1. INTRODUCTION

Each spectrographic mode has an absolute calibration, i.e., sensitivity (S) that can be used to derive the flux of a point source as a function of wavelength by F = C / S, where F is the flux of the source and C is the observed counts per second. S can be calculated from the count rates of spectrophotometric stars with known fluxes (see section 3) as:

 S = C / F    (1)

All observations discussed in this report were obtained in the A-1 (4.3") or the B-3 (1.0" circular) apertures. The aperture transmissions for point sources (Bohlin and Colina 1995) define calibrations for the 1" B-3 and other small apertures relative to the 4.3" aperture flux calibrations. With the exception of the red prism, the complete set of cycle 4 FOS calibration data is robust enough to characterize the time changes and to determine mean calibrations that are internally consistent to better than 2% for the more photometric high dispersion modes.

In practice, the uncertainties in an individual observation stem from low counting statistics, from uncertain dark count subtraction, from pointing errors, from light scattered off the gratings, and from scatter in the positioning of the spectrum on the FOS diode array. The calibration data for the eight bright calibration standard stars do not suffer from the first three of these possible errors, because of high signal levels and peak-up target acquisitions that are accurate to 0.026 arcsec in each axis. Since the installation of the COSTAR optics, the effect of OTA focus errors on FOS photometry has become negligible.

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