The Star Scanner is fully described by Fieseler, 2000, The Galileo Star Scanner as an Instrument For Measuring High Energy Electrons in the Jovian Environment, USC MS Thesis [FIESELER2000].
The Galileo spacecraft carries aboard it a photomultiplier tube based star scanner for the purpose of providing the spacecraft with an inertial attitude reference. This device has been subjected to the radiation environment within Jupiter's magnetosphere since 1995 and is providing measurements of the omnidirectional flux of 1.5 to 30 MeV electrons within about 12 Jupiter radii. The range of maximum sensitivity is roughly 4 to 15 MeV. The star scanner is measuring electrons in energy ranges similar to some channels of Galileo's Energetic Particles Detector (EPD) but the star scanner operates continuously thus providing a unique data set when EPD is not operating. The star scanner is generally not sensitive to pitch angle distribution. There is no data prior to the spacecraft reaching Jupiter.
The star scanner provides a single channel of data for measuring electrons termed 'background radiation count' (or sometimes 'raw background') along with other data on the health of the instrument, brightness of stars in the field of view and time.
Before Galileo's launch, an attempt was made to shield the star scanner's Photomultiplier Tube (PMT) from the particle environment at Jupiter. The attempt was not entirely successful resulting in this star scanner data set. The conclusion that the star scanner is sensing predominantly high energy electrons is based up multiple arguments.
1. Two analyses of the shielding around the star scanner PMT concluded that the star scanner should be effectively shielded from electrons below ~ 1 MeV and protons of several hundred MeV. The flux of such protons is generally much less than the ~1 MeV electrons.
2. Theoretical arguments [RUSSELL2001B] based on the fact that the star scanner measures longitudinal asymmetries at a given jovicentric distance. It is argued that these asymmetries would quickly smooth out unless drifting more or less with Io. This suggests ~10 to 15 MeV electrons.
3. Qualitative comparison of star scanner data with Pioneer and Voyager data.