Calibration and Data Reduction

8.7 Fluxes and Standard Magnitudes

The pipeline calibrated data are not flux calibrated and the data are in units of
Data Numbers (DN). However a flux calibration is implicit in the header. To get flux density, multiply DN by the value of the keyword PHOTFLAM in the calibrated (.c0h) science header file, and divide by the value of the keyword EXPTIME.

The magnitude of an object can be determined using the photometric zero-point keyword PHOTZPT as


where m is in the STMAG system which is based on a spectrum with constant flux per unit wavelength set to roughly match the Johnson system at V. The more conventional systems are based on Vega's spectrum. Table 8.1 was generated using SYNPHOT to provide rough conversions to the Johnson UBVRI and Cousins RI systems. Typical uncertainties are 5%, and probably much worse for the U filter. The correction depends on the spectrum of the object, hence the table was generated using a wide range of Bruzual models.

For example, to convert to the Cousins I band for an object on WF4, get PHOTZPT=-21.1 and PHOTFLAM=2.6044 x 10^-18 from the header. Then convert from WFPC2 counts to magnitudes in Cousins I using:

Note that the Cousins I filter is much closer to the F814W filter than Johnson I, as shown by the nearly constant correction as a function of spectral type (i.e. color term).

Table 8.1: Conversion from STMAG to Johnson UBVRI and Cousins RI.

This procedure will provide typical accuracies of about 0.05 mag (worse in the UV). More accurate photometry will require a variety of corrections (e.g., CTE effect, contamination and red leaks for the UV filters, variable gains on different chips, color terms, geometric distortions) which are discussed in detail in Holtzman et al. (P.A.S.P., 1995b).