Hubble Space Telescope
WFC3 Photometric Zeropoints

[08/10/2012] Note to users: The PHOTFLAM values given here are consistent with the values written to the image header for UVIS1 (sci,2 or ext4). The UVIS2 values computed by SYNPHOT differ from UVIS1 by ~2-3% for filters with pivot wavelength < 300nm and > 700nm. (The exception is F953N which differs by 7.7%) For all other filters, UVIS1 and UVIS2 agree to better than 1%. A single zeropoint should be used for both chips, as given in the tables below. We are working to resolve the discrepancy in SYNPHOT.

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 from a new reduction of all SMOV4, Cycle 17, and Cycle 18 observations of GD153, G191B2B, GD71, and P330E. The independent calibrations from the four stars agree to within 1% in most filters, and the photometric zeropoint is set to the average of the measurements. This is a substantial improvement over the previous calibration, where a filter-specific solution was not calculated (i.e., a smooth curve was across all wavelengths, ignoring small departures). For the first time, the new data sets have been processed using the new AstroDrizzle software and with the newly released WFC3 flat fields. For the UVIS camera, these flats are substantially better than the previous generation (i.e., more spatially stable given the removal of the "flare").

The results from this analysis will replace the existing calibration of the WFC3 photometric sensitivity in the file headers, and also define new throughput tables for synphot (e.g., the ETC) and CDBS. Changes in the zeropoints are less than 2% for most filters.

The methodology used to determine the zeropoints will be described in a forthcoming ISR.

Zeropoint tables after March 06, 2012:

For an infinite aperture:

IR

UVIS

For a 0.4 arcsec radius aperture:

IR

UVIS

Zeropoint tables prior to March 06, 2012:

Aperture Corrections and Encircled Energy Curves

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.

• 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

Users performing photometry on non-drizzled _flt images may wish to read the description of the pixel area map.