10.5 Inhomogeneous FCS Illumination of PHT-P Apertures

Caveat: In case the FCS calibration of an observation is performed in a non-standard aperture (see Section 5.4) strong deviations up to a factor $\simeq$ 2 of the correct responsivity and thus the derived fluxes can occur. The strongest deviations are expected for the largest apertures of the P1 filters.