PHOTOMETRIe CALLBRATION OF THE FAINT OBJECT
SPECTROGRAPH

J. D. Neili, R. C. Bohlin, and G. BÄrtig
Space Telescope Science In8tititute

FOS Instrument Science Repert CAL/FOS-o75
April 1992

Abstract

The absolute photometric calibration of the Faint Object Spectrograph
(FOS) is derived from observations of three spectrophotometric
standard stars (BD +2$D4211, BD±75D325, and IIZ-44) in die large 4.3“
entrance aperture for 14 detector-disperser conibinations (six for the
blue digicon, eight for the red). Observations of the three stars are
used from live epochs between 1991 .4 and 1992.0 for each of the blue
and red digicons. Absolute calibrations for die four smaller single
entrance apertures are coniputed from a set of transmission
measurements relative to the 4.3“ aperture at one epoch for each
side. Observations that span the time interval froni 1991.0 to 1992.2
show a decrease in the sensitivity for the blue digicon of about 10%
per year at all wavelengths, whule the red digicon has a sensitivity ~
that is constant to 5%, except in the wavelength range of 1800-2100Ä
where the decrease in sensitivity is also roughly 10% per year.