ISOPHOT - Photometric Calibration of Point Sources

We discuss the photometric calibration of ISOPHOT for point sources in the wavelength range from 3.2 to 240 μm for the aperture photometer (PHT-P) and the FIR camera section (PHT-C). To correct for temporal drifts of detector responsivities, all observations included reference measurements against stable internal sources. These have been absolutely calibrated in-orbit against well known celestial standards. In this process corrections were derived for instrumental effects like nonlinearities, signal-transients, time variable dark current or misalignments. For staring- or raster-observations of point sources in standard configurations, the scatter of the individual data points around the derived calibration relations is a measure of the consistency and typical accuracy of the calibration.