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2.3 Absolute and Relative Filter-to-Filter Accuracies for Extended Sources

With the establishment of wavelength dependent solid angles for the ISOPHOT-C detector pixels extended source photometry with the ISOPHOT-C arrays has been scientifically validated for OLP V7.0. A variety of off-source background photometry has been compared with COBE/DIRBE annual map data. Note that this cross-calibration compares 3 arcmin ISOPHOT beams with 0.7 deg COBE/DIRBE beams. So far no cross-calibration of an ISOPHOT map covering a full COBE/DIRBE beam has been performed.

The ISOPHOT-S spectral response function for extended sources is derived from the one for point sources by applying correction factors for the non-flat beam profile. Hence, all uncertainties of the point source spectral response function apply, cf. Table 2.2. On top of that a systematic positive flux correction of 10% for short wavelengths and 20% for long wavelengths has to be applied with the current spectral response function. This is due to an underestimate of the beam profile extent which is currently used in the calculations.

ISOPHOT-P extended source photometry has not yet been scientifically validated.