Revised detector quantum efficiency and filter transmission curves were delivered to SYNPHOT on March 22, 2012. The Sirianni et al. (2005, hereafter S05) published zeropoints should no longer be used. The results presented here reflect the most up-to-date absolute flux calibration and zeropoint values for the ACS CCDs.
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 drizzled pipeline products (_drz.fits or _drc.fits files). For photometry from non-geometrically corrected output products (_flt.fits, _flc.fits, _crj.fits, or _crc.fits files), the appropriate pixel area maps must be applied before flux calibration with the PHOTFLAMs.
Bohlin (2012, hereafter ISR-IV) revised the ACS flux calibration for the standard filters in the HRC and WFC cameras. This new calibration accounts for the loss of sensitivity over time through 2007.1, which is as much as 0.6% per year for the HRC F220W but is less than 0.3%/yr for F330W and longer wavelength filters in both CCD cameras (Bohlin, Mack, & Ubeda 2011). Retrieving old data from the archive will provide the new PHOTFLAM keyword value that is appropriate for the epoch of the observation. The smooth trend toward lower sensitivities has a discontinuity for WFC at 2006.5, when the operating temperature was lowered from -77C to -81C. Following SM4, there are insufficient data to define trends, so the sensitivities are constant after 2009.4.
All archival ACS/CCD data retrieved prior March 22, 2012 are populated with out-of-date photometric flux calibration PHOTFLAM keywords. Changes are due to the implementation of corrections to the changing sensitivity with time in table 1 of Bohlin, Mack, & Ubeda (2011) and to the updated QE and filter transmissions in table 5 of Bohlin (2012). After SM4 on 2009.4 when the ACS/WFC was revived, the gain was set to approximately reproduce the sensitivity at launch. Figure 3 of Bohlin, Mack, & Ubeda (2011) show that the redefined sensitivities were successfully reset to within +/- 1% of the initial values. Currently, there are not enough data to define trends with time, so that the post-SM4 sensitivities remain constant at the 2009.4 values. A QE update is required only for HRC, where data processing improvements mimic a time-independent sensitivity increase by as much as 1.9% for F220W. The filter transmission changes for the wide filters are typically less than 0.5% but reach as much as 3% for WFC F550M and 4% for HRC F344N.
For example, the PHOTFLAM value for F555W for the 2002.16 initial epoch for WFC changed from 1.926e-19 to 1.928e-19 on March 22, 2012 or by 0.1%, while for F555W just after the 2006.5 change to the lower operating temperature of -81C, the sensitivity is lower than the previous calibration by the factor 0.987. For the wide WFC filters, the maximum change of photflam is for F435W at 2007.1, where a loss of sensitivity of 1.3% combines with the 0.988 renormalization factor to make a total change of 2.5%. The WFC F435W is the one and only filter where the shape of the bandpass function has been updated.
The SBC table below and the WFC and HRC calculator give the current PHOTFLAM values and the magnitude zeropoints for the ACS cameras. These instrumental 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 systems are defined below and also in Chapter 3 of the Synphot User's Guide. By definition, Vega has magnitude 0 at all wavelengths in the VEGAmag system, and the estimate for the absolute flux of Vega is currently from alpha_lyr_stis_005.fits in the CALSPEC archive (see Bohlin (2007a) .
The ABmag and STmag magnitude systems are based on the absolute physical flux per unit frequency and per unit wavelength, respectively. A magnitude of zero in either system for wavelengths near the V passband corresponds roughly to the actual flux of Vega, because the zero points were originally based on historic estimates of the physical flux of Vega above the atmosphere. Because the VEGAmag is so commonly used, the average flux in the ACS bandpasses are tabulated here for 2002.16. At later times, small changes in the ACS sensitivity as a function of time and wavelength cause variations by as much as 0.1% in these average fluxes for the broader filters.
Average Flux of Vega in ACS Filters
The flux calibration and its zeropoints are for an "infinite" aperture, where infinite is defined by S05 to be for a 5.5 arcsec radius, because 5.5 arcsec is about the largest size possible in practice. Point source photometry in the infinite aperture is not possible, except for the bright and heavily exposed standard stars. The photometry from each image must be referenced to a 0.5 or 1.0 arcsec aperture and the aperture corrections of ISR-IV for 0.5 or 1.0 arcsec radius must be applied. For the longest wavelength filters, the aperture correction also depends on the spectral flux distribution for stars cooler than about M type. More detailed information on aperture corrections can be found in ISR-IV and S05 and at the Aperture Corrections page.
The ACS fluxes and zeropoints relevant to the calibrations of S05, Mack et al. (2007) , and Bohlin (2007b) are all now out of date. The current values given here should be used for absolute calibration of ACS WFC and HRC observations. The previous version of the SBC table is retained, as the SBC calibration has not been revised. Because of the changing sensitivity with time, similar tables for WFC or HRC are produced interactively by entering the observation date in the indicated field below. If the small changes in the PHOTFLAM values summarized above are important to your science program, please consider using the calculator, rather than reprocessing the observations.
- Flux : The average flux F in erg cm-2 s-1 Ang-1 over an ACS bandpass is F=N*PHOTFLAM, where N is the count rate in infinite aperture. For count rates N(ap) in smaller apertures, N=N(ap)/EE, where EE is the fractional encircled energy. (See the ACS Data Handbook and ISR-IV.)
- VEGAmag : Magnitude system where Vega has magnitude 0 at all wavelengths by definition. The vega magnitude of a star with flux F is -2.5 log10 (F/F_vega) where F_vega is the current flux spectrum of Vega from the CALSPEC archive.
STmag and ABmag: Both systems define the absolute physical flux density for a point source. The conversion is chosen so that the magnitude at V corresponds roughly to that in the Johnson system. In the STmag system, the flux density is expressed per unit wavelength, while in the ABmag system, the flux density is expressed per unit frequency. The definitions are:
- STmag = -2.5 Log F_lam -21.10
- ABmag = -2.5 Log F_nu - 48.60
where F_nu is expressed in erg cm-2 s-1 Hz-1, and F_lam in erg cm-2 s-1 Ang-1. An object with a constant flux distribution F_nu = 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 F_lam = 3.63 x 10-9 erg cm-2 s-1 Ang-1 will have STmag=0.
Photometric Keywords in the SCI extention of ACS images:
(keywords affected by the sensitivity curve update are highlighted)
- 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.
The header keywords PHOTFLAM and PHOTPLAM relate to the STMAG and ABMAG zeropoints through the formulae:
- STMAG_ZEROPOINT = -2.5 Log (PHOTFLAM) - 21.10
- ABMAG_ZEROPOINT=-2.5 Log(PHOTFLAM)-5 Log(PHOTPLAM)-2.408
- Bohlin, R. C. 2007a, in ASP Conf. Ser. 364, The Future of Photometric, Spectrophotometric, and Polarimetric Standardization, ed. C. Sterken (Ann Arbor, MI: Sheridan Books), 315, "HST Stellar Standards with 1% Accuracy in Absolute Flux"; also Astro-Ph 0608715
- Bohlin, R. C. 2007b, Instrument Science Report, ACS 2007-06, (Baltimore:STScI)
- Bohlin, R. C. 2012, Instrument Science Report, ACS 2012-01, (Baltimore:STScI)(ISR-IV)
- Bohlin, R. C., Mack, J., & Ubeda, L. 2011, Instrument Science Report, ACS 2011-03, (Baltimore:STScI)
- Mack, J., et al. 2007, Instrument Science Report, ACS 2007-02, (Baltimore:STScI)
- Sirianni, M., et al. 2005, PASP, 117, 1049S (S05)
Last modified on June 18, 2012.