IRAS Explanatory Supplement
VI. Flux Reconstruction and Calibration
C. Absolute Calibration
C.1 General Philosophy

The absolute calibration of the IRAS point source observations is tied directly to the absolute calibration by Rieke et al. (1984) of the ground based photometric system at 10 µm. Specifically, the 12 µm IRAS band is calibrated via measurements of a 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 a Lyr between 10 and 20 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 um 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 × f_v =  × 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.