Absolute photometry (PHT05/PHT25) vs multi-filter photometry (PHT03/PHT22)

Using the absolute photometry AOTs PHT05 or PHT25 one can minimize
uncertainties concerning

- dark signal
- FCS straylight contribution
- FCS signal

This can be done by including dedicated straylight and/or dark measurements
and by adjusting the integration time (>= 128 sec) such that a more stable
FCS signal is obtained.

For multifilter AOTs PHT03 and PHT22, only 1 FCS measurement of normally 32
sec is collected for each set of filters belonging to the same detector.
Therefore the FCS calibration of PHT03 and PHT22 is less accurate in case no
drift or straylight corrections are applied. The straylight correction depends
on an assumed sky-brightness during the measurement. In addition, in case
there is a non-linearity in the detector responsivity and several filters of
the same detector is used, additional non-linearity corrections will have to
be applied.

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