The absolute photometry for the three NICMOS cameras is derived by observing two primary absolute standards (G191-B2B and P330E) in all available filters and grisms. The white dwarf G191-B2B and the solar analog P330E provide NICMOS absolute calibration in the white dwarf and solar analog scales, respectively.
The white dwarf G191-B2B is the HST primary calibration in the ultraviolet and optical. In the near-infrared a model spectrum of this star normalized to Landolt V magnitude is used for the calibration. The uncertainties in the spectral energy distribution of the model spectrum vary from about 1% at 0.8 microns to about 3.5% at 2.5 microns. Details about the model spectrum of G191-B2B and accuracies can be found in Bohlin, Colina & Finley (1995; AJ 110, 1316) and Bohlin (1996; AJ 111, 1743).
The solar analog P330E has an spectral energy distribution in the optical range is, within the observational uncertainties (2%-3%), equal to that of the Sun. The near-infrared spectrum of P330E is covered by a state-of-the art model spectrum normalized in the optical to match the V magnitude of the spectrum obtained with the HST Faint Object Spectrograph (FOS). Full details about the optical and near-infrared spectrum of P330E used in the calibration of NICMOS can be found in Colina & Bohlin (1997; AJ 113, 1138) and Colina, Bohlin & Castelli (1996; AJ 112, 307).