The footprints have been normalized to 1 at the peak position. The model footprint is calculated from the convolution of the point spread function with the pixel surface. Comparison between the predicted and the measured footprint shows good agreement. This can be understood by the broadening of the point source profile due to infrared light scattered by the support legs. The variations are illustrated in Figure 4.9 where the scans over the largest aperture in the 7.3 and 12.0 m filters are presented.
Error bars indicate the signal reproducibility in the forward and backward scans.
Note that the background signal level has not been subtracted in all panels. Figure 4.9 clearly shows that the energy of a uniform extended source in a given aperture is not necessarily proportional to the area of the aperture. Moreover, the flux density of an extended source with a size similar to the aperture depends on the position of the peak of its infrared brightness distribution in the aperture. Not only the accuracy of the sky brightness measurements suffer from the irregular apertures, also the FCS calibration measurements require a separate calibration for the relation between power on the detector and aperture area for different levels of the FCS heating power. These in-orbit calibrations confirm the results of ground based measurements of the beam profiles.