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
Aperture Corrections and EE curves for WFC3/UVIS
Aperture Corrections and EE curves for WFC3/IR
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 WFC3 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 MJD
Modified 01/28/2011
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