SEDs of examples of the three source types are plotted as solid lines. The flux range from 0.1 to 10,000 Jy is well covered by stars, planets and asteroids. Models cover the wavelength range from 2 to 35-50 microns. Both programmes used Kurucz model atmospheres of Sirius and Vega as common zero-point for their broadband photometry. Some asteroids are well enough understood by thermophysical modelling in order to be used as FIR standards.
Some specific complications arise, since they are moving objects w.r.t.
Model SEDs are also available. For each PHT filter a reference wavelength is selected to which these flux densities are referred. The result of this calculation stills need to be corrected for the actual colour of the object, once this is known. The response of PHT-S detectors after each flux step is very slow when exposed to low illumination levels and transient effects are very important. In order to avoid this problem, as of OLP v8.4, a different approach was used to calibrate chopped PHT-S observations.