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9. HISTORY OF THE CYCLE 4 FLUX CALIBRATION
The previous cycle 4 absolute calibration for FOS was implemented in the pipeline processing on 1994 March 21. This original calibration was based on only one set of observations of G191B2B and used a preliminary definition of the WD flux scale (Bohlin 1994). The differences between the fluxes for any cycle 4 data processed with the original cycle 4 calibrations and the calibrations derived here are shown for the 4.3" A-1 aperture in Figure 7, where the average is used for the time variable red H19 and L15 sensitivities. The structure in the ratios is mostly due to the preliminary WD flux corrections used in 1994 March before the model fluxes were adopted for the standard star G191B2B. Small changes in the spline nodes, additional FOS data, and updated flats also contribute to the changes. For the smaller apertures, the differences have the same shapes, because the aperture corrections are quadratic fits as a function of wavelength for each spectral mode (Bohlin and Colina 1995). For an aperture as small as the 0.3" B-2, the difference in flux exceeds 10% shortward of 1300Å, because only theoretical aperture transmissions were available in 1994 March.
The new cycle 4 calibrations derived here should be implemented in the routine pipeline processing in 1995 August. If the new calibration has been used, the header keyword AIS_COR will be set to COMPLETED. If the changes in the FOS flux calibration described above are important for a particular science goal, then the archival data should be reprocessed. Information on obtaining FOS calibration reference files can be obtained via the world wide here under the "ADVISORIES" topic.