Note, all archival ACS/CCD data retrieved prior to August 12th, 2016 are populated with out-of-date photometric flux calibration keywords.
- The Zeropoints Calculator will automatically compute PHOTFLAM, STMAG, ABMAG, and VEGAMAG values for all filters given a user supplied date.
- The ACS zeropoints are derived for an infinite aperture (5.5" for WFC and HRC, 4" for SBC) and so the corresponding aperture corrections must be applied to photometry extracted with apertures smaller than 4" or 5.5" depending on the instrument.
Summary of the changes made in deriving the 2016 ACS/WFC 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.
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.
- Flux : The average flux F in erg cm-2 s-1 Å-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 16-03 section 7.3.
- 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/Fvega) where Fvega 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 Fλ -21.10
- ABmag = -2.5 Log Fν - 48.60
where Fν is expressed in erg cm-2 s-1 Hz-1, and Fλ in erg cm-2 s-1 Å-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 Å-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.)
- PHOTMODE: Observation configuration for photometric calibration.
- PHOTFLAM: inverse sensitivity (erg cm-2 s-1 Å-1).
- PHOTZPT: ST magnitude zeropopint (= -21.10).
- PHOTPLAM: pivot wavelength.
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:
- instr_STMAG_ZPT = -2.5 Log (PHOTFLAM) - 21.10
- instr_ABMAG_ZPT= -2.5 Log (PHOTFLAM) - 5 Log(PHOTPLAM)-2.408
Note: The ACS absolute flux calibration represented by the PHOTFLAM keyword of the ACS image headers (Section 2.4 of the ACS Data Handbook) is applicable to the distortion corrected pipeline products (*_drz.fits or *_drc.fits) produced by AstroDrizzle. In order to extract photometry from non-geometrically corrected pipeline products (*_flt.fits, *_flc.fits), the appropriate pixel area maps ( Section 5.1.3 of ACS DataHandbook) must be applied before flux calibration with the PHOTFLAM keyword.
- Avila, R.J., Chiaberge, M. 2016, Instrument Science Report, ACS 2016-05 (Baltimore:STScI)(ISR 16-05)
- Bohlin, R.C. 2016, Instrument Science Report, ACS 2016-03, (Baltimore:STScI)(ISR 16-03)
- Bohlin, R. C. 2012, Instrument Science Report, ACS 2012-01, (Baltimore:STScI)(ISR 12-01)
- Bohlin, R. C., Mack, J., & Ubeda, L. 2011, Instrument Science Report, ACS 2011-03, (Baltimore:STScI)(ISR 11-03)
- Bohlin, R. C. 2011, Instrument Science Report, ACS 2011-02, (Baltimore:STScI)(ISR 11-02)
- Tran, D, et al, 2002, On-Orbit Performance of the ACS Solar Blind Channel
- Sirianni, M., et al. 2005, PASP, 117, 1049S (S05)
Last modified on Nov 14, 2017.