PHOTOMETRIC CALIBRATION OF THE FAINT OBJECT SPECTROGRAPH


                 J. D. Neill, R. C. Bohlin, and G. Hartig
		     Space Telescope Science Institute


                 FOS Instrument Science Report CAL/FOS-077
                         Preliminary - June 1992


ABSTRACT

The absolute photometric calibration of the Faint Object Spectrograph (FOS) is
derived from observations of three spectrophotometric standard stars
(BD+28D4211, BD+75D325, and HZ-44) in the large 4.3" entrance aperture for 14
detector-disperser combinations (six for the blue digicon, eight for the red).
Observations of the three stars are used from five epochs between 1991.4 and
1992.0 for each of the blue and red digicons. Absolute calibrations for the
four smaller single entrance apertures are computed from a set of transmission
measurements relative to the 4.3" aperture at one epoch for each side.
Observations that span the time interval from 1991.0 to 1992.2 show a decrease
in the sensitivity for the blue digicon of about 10% per year at all
wavelengths, while the red digicon has a sensitivity that is constant to 5%,
except in the wavelength range of 1800-2100A where the decrease in sensitivity
is also roughly 10% per year.