IRAS Explanatory Supplement
VI. Flux Reconstruction and Calibration
C. Absolute Calibration
C.1 General Philosophy
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-Tau, with the assumption
that its absolute flux density is as given
in Rieke et al. Extrapolation to the
25 and 60 µm bands is achieved using models of stars, normalized
to observations of the Sun. In this latter respect, the IRAS calibration
differs in principle from the ground based calibrations of
Rieke
et al. and of Tokunaga (1984) who
assumed that the flux density of -Lyr
between 10 and 25 µm
varied as that of a 10,000 K blackbody. The extrapolation of the absolute
calibration from 60 to 100 µm is based on observations and model
calculations of asteroids whose absolute flux at 60 µm was obtained
using the stellar calibration.
The spectral response of the bands is sufficiently broad
(Section II.C.2) that it is necessary to specify
the continuum energy distribution of the source being observed when defining
flux densities at a given wavelength. The approach used for IRAS was to
assign effective wavelengths of 12, 25, 60 and 100 µm for the four
bands. The effective bandwidth of each band was then calculated such that the
quoted flux densities are correct if the source has an energy distribution
with a flux per logarithmic frequency interval v ×
fv =
×
f
which is constant with frequency
.
Any other continuum distribution, and in particular that of hot stars,
requires a color correction. This color correction, which ranges up to
50% for astronomically interesting continua, is discussed in detail in
Section VI.C.3 below.
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