Position offset correction

The footprints of the PHT-S pixels were peaked rather than flat-top, therefore observing a compact source outside the optical axis of the pixel could change the measured signal. Since the calibration was set up for the pixel centre, off-centre observations have to be corrected in order to derive their correct flux values. In addition, because the footprint profiles varied with wavelength (there were especially large jumps at 3.7 and 8.8 um) off-centre position lead to the appearance of spectral artifacts, too (an example is shown in Fig. 1). In order to correct for both the incorrect flux values and the spectral artifacts, we determined the offset of the source from the centre by comparing the accurate position of the object with ISO's coordinates. The 2-dimensional footprint map (we utilised the ones derived by Leinert et al. 2002) was then sampled at the offset position, and the measured flux was scaled with the ratio between footprint values at the centre and at the offset location. This procedure was repeated for each detector pixel independently. For accurate source positions we consulted the 2MASS catalog or SIMBAD; if no coordinates better than 1" were available then no correction was performed.

Fig.1: Example for spectral artifacts introduced by off-centre location of a point source in the PHT-S beam. The object (Ced 112 IRS\,4, ISO_id:62501217) was offset by +4.8" and -0.6" in satellite Y- and Z-direction, respectively. Vertical marks show the wavelengths where the most obvious artifacts, in the forms of discontinuities, appear.