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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, GRW+70D5824, and GD71) and the G-type sun-like standard P330E obtained between 2009 and 2019.

New 2020 Solutions:

  • Models: Models for the CALSPEC standard white dwarfs were improved and the Vega reference flux was increased (Bohlin et al. 2020). As a consequence, the standard white dwarf fluxes increase by ~2% for wavelengths in the range 0.15-0.4 micron and ~1.5% in the range 0.4-1 micron covered by the UVIS detector. 
  • Inverse sensitivity:  Corrected the image header 'inverse sensitivity keywords' to account for changes of ~0.1-0.2% per year according to the filter.
  • Chip sensitivity ratioImproved the chip-sensitivity ratio (PHTRATIO) by up to 1%, in agreement with early dithered star cluster and dithered standard white dwarf observations.
  • Encircled Energy: Improved the encircled energy (EE) correction by 1% in the ultraviolet filters and by 0.5% at wavelengths larger than 7,500 A, in close agreement with the 2009 EE values.
  • New ISR and paper:  ISR 2021-04 describes how the new UVIS inverse sensitivities were derived, while Calamida et al. (2022) details the updated calibrations for both detectors.
  • Jupyter notebooks: Notebooks that correct UVIS data for time sensitivity and that use stsynphot to calculate the latest UVIS and IR photometric zeropoints for any observation date are available through WFC3 Notebooks.

UVIS Inverse Sensitivity (Zeropoint) Tables

2020 Solution

The new calibration provides the inverse sensitivity values that take into account the sensitivity change with time of the UVIS detectors (see Figure 1). The IMPHTTAB carries the values for UVIS1 and UVIS2 via the keywords PHOTFLAM, PHTFLAM1, and PHTFLAM2 at the epoch of the observation.

 

 

Accordion

A plot with "MJD" on the x-axis and "% change, F814W" on the y-axis. There are many points on the graph, with different shapes representing the different targets (GRW70, GD153, GD71, G191B2B, P330E). Staring mode observations are black for Amp C and red for Amp D. GRW70 spatial scans are blue, and GD153 spatial scans are green; all scans are on Amp C.

Figure 1: Aperture photometry in a 10-pixel radius for five CALSPEC standards in the F814W filter versus observation date (in MJD). Monitoring observations of four white dwarfs GRW+70d5824 (filled circle), GD153(diamond), GD71 (square), G191B2B (triangle), and the G-type star P330E (horizontal triangle) are shown for two UVIS2 subarrays, where black points indicate drizzled (drc) photometry for amp C and red points for amp D. For comparison, photometry in the corresponding single calibrated (flc) images is shown in grey. Photometry on scanned images is overplotted for GRW+70d5824 (blue) and GD153 (green). The solid line shows the fit to the photometry for all five standards, for both staring and scanned images, and indicates a loss in sensitivity of 0.11% per year +/- 0.07% in this filter.

Plot reproduced from the WFC3 Data Handbook, Section 9.1.

 

 

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.

Figure 2 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.

Figure 3 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 (see UV Filter Modified Keywords for more background and for a table of relevant values). 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.

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; for examples on how to derive color transformations between different filters of the detector, see this Jupyter notebook.

 

Figure 2: 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.
Figure 2: 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. 
Figure 3: 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).
Figure 3: 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). 

 

Calibration History

Delivery Calibration Reference File Description Changes Documentation
Feb 10, 2021 Inverse sensitivities, EE   Time-dependent inverse sensitivities; updated inverse sensitivities (quad filters); new EE values for several filters  

ISR 2021-04: New time-dependent WFC3 UVIS inverse sensitivities

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

ISR 2017-14: WFC3/UVIS Updated 2017 Chip-Dependent Inverse Sensitivity Values

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

ISR 2017-07: WFC3 Chip Dependent Photometry with the UV Filters

Apr 20, 2016

PySynphot files wfc3uvis*syn.fits    

ISR 2016-07: Updated WFC3/UVIS Chip Dependent SYNPHOT/PYSYNPHOT Files

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

ISR 2016-04: UVIS 2.0: Chip-Dependent Flats

ISR 2016-05: UVIS 2.0: Ultraviolet Flats

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

ISR 2016-03: UVIS 2.0: Chip-dependent Inverse Sensitivity Values

Feb 23, 2016

Chip-dependent calibration   Implements chip-dependent photometric calibration and CTE corrects in pipeline  

ISR 2016-01: The Updated Calibration Pipeline for WFC3/UVIS: A Reference Guide for calwf3 (version 3.3)

Nov 15, 2013

First UVIS IMPHTTAB 
(infinite)
x5h1320fi_imp.fits HSTCAL replaces calls to STSDAS/synphot in calwf3 to populate PHOT keywords  

Software Update History for HSTCAL.CALWF3

Aug 29, 2012

In-flight Flats (binned) w*pfl.fits Dec 2011 solutions for 2x2 and 3x3 modes  

ISR 2012-04: Binned flat fields for the WFC3/UVIS Channel

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 Previous UVIS Photometric Calibrations

Dec 14, 2011

In-flight Flats v*pfl.fits

Flare removal

L-flat from Omega Centauri

3-5% improvement

ISR 2013-10: In-flight Corrections to the WFC3 UVIS Flat Fields

Nov 10, 2009

In-flight EE  

Based on deep observation in the F275W and F625W filters.

Other filters from optical models

 

ISR 2009-38: WFC3 SMOV Programs 11436/8: UVIS On-orbit PSF Evaluation

Nov 18, 2009

In-flight zeropoints (infinite)  

Polynomial correction with wavelength

GD153, GRW+70d5824

FLT*PAM

10-20% higher sensitivity than ground test data

ISR 2009-31: WFC3 SMOV Proposal 11450: The Photometric Performance and Calibration of WFC3/UVIS

Previous UVIS Photometric Calibrations

Apr 30, 2009

Jun 11, 2009

Jul  16, 2009

Ground Flats

t*pfl.fits

Thermal vacuum data, most filters

UV filters

Quads

 

ISR 2008-46: WFC3 UVIS Ground P-flats

ISR 2008-12: UVIS CASTLE Photometric Filter Flat Field Atlas

 

Last Updated: 03/22/2024

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