WFC3 Photometric Zeropoints

For WFC3, zeropoints represent the magnitude corresponding to a (flat-field corrected) count rate of 1 e ⋅ s ^-1, and thus can be used to convert instrumental to astrophysical units. The tables below summarize results determined during SMOV. The methodology used to determine the zeropoints is described in detail in ISR 2009-30 for IR and ISR 2009-31 for UVIS. Specifically, note that a smooth function was fit to the measured throughput across all filters for each camera, and therefore the measured counts for any given filter are generally consistent to 1-2% with the zeropoints in the table below. As discussed in the ISRs above, the WFC3/UVIS filter F336W exhibits counts that are ~5% higher than the zeropoint listed below. Another exception applies to filters on the UVIS camera with wavelengths greater than 8500 Angstroms, where post-SMOV calibration data demonstrates counts that are 5-15% higher than inferred from the default zeropoints.

IR

UVIS

Aperture Corrections and Encircled Energy Curves

• 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.

• 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