PHOTOMETRIC	CALIBRATION OF THE FAINT OBJECT SPECTROGR?~PH AND OTHEP. HST SCIENTIFIC TNSTRUMENTS

R.	0. Bohlin and J. D. Neill Space Telescope Science Institute

Instrument Science Report CAL/FOS-084
presented at the ST—ECF/STS0I HST Workshop in Baia Chia, 1992 July



ABSTRACT

The absolute photometric calibration of the Faint Object Spectrograph
(FOS> is derived from observations of three spectrophotometric
standard stars (BD+28 4211, BD+75 325, and HZ44) in the large 4.3
entrance aperture tor 14 detector disperser coxnbinatioris (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 tor tour smaller singile
entrance apertures are computed from a Set of transmission
measurements relative to the 4.3 aperture at one epoch tor 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 to
2100A, where the decrease in sensitivity is also roughly 10% per
year. After correcting for these sensitivity losses and after
considering the sensitivity changes caused by the OTA focus changes,
the FOS spectrophotometry in the 4.3 aperture has an internal
consistency of a few percent.


Preliminary GHRS flux distributions derived from observations of
BD-I-28 4211 in the Large Science Aperture are consistent with the FQS
to 5%. Since the primary WFPC. flux standard is BD+75 325, the
absolute calibrations of the default chips WF2 and PC6 are also on the
same absolute flux scale to the accuracy that the changing sensitivity
can be traoked. Longward of the F336W filter, WFPC photometry is good
to 5-10%.