Measurements of the sky background using the HST Faint Object Camera.

Abstract

The results of a novel method of measuring the three dominant components of the sky brightness from the Hubble Space Telescope orbit are reported. The shadows cast by an occulting finger in the Faint Object Camera (FOC), when the field is dispersed with an objective prism, are separated into three features: the geocoronal Lyα and neutral oxygen airglow emission lines in the far ultraviolet, and the zodiacal light continuum above 3000 A. These are measured as functions of the relevant angles in the spacecraft-earth-sun geometry. The measurements are compared with predictions derived from models. Satisfactory agreement is found when the revised f/96 detector quantum efficiency of the FOC is used. The zodiacal light is well within the expected range of 70-210 S10 units for the observed ecliptic coordinates. The Lyα emission background is 25% lower than expected for all solar zenith distances. There is an excess of OI emission, by a factor of two above the predictions. There is a weak correlation between the Lyα emission and the target zenith distance.