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3.5 Fine Calibration Sources (FCSs)  

The internal fine calibration sources act as stable reference sources and are used for photometric calibration of PHT-P or PHT-C observations, see Chapter 5. In terms of data collection, internal calibration measurements are similar to the celestial measurements except that they utilize one of the internal FCSs by imaging it via the chopper mirror, see also section 3.6. All ISOPHOT AOTs except for PHT40 contain one or more FCS measurements depending on the observing mode(s) requested.

There are two identical FCSs which can be operated independently. Each FCS consists of two Thermal Radiation Sources (TRSs): TRS1 is operated in reflecting mode via a mirror whereas TRS2 is operated in a transmitting mode via a beam splitter giving a factor of about 1000 lower flux level than for TRS1. The TRSs are heated diamond wafers, useable at temperatures up to 300 K. By controlling the source temperature between 5 K and 300 K with an accuracy of 0.1 K, different flux levels in the range 5 to W on the detector can be applied to all detectors except for PHT-SS (see section 5). The temperature is accomplished by tuning the heating power on the FCSs.

During the entire ISO mission FCS1 was calibrated against astronomical standards. In practice, FCS1 is used as the main reference source, its emission was directly compared with the flux of celestial standards. FCS2 has been cross calibrated against FCS1. This 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 is 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, several FCS measurements are collected, see section 3.7. For observations using raster mode where the execution of the AOT may be relatively long, the actual measurement is bracketed by two internal calibration measurements.

In AOT PHT40 (spectrophotometry with PHT-S) no FCS measurements are taken because the colour temperature of the FCS is too low to allow an homogeneous illumination of both PHT-SS and PHT-SL. In particular, in order to illuminate PHT-SS sufficiently, PHT-SL would have to be heavily saturated which could lead to long term drifts of the PHT-SL detector afterwards.