FOS Instrument Science Report #125

"Pre-COSTAR Photometric Calibration of the Faint Object Spectograph"
by D.J. Lindler and R.C. Bohlin

Abstract excerpted from the report:

The pre-COSTAR absolute photometric calibration of the Faint Object 
Spectograph is derived from observations of five spectophotometric standard
stars (G191B2b, BD+75D325, HZ-44, BD+33D2642, and BD+28D4211) in the 4.3"
entrance aperture for 14 detector-disperser combinations (six for the blue
digicon and eight for the red).  Observations of the five stars over the 
period of January 1991 through December 1993 show variations of the sensitivity
with time.  There is a systmatic decline of the combined efficiency of 
the FOS optics and detectors, as well as a dependence of throughput on the
excursions of the OTA focus from the established "best focus" position.  The
systematic sensitivity loss is typically 10% but is as large as 27% for the 
red H19 grating mode.  During 1992 and 1993, the loss of sensitivity proceeded
at a lower rate than in 1991.

A correction algorithm is presented that approaches an internal photometric
precision of 1% in the high dispersion gratings for our ensemble mean of many
calibration spectra, which implies a photometric accuracy for an individual
stellar observation that is limited by the photon statistics, pointing
accuracy, OTA breathing and aperture size of the particular spectrum.







For more information, please contact the above authors.