MEASUREMENTS OF THE SKY BACKGROUND USING THE HST FAINT OBJECT CAMERA

The results of a novel method of measuring the three dominant componen ts of the sky brightness from the Hubble Space Telescope orbit are rep orted. The shadows cast by an occulting finger in the Faint Object Cam era (FOC), when the field is dispersed with an objective prism, are se parated into three features: the geocoronal Ly alpha and neutral oxyge n airglow emission lines in the far ultraviolet, and the zodiacal ligh t continuum above 3000 Angstrom. These are measured as functions of th e relevant angles in the spacecraft-earth-sun geometry. The measuremen ts are compared with predictions derived from models. Satisfactory agr eement is found when the revised f/96 detector quantum efficiency of t he FOC is used. The zodiacal light is well within the expected range o f 70-210 S10 units for the observed ecliptic coordinates. The Ly alpha emission background is 25% lower than expected for all solar zenith d istances. There is an excess of OI emission, by a factor of two above the predictions. There is a weak correlation between the Ly alpha emis sion and the target zenith distance.