B. Schulz 1, S. Huth 1,2, U. Kinkel +1,2, D. Lemke 2, J. Acosta 1,2, M. Braun 1,2, H. Castañeda 1,2, L. Cornwall 1, C. Gabriel 1, I. Heinrichsen 1, U. Herbstmeier 2, U. Klaas 1,2, R. Laureijs 1, & T. Müller 1,2
1 ISO Science Operations Centre, Astrophysics Division of ESA, Villafranca, P.O. Box 50727, 28080 Madrid, Spain
2 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
We discuss the ISOPHOT photometric calibration for point sources over a wavelength range from 3.2 to 240 micron for the aperture photometer and the FIR camera section. The detectors show drifts and transients due to cosmic radiation and slow response to changes of IR-flux. We therefore used stable internal reference sources (FCS) to calibrate the science observations. These adjustable reference sources were absolutely calibrated against stars, planets and asteroids, that cover almost the full accessible flux/wavelength space of the instrument. We present the calibration strategy for point-sources, covering a flux-range from 60 mJy up to 4500 Jy and discuss the measurements and the uncertainties involved. We corrected for non-ideal effects like non-linearities, signal dependencies on readout timing parameters and transients. A set of formulae is developed, that describe the calibration from signal-level to flux densities, introducing new matrices of calibration factors. These describe instrumental effects that were discovered during in-orbit calibration and are probably due to misalignments, diffraction effects and spatial inhomogeneities in the filter transmissions. Comparing the measured data with the derived calibration curves, we derive accuracies that can be achieved in scientific observations. The resulting calibration tables and algorithms have been implemented in the most recent versions of the software for offline processing and interactive analysis.
+ deceased 1997