Introduction and purpose of PHT05 and PHT25

Astronomical Observing Templates (AOTs) PHT05 and PHT25 were designed to
determine the absolute brightness of the sky in the wavelength range between
3.3 and 240 micron. Target examples are the zodiacal light, the galactic
background, and the extragalactic background. These type of sources are
relatively faint and cannot be measured using the standard differential
technique which applies an on- and off- position. This means that the target
signal should be extracted from the total instrumental signal which includes
the dark signal. The absolute photometry AOTs can also be applied for point
sources in case a very accurate determination of the source flux is necessary;
see also "Absolute photometry (PHT05/PHT25) vs multi-filter photometry (PHT03/PHT22)".

To convert the target signal into a fluxdensity, a calibrated comparison
signal is measured from an internal grey body, the Fine Calibration Source
(FCS). The FCS itself has been calibrated by direct comparison with celestial
standards during the mission.

In orbit tests have shown that the signal from an FCS measurement does not only
contain components from the FCS itself and dark current but also a straylight
component from the sky entering the instrument. This is due to the absence of
a mechanical shutter to block the external beam. Both the dark signal as well
as this straylight signal should be removed from the total FCS signal before
comparison with the target signal.

The straylight and dark signal components can now be separated in PHT05
and PHT25 by including a dark measurement and a "cold FCS" measurement with
no heating power applied to the FCS.

A signal separation can only be achieved as long as the target signal, FCS
straylight, as well as the dark signal are stable during an observation. This
has put additional requirements on the minimum integration time for each
measurement.

Note: the dark signal is caused by the detector dark current and a continuous
stream of very low level radiation hits close to the detector noise level. The
dark signal differs per detector pixel and can vary in time depending on the
ionizing radiation conditions.

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