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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).
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