Instrument Overview
    ===================
    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].
 
 
    INTRODUCTION
 
    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.
 
 
    Calibration:
 
    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. The star scanner measures a strong decrease in flux at
    the Io L-shell similar to that seen in 5 MeV and 8 MeV electron data
    from Pioneer. Although there are proton channels that show this
    effect, they also show a noticeable flux decrease at the Europa L-
    shell which is absent in both the Pioneer  5 MeV and 8 MeV electron
    data as well as star scanner data.
 
    4.  Quantitative comparison with Galileo Energetic Particles
    Detector (EPD) data and Galileo Heavy Ion Counter (HIC) data.
    Extremely strong correlations were found with the EPD DC3 channel
    measuring > 11 MeV electrons. and with  the EPD B1 channel measuring
    ~1.5 to 10.5 MeV electrons. A very strong correlation was also noted
    with the EPD DC2 channel measuring > 2 MeV electrons.
 
    5.  Pre-flight testing of the unshielded PMT found that it was
    sensitive to ~1 MeV electrons.
 
    Using the above analyses, it was found that the range of energies
    the star scanner is measuring could be bracketed between 1.5 and 30
    MeV. This, of course, does not imply that the star scanner would not
    react to a 40 MeV electron or above. It is just that the flux of
    these higher energy electrons drops off quickly with increasing
    energy in the Jovian environment.  The star scanner response was
    modeled using the [DIVINE&GARRETT1983] model for flux in the Jovian
    environment which predicted that the > 80% of the star scanner's
    response was caused by electrons under 30 MeV.
 
    The star scanner is a linear detector at the lower fluxes (<1000)
    but becomes non-linear at higher fluxes due to saturation effects. A
    derived channel called 'compensated counts' is included which
    corrects for this non-linearity and other deterministic biases in
    the data.
 
    Also using the above analyses, it is believed that the star scanner
    is most sensitive to electrons in the range 4 to 15 MeV.  An attempt
    was made to convert star scanner counts to a flux of electrons by
    correlating against the EPD DC3 channel and the Divine Model.  In
    all cases, a dependency on jovicentric distance was noted.
 
    The equation:
 
            Flux (#electrons cm*-2 sec*-1) = 1755 * CC * (RJ1.1208)
 
    where RJ is radial distance from Jupiter, and CC is compensated
    counts available in the star scanner text files. This conversion is
    preliminary and thought to be correct only within a factor of five.
    The corrected data from the star scanner (compensated counts) are
    believed to be self-consistent; it is the conversion to flux that is
    problematic.  Additional calibration work is on-going and will be
    updated here as work progresses.
 
 
    Operational Considerations:
 
    There are several situations which create loss of data. The most
    likely of these is extended periods of no station coverage. All
    other period where the data is missing or suspect are noted in the
    'notes' column of the star scanner text files. These include:
 
    1. Brief periods (measured in minutes) near the point of closest
    approach to a Galilean satellite in several encounters where the
    star scanner shutter was closed to provide bright light protection.
    The star scanner was not designed to return telemetry in this
    condition.
 
    2. Periods where the spacecraft antenna was turned sufficiently far
    from the earth to cause loss of telemetry.
 
    3. There is an operational mode termed 'OSAD' for One Star Attitude
    Determination where only a single bright star is being intentionally
    observed by the star scanner. This situation increases the noise in
    the background radiation data noticeably but does not otherwise harm
    the data.
 
    4. Periods where Bright Body vectors were active. These are periods
    during each rotation when the star scanner's is effectively shut
    down to protect the photomultiplier tube from bright light sources.
    These periods usually have no effect on the data set but, in a few
    instances, can block a star from the field of view and thus reduce
    the sampling of the data set.
 
    It is noted that three of the four Io fly-bys appear to have
    'spikes' of  increased flux right at Io closest approach. For orbits
    I25 and I27 at least, it appears the star scanner was sensing bright
    light reflected from Io rather than measuring a feature of the
    environment. In other words, the bright body vectors were not
    effective due to prior spacecraft anomalies. It cannot be ruled out
    that these spikes don't partially or entirely reflect a feature of
    the electron environment since a spike was apparently seen in the J0
    orbit, nevertheless, this spiked data must be used with extreme
    caution.
 
 
    Detector:
 
    There are two possible sources of the radiation signal within the
    star scanner. Either direct electron stimulation of the photocathode
    of the photomultiplier tube or light production by fluorescence
    and/or Cerenkov radiation in the lenses that focus the light on the
    photomultiplier. Pre-flight testing found that up to approximately
    15% of the signal was expected to come from the lenses. This can not
    be verified in flight as there is no way to distinguish the two
    signals. This does not cast doubt on the fact that the star scanner
    as a whole is detecting electrons as described above but it does
    mean that no meaningful geometric factors can be derived for the
    detector.
 
    The PMT were specially modified by JPL starting with a 13 stage tri-
    alkali off-the-shelf photo-multiplier tube supplied by EMR
    photoelectric (model #549-01090). There are three lenses, but only
    the crown glass Ohara SK18 is important as it is the least shielded
    and closest to the photomultiplier tube. As this is the last lens in
    the optical train, there is no focusing of any light generated
    within this element. See [Fieseler, 2000] for more details.
 
 
    Measured parameters and onboard processing:
 
    The star scanner only provides the single measurement of radiation
    in the unit of 'counts'. This actually is the average of 32 of the
    most recent measurements that has been held in a special buffer.
    Each measurement lasts for 3.2 milliseconds and are staggered by
    four independent accumulators such that each measurement starts 0.8
    millisecond after the previous. Thus the data that is downlinked is
    actually an average taken over the previous 25.6 milliseconds. The
    star scanner also provides the end time of this event, a code word
    providing the health of the star scanner, the intensity of the most
    recently seen star. Spacecraft twist information is also calculated
    at the time the telemetry is prepared for downlink.
 
 
    References
    ----------
    DIVINE&GARRETT1983
 
        Divine, N. and H. Garrett, Charged Particle Distributions in
        Jupiter's Magnetosphere, J. Geophys. Res., 88, A9, 1983.
 
    FIESELER2000
 
        Fieseler, P., The Galileo Star Scanner as an Instrument for
        Measuring Energetic Electrons in the Jovian Environment, MS
        Thesis, University of Southern California, Los Angeles, 2000.
 
    RUSSELLETAL2001B
 
        Russell, C.T., P.D. Fieseler, D. Bindshadler, Z.J. Yu, S.P. Joy,
        K.K. Khurana, and M.G. Kivelson, Large scale changes in the
        highly energetic charged particles in the region of the Io
        torus, Adv. Space Res., 28, 1495, 2001.