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ACS Zeropoints

WARNING: These zeropoints are out of date and given only for reference.

Click here to go directly to ACS Zeropoint tables.

Improved Detector Quantum Efficiency curves were delivered to SYNPHOT on Sep 19, 2007. Sirianni's published zeropoints should no longer be used for calibration. Instead, the tables below reflect the most up-to-date zeropoint values for the ACS CCDs.

The zeropoints discussed here and contained in the ACS image headers refer to the drizzled pipeline products (_drz.fits files) which are corrected for geometric distortion. To perform photometry on non-geometrically corrected output products (_flt.fits files), the appropriate pixel area maps must be applied.

The tables below give the photometric zeropoints for the ACS CCD cameras calculated using SYNPHOT. The zeropoints are defined as the magnitude of a star which produces a count rate of 1 electron per second in a given filter. The STmag, ABmag and VEGAmag magnitude systems are defined below and also in Chapter 3 of the Synphot User's Guide. In short, VEGAmag is a standard magnitude system, for which Vega by definition has magnitude 0 at all wavelengths. The ABmag and STmag magnitude systems are based on constant flux per unit frequency and per unit wavelength, respectively. The zero points for these two systems are set for convenience so that Vega has a magnitude of approximately 0 in both systems for the Johnson V passband.

The ACS sensitivity files are revised periodically, and the HST Archive will appropriately populate the photometric keywords written to the image header. These keywords are defined in detail in Section 2.4 of the ACS Data Handbook. Reretrieving your data from the archive will provide an identical dataset, and only a few keywords values will change. In lieu of obtaining new archival products, you may simply use the revised photometric keywords given in the tables below.

Please note that SYNPHOT zeropoints are given for an "infinite" aperture. If you are performing surface photometry, you can use the zeropoints as they are. However, for point source photometry an "infinite" aperture is clearly not possible and an aperture correction is needed. Unfortunately there is not a unique correction from a given aperture (say 0.5") to an infinite aperture for all filters. The correction depends on the filter and, for a few red filters, the spectrum of the object. More detailed information on aperture corrections can be found in Sirianni et al., 2005, PASP, 117, 10495.

Revised ACS zeropoints posted May 19, 2009

The zeropoints published in Sirianni et al. 2005 and in ISR07-02 are now out of date. The zeropoints presented in the tables below reflect the following changes:

1. An improved photometric calibration of the ACS CCD cameras has been computed by Bohlin see ACS-ISR 07-06, from analysis of spectrophotometric standard stars. The revised QE curves impact all CCD observations, regardless of observation date.

2. An improved stellar spectrum of the HST flux standard Vega (Bohlin 2007). was delivered to SYNPHOT in Sept 2006. This impacts the VEGAmag zeropoints .

3. Following the recovery of ACS with the side-2 electronics in July 2006, the temperature of the WFC detector was lowered from -77C to -80C, in an effort to mitigate the impact of CTE and hot pixels. As predicted, the temperature change resulted a slight loss in detector sensitivity. A revised set of photometric zeropoints has been computed for all WFC observations obtained after July 4, 2006. (see ACS ISR 07-02)

All archival data retrieved prior Sept 19, 2007 will be populated with out-of-date photometric keywords. Instead, the tables given here should be used for absolute calibration of ACS CCD observations.

NOTE: The total throughput curve (HST+ACS) can be downloaded by clicking on the filter name

ACS/WFC Zeropoints at -77 C

Note: For all data obtained before July 4, 2006
FILTER PHOTPLAM PHOTFLAM PHOTBW STmag VEGAmag ABmag
F435W4317.9093.109634E-19293.443525.1682325.792825.68392
F475W4745.2931.786176E-19420.137825.7701926.184426.08093
F502N5022.9695.254536E-1828.7279222.0985522.3749722.28579
F550M5581.3153.797236E-19163.149624.9513724.8854624.90975
F555W5360.1421.925796E-19360.068525.6884825.7438325.73465
F606W5919.3697.766405E-20672.375326.6744426.4202826.50512
F625W6311.0061.172743E-19415.447626.2270125.7553625.91856
F658N6583.9771.959716E-1837.0104823.1694922.3932822.7691
F660N6599.3825.241119E-1835.2203922.1009421.4157821.69548
F775W7693.2619.844788E-20434.593826.4169925.290625.67849
F814W8059.8586.925557E-20653.003226.7988725.5356125.95928
F850LP9036.4191.507213E-19527.200625.9545624.3471124.86663
F892N8914.8291.507729E-1872.4120123.4539421.9062922.39543

ACS/WFC Zeropoints at -81 C

Note: For all data obtained after July 4, 2006
FILTER PHOTPLAM PHOTFLAM PHOTBW STmag VEGAmag ABmag
F435W4318.8623.184049E-19293.373625.1425625.7669525.65777
F475W4746.9211.820979E-19420.155125.7492426.1625226.05923
F502N5022.9775.344764E-1828.6430122.0800622.3564822.2673
F550M5581.4333.848120E-19163.096624.9369324.8709424.89525
F555W5360.9721.954235E-19360.093125.6725625.7274725.7184
F606W5921.1247.862481E-20672.24926.6610926.4059826.49113
F625W6311.3691.185525E-19415.375926.2152425.7433925.90667
F658N6583.9881.979690E-1836.8474623.1584822.3822822.7581
F660N6599.365.294996E-1834.9526722.0899821.4048121.68452
F775W7692.4179.969696E-20434.408726.403325.2772825.66504
F814W8056.9487.033186E-20651.949826.7821325.5199425.94333
F850LP9033.1081.542299E-19525.713225.9295924.3230524.84245
F892N8914.8131.539083E-1872.3173823.4320421.8845122.37353

ACS/HRC Zeropoints at -80 C

Note: For all HRC observations, regardless of date.
FILTER PHOTPLAM PHOTFLAM PHOTBW STmag VEGAmag ABmag
F220W2255.2868.072094E-18187.112721.6325521.8798623.55868
F250W2715.8464.756412E-18239.40622.2067922.2379423.72936
F330W3362.6922.225162E-18173.720423.0316122.9130724.09021
F344N3433.7112.130053E-1727.5464220.5791220.4463121.59233
F435W4310.9665.342467E-19309.552724.5806525.1933725.09983
F475W4775.6372.923535E-19418.800925.2352325.6315325.53213
F502N5022.98.003152E-1836.4577221.6417221.9181121.82902
F550M5579.8225.961178E-19165.188224.461724.396624.42066
F555W5355.8493.007370E-19357.17725.2045325.2623725.25245
F606W5887.5331.273481E-19664.714226.1375125.9006925.9799
F625W6295.3291.966458E-19415.096825.665825.2026325.36276
F658N6583.7043.347227E-1857.9108622.5882621.8119622.18797
F660N6599.0918.957878E-1874.6853221.5189720.8337821.11362
F775W7664.8031.946614E-19431.988325.6768124.5630224.94635
F814W8115.451.268850E-19702.409126.1414824.8614725.28697
F850LP9144.6312.289323E-19538.778425.5007323.865524.38695
F892N8916.0822.448235E-18101.269222.9276221.3810521.8688

ACS/SBC Zeropoints

Note: For all SBC observations, regardless of date.
FILTER PHOTPLAM PHOTFLAM PHOTBW STmag VEGAmag ABmag
F115LP1406.6471.491654E-17151.367520.9658721.2052223.91702
F125LP1438.1961.721834E-17142.876620.8100321.1815323.71301
F140LP1527.9912.712900E-17126.690220.3164220.9189623.08787
F150LP1612.2164.391667E-17113.067819.7934520.442322.4484
F165LP1762.5361.359691E-1698.3406418.5664519.1548121.0278
F122M1273.7452.809779E-1690.879317.7786617.0078420.94535
PR110L1430.091.779840E-17151.908220.7740721.1058223.68932
PR130L1439.4011.763869E-17143.898320.7838621.1560823.68501

Photometric Systems:

Photometric Keywords in the SCI extention of ACS images:

(keywords affected by the sensitivity curve update are highlighted) The header keywords PHOTFLAM and PHOTPLAM relate to the STMAG and ABMAG zeropoints through the formulae:

Last modified Nov 17, 2010.