PRE-COSTAR FOS APERTURE THROUGHPUT VARIATIONS
                        DUE TO OTA FOCUS CHANGES

                     D. J. Lindler and R. C. Bohlin

               FOS Instrument Science Report CAL/FOS-102
                               93 August

      ABSTRACT

      Since the throughput of the FOS entrance apertures depends on
      the HST focus, estimates of absolute fluxes from observed count
      rates require a correction for focus effects.  For the larger
      apertures, the throughput is nearly independent of wavelength,
      while for the smaller apertures, the dependence on wavelength is
      important.  Differences in the red and blue side transmissions
      are typically less than 1% for the same aperture and wavelength.
      Since January 1991, the historic record of the effect of
      desorption on the HST focus shows deviations of -25 to +15
      microns from the nominal value.  The 4.3 arcsec aperture
      throughput observed by a 1.43 arcsec high diode array varies by
      approximately 10% over this range.  The 1.0 arcsec aperture
      suffers the worst photometric throughput variations of nearly
      20%.  Throughput corrections are modeled using calculations of
      the aberrated PSF at the FOS entrance apertures in conjunction
      with a desorption model and a history of secondary mirror
      movements.
      This correction procedure will leave OTA breathing as a dominant
      source of photometric uncertainty, which can still be almost 10%
      in the 1.0 arcsec aperture.