Michael R. Rosa, Florian Kerber (STECF (Space Telescope European Coordinating Facility), c/o ESO, Garching)
The Faint Object Spectrograph (FOS),decommissioned early 1997 from HST, produced a wealth of science data sets. The entire archive of data is now being evaluated to provide a coherent view on the calibration, leading to a improved scientific value of the FOS Post Operational Archive. We illustrate the impact of the geomagnetic field and trapped particles on the wavelength zero points and the dark count rates. Our predictive software solutions for re-calibration of the data are based on physical models of the effect of B-field distortions in Digicons, the trapping of particles, geophysical and magnetospheric data archives and HST orbital software.