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New UVIS and IR Photometric Zeropoint Tables Available

IR
IR
UVIS
UVIS

Photometric Systems:

  • 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.

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) - PHOTZPT = -2.5 Log (PHOTFLAM) - 21.10
  • ABMAG_ZEROPOINT = -2.5 Log (PHOTFLAM) - 21.10 - 5 Log (PHOTPLAM) + 18.6921
Users performing photometry on non-drizzled _flt images may wish to read the description of the pixel area map.

Created 09/09/2009
Modified 10/06/2009
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