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Overview

The new IR inverse sensitivities are based on observations of the four white dwarf standard stars, namely GD153, G191B2B, GD71, GRW+70D5824, and the G-type star,  P330E,  collected between 2009 and 2023. Changes relative to the 2020 values are described below.

New 2024 solution:

  • Inverse sensitivities: values have been updated to correct for time-dependent sensitivity losses measured to be up to 2% since installation, according to the filter.
  • Calibration pipeline: a new image photometry reference table (IMPHTTAB) was delivered to populate the FITS file image headers with time-dependent inverse sensitivity keywords. A new version of calwf3  has been released to enable the image processing with the time-dependent IMPHTTAB. 
  • New ISRs: ISR 2024-06 describes how the IR photometric sensitivity loss was evaluated using multiple observing methods and targets. ISR 2024-13 details the creation and delivery of the time-dependent IMPHTTAB.

IR Inverse Sensitivity (Zeropoint) Tables

2024 values:

Please note that we only provide values for an infinite aperture of radius 6 arcsec and for a reference epoch of MJD = 55008. Time-dependent values (for STMAG and ABMAG) can be calculated from the image headers; see notebook for calculating time-dependent VEGAMAG zeropoints.

Accordion

Callout

For previous WFC3/IR Zeropoint (Inverse Sensitivity) solutions, please visit:

Previous IR Photometric Calibrations

End callout

 

Infrared Repeatability

The repeatability of photometric measurements stars over the course of one to a handful of orbits is found to be approximately +/- 1.0% (ISR 2024-06; ISR 2024-01), despite Poisson noise terms being quite a bit smaller than 1%.  This is due to self-persistence from previous images in the visit, and can be mitigated by dithering, by at least 10 pixels between exposures, to ensure that stars do not fall on the same pixels.  Using this dither strategy, stars with very small Poisson noise components achieve repeatability of approximately 0.5% (ISR 2021-05; ISR 2019-07).

GD153-F160W
Figure 1: Aperture photometry measurements and residuals for the spectrophotometric standard star GD153 measured over several years by WFC3/IR in the F160W filter. Top plot: the dashed y-axis line at 1.00 corresponds to the mean value FLT measurements. Each point represents a single exposure of the star, and the error bars are the computed Poisson and background error of the photometry (and does not include other noise terms such as calibration uncertainty). Data were fit using linear regression; the resulting line of best fit has a slope of -0.083 +/- 0.022 %/year. Bottom plot: data have been detrended using the linear model, which reduces the scatter of the data. 1-sigma and 2-sigma ranges about 0.0 are indicated with dark and light shaded areas respectively. Note: a large majority of this data did not use the dithering strategy discussed above. Plot reproduced from ISR 2024-06.

Count-rate non-linearity

Previous analyses have shown that WFC3-IR suffers a count-rate dependent non-linearity of about 1% per dex, an order of magnitude smaller than the prior HgCdTe detector, NICMOS, flown on HST, but large enough to potentially limit the accuracy of photometry. In Riess et al. 2019 (ISR 2019-01) we present new and more precise measurements of count-rate non-linearity (CRNL) through a combination of comparisons of cluster star photometry between WFC3-IR and WFC3-UVIS and by using observed and synthetic magnitudes of white dwarfs. We further extend the measured range of CRNL to higher count rates by comparing magnitudes between the ground and WFC3-IR for LMC and Milky Way Cepheids. Combining these results with all previous measurements and those from the WFC3 grism provides a consistent and improved characterization of the CRNL of WF3-IR, of 0.75% +/- 0.06% per dex, with no apparent wavelength dependence, measured across 16 astronomical magnitudes. To illustrate the value of the precision reached for the CRNL, we show it is sufficient to compare the photometry of sources along a distance ladder calibrated by Gaia parallaxes and produce a 1% determination of the Hubble constant.

Due to the difficulty of measuring CRNL for WFC3-IR on orbit, a broad set of approaches have been used. The initial, on-orbit calibration of CRNL was produced by Riess (2010) who compared the magnitudes of stars in clusters between two overlapping passbands, one from an instrument without CRNL (the CCDs of ACS WFC) or corrected for CRNL (NICMOS) and WFC3-IR.

In Riess et al. (2019) we present additional sets of measurements which provide greater precision in the on-orbit determination of the WFC3-IR CRNL. First we present new measurements comparing cluster star magnitudes in bands where CCD’s overlap with WFC3-IR, F850LP, and F098M using a combination of deep and shallow frames to extend the dynamic range. We also provide calibrations of the CRNL at brighter magnitudes (F160W=6-14) using comparisons between the ground and WFC3-IR of the photometry of Cepheids in the Large Magenallanic Cloud and the Milky Way. We then use an independent approach where we compare the near-infrared photometry of hot, DA White Dwarf (WD) to models constrained by optical measurements.

We find the CRNL in WFC3-IR to be 0.0077 +/- 0.0008 mag/dex (or 0.0075 +/- 0.006 mag/dex including the grism measurements), characterized over 16 magnitudes with no apparent wavelength dependence and independent corroboration. This result is consistent with the initial determination by Riess (2010) of 0.010 +/- 0.002 mag/dex but the present results are much more precise. This result may be used to correct IR photometry by using the difference in apparent flux (in dex) between where the WFC3-IR zeropoint is set (~12th mag) and the target source (fainter sources appear even fainter and thus are corrected to be brighter).

 

Count-Rate Non-Linearity 1
Figure 2: A summary of the CRNL for WFC3-IR photometry measured over 16 astronomical magnitudes from various sources described in the text.​​​​

 

Count-Rate Non-Linearity 2
Figure 3: Magnitude ladder used to measure the CRNL for WFC3 over 16 astronomical magnitudes. Bottom plot shows residuals from the best fit 0.0077 +/- 0.0008 mag/dex result presented in Riess et al. (2019).

 

Calibration History

2020: Updated Inflight Calibration 

Delivery Calibration Reference File Description Change Documentation
Oct 15, 2020 IMPHTTAB
(Updated zeropoints)
4af1533ai_imp.fits

GRW+70l GD153, GD71, G191B2B, P330E

Monitoring data 2009 - 2019

<0.5% for wide filters

<2% for medium- and wide filters

~1.5% change from new white dwarf models

ISR 2020-10: Updated WFC3/IR Photometric Calibration

Bohlin et al 2020 AJ 160 21

Oct 06, 2020

SYNPHOT files

wfc3_ir_f*_007_syn.fits

wfc3_ir_cor_004_syn.fits

wfc3_ir_aper_002_syn.fits

2020 Inflight filter curves

2009 Inflight QE correction

2009 Inflight EE

   
Oct 15, 2020

PFLTFILE 

(Inflight flats)

4*_pfl.fits Filter-dependent Sky Flats ~0.5% residuals at the detector center and <2% at the edges ISR 2021-01: WFC3/IR Filter-Dependent Sky Flats

2012: Updated Inflight Calibration 

Delivery Calibration Reference File Description Change Documentation

Nov 19, 2012

First IMPHTTAB for WFC3/IR wbj1825ri_imp.fits

calwf3 no longer calls synphot to populate PHOT* keywords

(Solutions at infinity)
Uses Mar 2012 solution Software Update History for HSTCAL.CALWF3

Mar 06, 2012

Revised Inflight Zeropoint Tables Posted on website only Filter-dependent correction
Master DRZ frames per filter
Uses 2010 updated flats
(Solutions at infinity and 3 pix)
 

Prior Zeropoint Tables

ISR 2011-08: The Photometric Performance of WFC3/IR: Temporal Stability Through Year 1

Sept 28, 2012

SYNPHOT files

wfc3_ir_f*_004/005_syn.fits

wfc3_ir_cor_004_syn.fits

wfc3_ir_aper_002_syn.fits

2012 Inflight filter curves

2009 Inflight QE correction

2009 Inflight EE

   
Dec 07, 2010

PFLTFILE

(Inflight Flats)

u*pfl.fits

Ground flats corrected by a 'grey' delta-flat (based on the average sky flat for all filters)

~2% spatial residuals

ISR 2011-11: Sky Flats: Generating Improved WFC3 IR Flat-fields

2009: Early Calibration (Inflight + Ground)

Delivery Calibration Reference File Description Change Documentation

Sep  23, 2009

Nov 10, 2009

Mar 12, 2008

SYNPHOT files

wfc3_ir_cor_004_syn.fits

wfc3_ir_aper_002_syn.fits

wfc3_ir_f*_002/003_syn.fits

2009 Inflight QE correction

2009 Inflight EE

2008 Ground filter curves

  ISR 2009-37: WFC3 SMOV Programs 11437/9: IR On-orbit PSF Evaluation
Nov 18, 2009 Inflight Zeropoint Tables ISR 2009-30 table 4 Inflight polynomial correction with wavelength
GD153, P330E
FLT photometry
(Solutions at infinity and 3 pix)
10-15% higher sensitivity than ground test data ISR 2009-30: WFC3 SMOV Proposal 11451: The Photometric Performance and Calibration of WFC3/IR
Dec 10, 2008

PFLTFILE

(Ground-based  Flats)

s*pfl.fits

Thermal vacuum data

 

ISR 2008-28: WFC3 IR Ground P-Flats

Last Updated: 05/21/2025

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