Calibration and Data Reduction

8.8 Color Transformations of Primary Photometric Filters

The WFPC2 UBVRI system is fairly close as regards effective wavelengths to the Johnson UBVRI system, but cross-calibration is necessary to convert to
Johnson magnitudes. See the IDT OV/SV Report and Harris et al., A.J. 101, 677 (1991) for examples in the case of WF/PC-1. Figures 8.1 through 8.5 show the results of regression fits between these two systems on the main sequence stars in the Bruzual, Persson, Gunn, Stryker atlas that is installed in the calibration database system (CDBS).

Figure 8.1: F336W-F439W against Johnson U-B for the BPGS Atlas of MS Dwarf Spectra.

Figure 8.2: F439W-F555W Against Johnson B-V.

Figure 8.3: F555W-F814W Against Johnson V-I_c.

Figure 8.4: F555W-F675W Against Johnson V-R_c.

These fits should be used with caution for quantitative work. The zero-points in all cases are defined so that Vega's spectrum integrated over the bandpass is exactly magnitude zero (VEGAMAG in XCAL). The zero-points of the canonical Johnson-Cousins system differ from this by up to 0.02 magnitudes. The zero-points thus defined for the HST filters do not coincide with the STMAG definition used in the previous section. In addition, the ground based filter curves used, which are taken from Bessel (P.A.S.P. 102, 1181), give a good approximation to the standard Johnson-Cousins system, but are not as accurate as taking Landolt's curves and applying his color corrections to transform to the standard system. The latter procedure was used to derive the transformations given in Holtzman et al. (P.A.S.P., 1995b), which also discusses the changes in the transformations that result from source spectrum variations (such as metallicity and gravity effects).

Figure 8.5: F675W-F814W Against Cousins R_c-I_c.