The fine calibration sources (FCSs) are two internal radiation sources which will be used as photometric standards to monitor the time evolution of the detector responsivities. They provide a homogeneous illumination of the field of view with the same f-ratio as the telescope and with all optical elements in the beam, as used for observations. Their flux levels are set to a level representative of the astronomical target. The FCSs are grey bodies with emissivity of 0.7.
The FCSs will be used to calibrate fluxes measured for astronomical sources with respect to those for a standard star. In the so-called Absolute Photometry mode the FCS is used as a reference for measuring absolute sky fluxes by chopping between the FCS and the celestial position.
The FCSs, however, cannot be used for spectrophotometric measurements because the dynamic range of both PHT-S detectors cannot be adjusted to the steep spectral emission of the FCSs.