The internal fine calibration sources acted as stable reference sources and were used for photometric calibration of PHT-P or PHT-C observations, see Chapter 5. In terms of data collection, internal calibration measurements were similar to the celestial measurements except that they utilized one of the internal FCSs by imaging it via the chopper mirror, see also Section 2.8. All ISOPHOT AOTs except PHT40 contained one or more FCS measurements depending on the observing mode(s) requested.
There were two identical FCSs which could be operated independently.
Each FCS consisted of two Thermal Radiation Sources (TRSs): TRS1 was
operated in reflecting mode via a mirror, whereas TRS2 was operated in a
transmitting mode via a beam splitter giving a factor of about 1000 lower
flux level than for TRS1. The TRSs were diamond wafers, heated to
temperatures up to 
K. By controlling the source temperature between
5 K and 300 K with an accuracy of
K,
different flux levels in the range 5
to
W on the detector could be applied to all
detectors except PHT-S (see Chapter 5).
The temperature was accomplished by tuning the electrical heating
power of the FCSs.
During the entire ISO mission FCS1 was calibrated against astronomical standards. For standard astronomical observations only FCS1 was used as reference source, its emission was directly compared with the flux of celestial standards. For P1 and P2, FCS2/TRS1 was cross-calibrated against FCS1/TRS1. After the change of illumination behaviour of TRS2 in FCS1 on revolution 94, it was decided to independently calibrate FCS2/TRS2 by a set of measurements against celestial standards with P3, C100 and C200 detectors. The calibration against celestial standards was accomplished by determining the heating power (in mW) that should be commanded on the FCS in order to mimic the power on the detector from a celestial calibration source (in W). Details of the calibration procedure can be found in Chapter 5.
One FCS measurement was obtained at the end of a sequence of measurements with a given detector subsystem for single pointing photometric observations. In case of multi-filter observations using more than one detector subsystem, one FCS measurement per subsystem was collected, see Section 3.10. For observations using raster mode where the execution of the AOT could be relatively long, the actual measurement was bracketed by two internal calibration measurements.
In AOT PHT40 (spectrophotometry with PHT-S) no FCS measurements were taken, because the colour temperature of the FCS was too low to allow a homogeneous illumination of both PHT-SS and PHT-SL. In particular, in order to illuminate PHT-SS sufficiently, PHT-SL would have had to be heavily saturated which would have led to long term drifts of the PHT-SL detector afterwards.