Overview

IR photometry is currently based on derived quantities from observations of the three HST white dwarf standard stars,  GD153, G191B2B, GD71, and the G-type star,  P330E,  acquired between 2009 and 2012. Data were reduced with CALWF3 and  processed with AstroDrizzle.The independent calibrations from the four stars agree to within 1% in most filters.  The inverse sensitivity per filter,  PHOTFLAM, is based on the average of the measurements.

The tables below are the derived quantities from observations of GD153, G191B2B, GD71, and P330E acquired between 2009 and 2012. The independent calibrations from the four stars agree to within 1% in most filters, and the photometric zeropoint is set to the average of the measurements. The data sets were processed with AstroDrizzle.

IR Inverse Sensitivity (Zeropoint) Tables

The 2012 PHOTFLAM values are provided for the infinite aperture (defined as having a radius of 6 arcseconds) and  for an aperture of radius r=0.4 arcseconds (corresponding to a radius of about 3 pixels)

IR values of the inverse sensitivity (photflam) and photometric zeropoints in the STmag, ABmag and VegaMag systems

Infrared Repeatability

The repeatability of photometric measurements stars over the course of one to a handful of orbits is found to be approximately +/- 1.5%, 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%.  More information can be found in ISR 2019-07

 

GD153-F160W
Figure: Aperture photometry measurements of the spectrophotometric standard star GD153 measured over several years by WFC3/IR in the F160W filter. The 1.00 line on the y axis corresponds to the median value FLT measurements. Each point represents a single exposure of the star, and the various colors/colorbar represent the exposure time of the observation. The error bars are the computed Poisson and background error of the photometry (and does not include other noise terms such as calibration uncertainty). Note that a large majority of this data did not use the dithering strategy discussed above.

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 (WFC3-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: 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: 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).

 

Prior Calibration

Delivery Calibration Reference File CALWF3 version Description Change Documentation
Dec 10, 2008 Ground Flats s*pfl.fits v1.4+ Thermal vacuum data   2008-28

Nov 18, 2009

In-flight Zeropoints

(infinite)

   

Polynomial correction with wavelength

GD 153, P330E

FLT*PAM

10-15% higher sensitivity than ground test data

2009-30

Nov 10, 2009

In-flight EE

   

F098M, F105W and F160W

Other filters from optical models

 

2009-37

Dec 7, 2010

In-flight Flats

u*pfl.fits

 

Ground flats corrected by a 'grey' delta-flat

Computed from average sky flat for all filters

+/- 2% correction

2011-11

Mar 06, 2012

Revised Zeropoints

(Infinite and 3 pix)

   

Filter-dependent correction

GD153, GD71, G191B2B, P330E

Master DRZ frames, per filter

 

Available via this WFC3 webpage only: Kalirai et al.

(see above tables)

Nov 19, 2012

First IMPHTTAB for IR

(infinite)

wbj1825ri_imp.fits

v3.1.6+

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

 

HSTCAL Webpage

 

LAST UPDATED: 10/09/2019

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