts (for details see ISR 2017-07). For cooler red sources, the difference can be larger, and a correction to the UVIS2 magnitudes has to be applied. For details about the UV filter color term corrections please see ISR 2018-08.

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.

## Ultraviolet color terms

The UVIS1 and UVIS2 detectors have different quantum efficiencies in the ultra-violet (UV) regime (lλ < 4,000 Å): count rate ratios change as a function of the spectral type of the source. When calibrating photometry of stars cooler than Teff ~ 30,000 K in the UV filters (e.g. when observing open and globular clusters, resolved local group galaxies, Galactic stellar populations), color term transformations need to be applied to UVIS2 magnitudes. Sources of any spectral type observed on one detector only will not require any magnitude offset.

The left panel of the figure below shows the synthetic ST magnitude difference, UVIS1 – UVIS2, for a sample of CALSPEC stars of varying spectral type (the white dwarfs (WDs) include G191B2B, GD71 and GD153), as computed for three UV filters, F218W, F225W, F275W. Cool red sources such as the G-type star P330E measured on UVIS2 have a magnitude offset relative to UVIS1 up to ~0.08 mag when observed with the F225W filter. F-type stars such as HD160617 have a magnitude offset of ~0.04 mag.

The right panel of the figure shows the ST magnitude difference for the F225W and F275W filters for ω Cen Extreme Horizontal Branch (EHB), Horizontal Branch (HB), Main Sequence (MS), and Red-Giant Branch (RGB) stars measured on different detectors and amplifiers, A (UVIS1) and C (UVIS2). WFC3 observations of ω Cen validate the results of synthetic photometry: red stars (RGBs) observed on UVIS2 have a magnitude offset up to 0.08 mag relative to UVIS1.

Calamida et al. 2018 ( WFC3 ISR-2018-08) provides lookup tables with color term transformations to be applied to UVIS2 magnitudes when observing with three UV filters, F218W, F225W, and F275W. UVIS2 magnitudes are scaled to UVIS1 by the WFC3 calibration pipeline (cal_wf3) using UVIS2 to UVIS1 modified inverse sensitivity ratio, PHTRATIO. PHTRATIO is derived using photometry of the CALSPEC WDs and is valid for hot stars, Teff > 30,000 K. For cooler stars, when observing with UV filters, PHTRATIO is not equal to the ratio of the two detectors' count rates but changes with the stellar spectral type. Photometry for cooler stars measured on UVIS2 needs to be corrected by applying a magnitude offset according to their color, if available, or temperature or spectral type.

Before applying the offset to magnitudes measured on UVIS2, photometry must be calibrated using UVIS1 inverse sensitivities provided here.

STMag (UVIS1) = -21.1 -2.5 x log(PHOTFLAM)

STMag (UVIS2) = -21.1 -2.5 x log(PHOTFLAM) + Delta (Mag)

where Delta (Mag) = Mag(UVIS1 – UVIS2) is listed in lookup tables in WFC3 ISR-2018-08. For more details and to consult the color transformation tables please see the referred ISR.

## Prior Calibration

Delivery Calibration Reference File CALWF3 version Description Change Documentation

Apr 30, 2009

Jun 11, 2009

Jul  16, 2009

Ground Flats

t*pfl.fits

v1.4+

Thermal vacuum data, most filters

UV filters

Nov 18, 2009

In-flight Zeropoints (infinite)

Polynomial correction with wavelength

GD153, GRW+70d5824

FLT*PAM

10-20% higher sensitivity than ground test data

2009-31

Nov 10, 2009

In-flight EE

Based on deep observation in the F275W and F625W filters.

Other filters from optical models

2009-38

Dec 14, 2011

In-flight Flats

v*pfl.fits

Flare removal

L-flat from Omega Centauri

3-5% improvement

2013-10

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:

Aug 29, 2012

In-flight Flats (binned)

w*pfl.fits

Dec 2011 solutions for 2x2 and 3x3 modes

(2012-04)

Nov 15, 2013

First UVIS IMPHTTAB
(infinite)

x5h1320fi_imp.fits

v3.1.6+

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

HSTCAL Webpage

Feb 23, 2016

Chip-dependent calibration

v3.3+

Description of the new methodology

2016-01

2016-02

Feb 23, 2016

Zeropoints
(10 pix)

zcv2057li_imp.fits

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

GD153, GD71, G191B2B

3-4% change from 2012

2016-03

Feb 23, 2016

Flats

UV Flats

z*pfl.fits

L-flat from CTE-corrected Omega Cen

Temperature-correction

1% for most filters

Up to 3% for UV filters

2016-04

2016-05

Apr 20, 2016

PySynphot files

wfc3uvis*syn.fits

2016-07

Nov 21, 2016

UV zeropoints
(10 pix)

0bi2206ti_imp.fits

Equalize UV count-rate across chips for blue sources

2% in F225W for

1% in F218W, F275W

2017-07

Jun 12, 2017

Zeropoints
(infinite)

1681905hi_imp.fits

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

~10% due to change in standard aperture

2017-14

LAST UPDATED: 10/09/2019