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ACS Zeropoints and Flux Calibration

Note, all archival ACS/CCD data retrieved prior to August 12th, 2016 are populated with out-of-date photometric flux calibration keywords.

ACS WFC and HRC Zeropoints Calculator

Aperture Correction Tables

SBC Zeropoints Table

Outdated WFC and HRC Zeropoints

The results presented by the ACS zeropoint calculator are derived using the most recent infinite aperture (5.5") flux calibrations and zeropoint values for the ACS CCDs detailed in Bohlin (2016, hereafter ISR 16-03).

ISR 16-03 continues the work documented in ISR 11-02, ISR 11-03, and ISR 12-01 by performing the most precise set of flux calibrations for the HRC and WFC cameras to date. The data are comprised of ~18 orbits of the primary hot White Dwarf (WD) stars used to define the ACS flux calibration and its change over time, G191B2B, GD153, GD71, spanning a ~7 year period from August of 2009 to March of 2016. The average decline in sensitivity is 0.061% ± 0.007% per year across all filters, which differs at most by < 1σ from the measured rate of 0.035% ± 0.037% for F850LP; therefore, the average loss rate is adopted for all WFC data taken after SM4 (2009.4). Even with gain adjustments made after the revival of the ACS WFC to match post-SM4 photometry to 2002.4 epoch photometry of 47 Tuc for F606W (ISR 11-03), an additional (small) shift in sensitivity by 0.997 improves the match at F606W. These adjustments range from 0.996 to 1.007 for the other filters. After applying corrections for sensitivity loss and renormalizing, the new observations and newly reprocessed data for F435W and F814W required shifts to each filter's bandpass transmission function to reduce the maximum error. Once all of the adjustments are made, the fully corrected photometry at the WFC1-1K reference point in the eight broadband filters achieves the 1% precision goal.

The ACS absolute flux calibration is represented by the PHOTFLAM keyword of the ACS image headers (see Section 2.4 of the ACS Data Handbook) and is applicable to the geometrically corrected drizzle pipeline products (*_drz.fits or *_drc.fits). In order to extract photometry from non-geometrically corrected pipeline products (*_flt.fits, *_flc.fits), the appropriate pixel area maps must be applied before flux calibration with the PHOTFLAM keyword.

Comparing the WFC PHOTFLAM results of ISR 16-03 with Sirianni et al. (2005, hereafter S05) for 2002.16 epoch reveals differences from 5% smaller for F660N to 1% larger for F550M, while the eight broadband filters are 1-3% smaller. For HRC, differences range from 0-3% smaller, except for F344N, where the new calibration is 3% larger. These differences are due mostly to changes in the CALSPEC reference spectral energy distributions (SEDs) and to a refined and expanded set of ACS observations of these standard stars.

Photometric Systems:

where Fν is expressed in erg cm-2 s-1 Hz-1, and Fλ in erg cm-2 s-1-1. An object with a constant flux distribution Fν= 3.63 x 10-20 erg cm-2 s-1 Hz-1 at all wavelengths will have ABmag=0 at all wavelengths, and similarly an object with Fλ= 3.63 x 10-9 erg cm-2 s-1-1 will have STmag=0.

Photometric Keywords in the SCI extention of ACS images:

(Keywords affected by the sensitivity curve update are in bold font.)

The PHOTFLAM and PHOTPLAM header keywords are used to derive the instrumental zeropoint magnitudes, which are defined to be the magnitude of an object that produces one count per second. The instrumental zeropoint magnitudes are defind by the following formulae:

Average Flux of Vega in ACS Filters

ACS/WFC ACS/HRC ACS/SBC
FILTER Vega Flux
F435W 6.377e-09
F475W 5.488e-09
F502N 4.635e-09
F550M 3.395e-09
F555W 3.827e-09
F606W 2.853e-09
F625W 2.347e-09
F658N 1.808e-09
F660N 1.962e-09
F775W 1.274e-09
F814W 1.101e-09
F850LP 8.586e-10
F892N 9.291e-10
FILTER Vega Flux
F220W 4.224e-09
F250W 3.797e-09
F330W 3.207e-09
F344N 3.204e-09
F435W 6.668e-09
F475W 5.356e-09
F502N 4.634e-09
F550M 3.397e-09
F555W 3.839e-09
F606W 2.882e-09
F625W 2.365e-09
F658N 1.804e-09
F660N 1.961e-09
F775W 1.288e-09
F814W 1.077e-09
F850LP 8.761e-10
F892N 9.297e-10
FILTER Vega Flux
F115LP 6.075e-09
F122M 1.079e-09
F125LP 6.4078e-09
F140LP 6.630e-09
F150LP 7.021e-09
F165LP 6.535e-09

All WFC and HRC flux calibrations and zeropoints are given for an infinite aperture, where the radius of the infinite aperture is defined to be 5.5" following the convention set by S05. Any photometric measurements made using an aperture smaller than the defined "infinite" aperture must have the proper WFC/HRC derived aperture corrections applied to account for source flux falling outside the chosen aperture radius; refer to the Flux bullet in the Photometric Systems section.

ACS/SBC Zeropoints

For all SBC observations, regardless of date.
FILTER PHOTPLAM PHOTFLAM STmag VEGAmag ABmag
F115LP1406.6471.492e-1720.96621.20523.917
F125LP1438.1961.722e-1720.81021.18123.713
F140LP1527.9912.713e-1720.31620.91923.088
F150LP1612.2164.392e-1719.79420.44222.448
F165LP1762.5361.360e-1618.56719.15521.028
F122M1273.7452.810e-1617.77917.00820.945
PR110L1430.091.780e-1720.77421.10623.689
PR130L1439.4011.764e-1720.78421.15623.685

All SBC flux calibrations and zeropoints are given for an infinite aperture, where the radius of the infinite aperture is defined to be 4". Any photometric measurements made using an aperture smaller than the defined "infinite" aperture must have the proper SBC derived aperture corrections applied to account for source flux falling outside the chosen aperture radius; refer to the Flux bullet in the Photometric Systems section.

REFERENCES

Last modified on Nov 17, 2016.