Ultraviolet to optical diffuse sky emission as seen by the Hubble Space
  Telescope Faint Object Spectrograph
  We present an analysis of the blank sky spectra observed with the Faint
Object Spectrograph on board the Hubble Space Telescope. We study the diffuse
sky emission from ultraviolet to optical wavelengths, which is composed of the
zodiacal light (ZL), diffuse Galactic light (DGL), and residual emission. The
observations were performed toward 54 fields distributed widely over the sky,
with the spectral coverage from 0.2 to 0.7 um. In order to avoid contaminating
light from the earthshine, we use the data collected only in orbital nighttime.
The observed intensity is decomposed into the ZL, DGL, and residual emission,
in eight photometric bands spanning our spectral coverage. We found that the
derived ZL reflectance spectrum is flat in the optical, which indicates major
contribution of C-type asteroids to the interplanetary dust (IPD). In addition,
the ZL reflectance spectrum has an absorption feature at ~0.3 um. The shape of
the DGL spectrum is consistent with those found in earlier measurements and
model predictions. While the residual emission contains a contribution from the
extragalactic background light, we found that the spectral shape of the
residual looks similar to the ZL spectrum. Moreover, its optical intensity is
much higher than that measured from beyond the IPD cloud by Pioneer10/11, and
also than that of the integrated galaxy light. These findings may indicate the
presence of an isotropic ZL component, which is missed in the conventional ZL
models.
