ACS Zeropoints

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 magnitude 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 Sep 19, 2007

The zeropoints published in Sirianni et al. 2005 and in ISR07-02 are now out of date.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.

COMING SOON! Improved SBC Zeropoints!

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

ACS/WFC Zeropoints at -77 C

• VEGAmag : SYNPHOT standard magnitude system, for which Vega by definition has magnitude 0 at all wavelengths. The vega magnitude of a star with flux F is -2.5 log10 (F/F_vega) where F_vega is the calibrated spectrum of Vega in Synphot.
• STmag and ABmag: both systems define an equivalent flux density for a source, corresponding to the flux density of a source of predefined spectral shape that would produce the observed count rate, and convert this equivalent flux to a magnitude. The conversion is chosen so that the magnitude in V corresponds roughly to that in the Johnson system. In the STmag system, the flux density is expressed per unit wavelength, and the reference spectrum is flat in F_lam , while in the ABmag system, the flux density is expressed per unit frequency , and the reference spectrum is flat in F_nu. The definitions are:
  • STmag = -2.5 Log F_lam -21.10
  • ABmag = -2.5 Log F_nu - 48.6
  where F_nu is expressed in erg cm^-2 s^-1 Hz^-1, and F_lam in erg cm^-2 s^-1 Ang^-1. Another way to express these zeropoints is to say that an object with F_nu = 3.63 x 10^-20 erg cm^-2 s^-1 Hz^-1 will have magnitude AB =0 in every filter, and an object with F_lam = 3.63 x 10^-9 erg cm^-2 s^-1 Ang^-1 will have magnitude ST=0 in every filter.

• PHOTMODE: Observation configuration for photometric calibration.
• PHOTFLAM: inverse sensitivity (erg cm^-2 s^-1 Ang^-1).
• PHOTZPT: ST magnitude zeropopint (= 21.10).
• PHOTPLAM: pivot wavelength.
• PHOTBW: rms bandwidth of filter plus detector.

• STMAG_ZEROPOINT = -2.5 Log (PHOTFLAM) - PHOTZPT = -2.5 Log (PHOTFLAM) - 21.10
• ABMAG_ZEROPOINT = -2.5 Log (PHOTFLAM) - 21.10 - 5 Log (PHOTPLAM) + 18.6921

Last modified November 6, 2007.