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5.4 Aperture calibration  

For a given filter of the PHT-P subsystems, the FCS calibration has been determined from observations always taken with the same standard aperture (section 4.5.4). In OLP Version 7, 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 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, probably due to spatial inhomogeneities of the system transmission.

The assessment is ongoing. In OLP Version 7 no corrections are made for these deviations. Corrections are foreseen in a future version of the software.