ISOPHOT – Photometric calibration of point sources

¹ ISO Data Centre, Astrophysics Division of ESA, Villafranca, PO Box 50727, 28080 Madrid, Spain
² ISO Science Operations Centre, Astrophysics Division of ESA, Villafranca, PO Box 50727, 28080 Madrid, Spain
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Instituto de Astrofisica de Canarias, 38200 La Laguna, S/C Tenerife, Spain
⁵ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁶ Radio Astronomy Laboratory, 601 Campbell Hall, University of California, Berkeley, CA 94720, USA
⁷ Vanguard Research, Inc. Suite 204, 5321 Scotts Valley Drive, Scotts Valley, CA 95066, USA
⁸ Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK
⁹ Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
¹⁰ Infrared Processing and Analysis Center, California Institute of Technology, MS 100/22, Pasadena, CA 91125, USA
¹¹ 211 Bryant Space Science Center, PO Box 112055, Dpt. of Astronomy, Univ. of Florida, Gainesville, FL 32611-2055, USA
¹² MIT, Dept. of Earth, Atmospheric and Planetary Sciences, Bldg. 54-420, 77 Massachusetts Ave., Cambridge MA 02139, USA

Corresponding author: B. Schulz, bschulz@iso.vilspa.esa.es

Received: 1 August 2001
Accepted: 29 October 2001

Abstract

All observations by the aperture photometer (PHT-P) and the far-infrared (FIR) camera section of ISOPHOT included reference measurements against stable internal fine calibration sources (FCS) to correct for temporal drifts in detector responsivities. The FCSs were absolutely calibrated in-orbit against stars, asteroids and planets, covering wavelengths from 3.2 to 240 μm. We present the calibration concept for point sources within a flux-range from 60 mJy up to 4500 Jy for staring and raster observations in standard configurations and discuss the requisite measurements and the uncertainties involved. In this process we correct for instrumental effects like nonlinearities, signal transients, time variable dark current, misalignments and diffraction effects. A set of formulae is developed that describes the calibration from signal level to flux densities. The scatter of 10 to 20% of the individual data points around the derived calibration relations is a measure of the consistency and typical accuracy of the calibration. The reproducibility over longer periods of time is better than 10%. The calibration tables and algorithms have been implemented in the final versions of the software for offline processing and interactive analysis.