3. SPECTROPHOTOMETRIC STANDARD STARS

Uncertainty in the absolute flux F of the standard stars was the dominant error in previous FOS absolute flux calibrations. Random photometric errors and systematic uncertainties are present in the previous reference standards, which are composed of IUE fluxes in the UV (Bohlin et al. 1990) and ground based spectra from Oke (1990) in the visual. Since all FOS wavelengths are measured in vacuum, the Oke wavelengths are transformed from air to vacuum values. Photometric errors are typically a few percent and range up to 10% in the difficult region for both IUE and Oke around 3200A, while the absolute IUE flux scale is low by 5-10% in the UV in comparison to models for pure hydrogen white dwarfs (WD).

Now, all the FOS flux standards are pure hydrogen WD models or are based on FOS photometry relative to the WD G191B2B, as detailed in Lindler & Bohlin (1994). Absolute flux distributions for the four WD models appear in Bohlin, Colina, & Finley (1995). FOS photometry relative to G191B2B establishes the fluxes for the four non-WD standards; i.e. FOS data is used as the standard below 3850A, while the higher S/N Oke spectra are bent to the broadband FOS shape at longer wavelengths. This set of eight spectrophotometric standard stars, G191B2B (WD0501+527), BD+75xfb 325, HZ44, BD+33xfb 2642, BD+28xfb 4211, GD71, GD153, and HZ43 is used for the sensitivity calibration of the FOS.

To summarize the current status of the standard star spectrophotometry, the external accuracy of the absolute flux scale is determined by how well the G191B2B model predicts the actual stellar flux distribution and is estimated to be within about 3% of the truth in the UV after normalizing to the Landolt visual photometry (Bohlin, Colina, & Finley 1995). Since the differences between the FOS absolute photometry for the average spectra of these eight stars and Landolt (1992 & private communication) B and V photometry show an rms scatter of less than 1% (cf. Colina and Bohlin 1994), both the Landolt data and the FOS spectra are excellent photometry. See Bohlin (1995) and Lindler & Bohlin (1994) for more details. For a normal FOS science observation of a point source, the accuracy of absolute fluxes is limited by the uncertainties in the observation itself and not by uncertainties in the flux calibration files.

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