5.4 Aperture Calibration

For a given filter of the PHT-P subsystems, the FCS calibration was obtained from observations always taken with the same standard apertures or default apertures:

• $23''$ aperture for P1 filters with a reference wavelength less or equal than 7.7 ${\mu}$m,
• $52''$ aperture for P1 filters with reference wavelengths longer or equal than 10 ${\mu}$m,
• $79''$ aperture for P2 filters, and
• $180''$ aperture for the P3 filters.

The applicability of the calibration to any given aperture is based on the following assumptions:

1. the apertures are perfectly circular or rectangular,
2. the effective solid angle of an aperture is proportional to its area,
3. the solid angle of an aperture is independent of filter,
4. the FCS in-band power is proportional to the area of an aperture.

The physical dimensions of the apertures were measured on ground and are given in Section A.1.2. From available calibration measurements deviations from these simple assumptions are clearly indicated:

• Aperture sequences on the FCS indicated that assumption 4 is not valid. Deviations up to 100%, 15%, and 40% were measured for P1, P2, and P3 respectively; the deviations also show a dependency on FCS heating power.
• Observations of flat sky areas have indicated a disproportional increase of signal for the largest apertures (size $>$ 120$''$) arguing against assumption 2.
• Scans of point sources for a given aperture give profiles that show large deviations from the expected profiles based on assumption 1.

For a subset of PHT04 observations the PHT off-line processing provides an analysis based on an empirical calibration scheme (Müller 2000a, [41]; Müller 2000b, [42]). In the analysis the normalised source signals of an aperture sequence are compared with the normalised signals of a calibration (point) source. Subsequently, the algorithm determines whether the source is point-like or extended, see Section 7.9.