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.