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Instrument Information

    Instrument Information
      Instrument Id                  : UVS
    Instrument Information
      Instrument Host Id             : GO
      Instrument Name                : ULTRAVIOLET SPECTROMETER
      Instrument Id                  : UVS
      Instrument Type                : ULTRAVIOLET SPECTROMETER
      Instrument Host Id             : { VG1, VG2 }
      Pi PDS User Id                 : ALBROADFOOT
    Instrument Description
      Instrument Name                : ULTRAVIOLET SPECTROMETER
      Instrument Type                : ULTRAVIOLET SPECTROMETER
      Build Date                     : N/A
      The Galileo Ultraviolet Spectrometer investigation will use data
      Instrument Mass                : 4.52
      obtained by two instruments.  The Ultraviolet Spectrometer (UVS)
      Instrument Length              : 43.18
      covers the wavelength range from 113 to 432 nm and was the original
      Instrument Width               : 14.78
      instrument selected for the Galileo Orbiter.  The Extreme
      Instrument Height              : 17.15
      Ultraviolet Spectrometer (EUV) was added to the Orbiter payload
      Instrument Serial Number       : 3
      after the Challenger accident in 1986.  The UVS instrument is
      Instrument Manufacturer Name   : N/A
      described in this document;  the EUV instrument will be discussed in
      a separate document.
    Instrument Description
      The UVS instrument consists of a Cassegrain telescope and an
      Ebert-Fastie scanning spectrometer.  Spectral scanning is
      The Voyager 1 and 2 Ultraviolet Spectrometers are nearly
      accomplished using a fully programmable diffraction grating drive.
      identical instruments.  This discussion applies to both, except
      Three separate photomultiplier detectors, located in the exit focal
      plane of the spectrometer, are used to cover the entire
      in a few instances in which important differences between the
      two are noted explicitly.  The Voyager 1 Ultraviolet
      ultraviolet-near-visible spectrum from 113 to 432 nm.  Spectral
      Spectrometer (UVS) is a compact Wadsworth mounted objective
      scanning, instrument command and control, data formatting, and
      grating spectrometer that covers the wavelength range of 0.0535
      spacecraft interface are all normally controlled by a microprocessor
      to 0.1702 micron (0.0513 to 0.1680 micron for the Voyager 2
      within the instrument.  A hardware-controlled logic circuit, called
      UVS).  It records the entire spectrum within this range in a
      Cold Start Mode, controls scanning at power on in the event normal
      single exposure.  It has no moving parts.  A mechanical
      commanding capability is inadvertently lost.  The UVS instrument
      'collimator' consisting of a series of 13 aperture plates
      components are summarized in Table 1 and detailed in subsequent
      sections of this document.
      defines the main 'airglow' field of view (FOV) of 0.10 degrees
      degrees full-width-half-maximum (FWHM) x 0.87 degrees in length.
                                TABLE 1
      Light passing through the collimator strikes the concave
                   Summary of Galileo UVS characteristics
      diffraction grating at near normal incidence.  The grating
      disperses and focuses light onto a 1-d array detector that
      records individual photoevents.  An auxiliary field of view for
              Focal length              250 mm
      solar occultation experiments is offset 20 degrees from the
              Focal ratio               f/5
      airglow field by a small mirror near the front of the
              Aperture                  50.3 mm x 52.8 mm
      collimator. Using this 'occultation port', the UVS can view the
              Unobscured area           13.89 cm**2
      Sun without pointing the main field, and those of other
      coaligned instruments, directly at the Sun.  The occultation
      field is 0.25 FWHM x 0.87 degrees.  A sunshade prevents
              Focal length              125 mm
      illumination of the main entrance aperture by the sun during
      occultation observations.
                    Ruling              2400 lines / mm
                    Blaze angle         16.75 deg
      The UVS has two spectral resolutions depending on the nature of
      the source.  An extended monochromatic source that fills the
      FOV ideally produces a triangular intensity distribution of 0.1
             G channel                  EMR 510G-09 CsI photocathode
      degree FWHM.  (The actual response function is slightly rounded
      at the top and base, but a triangle is a satisfactory
             F channel                  EMR 510F-06 CsTe photocathode
      approximation for most applications.) The 0.1 degree
             N channel                  EMR 510N-06 KCsSB photocathode
      corresponds to width of 3.5 anodes, or 0.0033 microns.  This
      Nominal wavelength range
      inherent spectral resolution may often be improved by spectral
      analysis.  A monochromatic point source is imaged onto a width
             G channel                  113.3 - 192.1 nm  second order
      of about 1 anode for a practical resolution of about 0.0015
             F channel                  162.0 - 323.1 nm  first order
      microns.  Precise measurements of the relative response as a
             N channel                  282.0 - 432.0 nm  first order
      function of position within the FOV have been made by rastering
      Nominal resolution
      the FOV across a star.  At wavelengths longward of 0.1350
             G channel                  0.67 nm
      microns there is a slight (~10%) asymmetry in the response on
      either side of the center of the FOV.
             F channel                  1.36 nm
             N channel                  1.27 nm
      The effective sizes of the entrance apertures are (airglow
      Field of view
      port) 21.2 and (occultation port) 0.75 cm**2.
             G and N channels           0.1 x 1 deg
             F channel                  0.1 x 0.4 deg
      The anode array is scanned at a rate of 3125 scans per second
      and the results are added into an internal memory.  The UVS
      Exit slit solid angle
      transmits the contents of this memory to the flight data system
             G and N channels           3.05E-5 steradians
      (FDS) on command of the FDS.  The FDS retrieves values for a
             F channel                  1.20E-5 steradians
      pair of channels each 5 msec, and so reads a complete spectrum
      from the UVS in 0.32 sec.  For the fastest transmitted data
      rate (OC-1, see below) used for occultation observations, this
             Mass                       5.2 kg
      readout proceeds continuously, producing a series of spectra
             Power consumption          2.4 W
      separated by 0.32 sec.  For lower data rates, the memory is
             Heater power consumption   4 W
      read in bursts of 0.32 sec separated by the appropriate
      intervals.  During these intervals, the UVS integrates the
      spectrum in its internal memory.  As the data is transferred to
    Instrument Optics
      the FDS it is logarithmically compressed from 16 to 10 bits.
      The optical design for the UVS telescope is a Dall-Kirkham
      The FDS determines the rate at which spectra are read from the
      UVS after being integrated internally in the instrument memory.
      configuration (elliptical primary mirror and spherical secondary
      Most planetary observations are made at one of two data rates,
      mirror) with an effective focal length of 250 mm and a focal ratio
      OC-1 (0.32 sec spectra, for occultation measurements) and GS-3
      of f/5.  In order to measure accurate limb profiles, the telescope
      (3.84 sec spectra, for emission spectroscopy).  Slower rates
      has been equipped with an external sunshade and an extensive baffle
      are used from time to time.  Rates and their designations are:
      system for rejection of off-axis scattered light.  The field of view
      is wavelength-dependent, being limited by the spectrometer entrance
          Name            Mode #           Integration time (sec)
      slit to 1 degree by 0.1 degree for two of the detectors (G channel -
            OC-1            1                0.32
      113 to 192 nm and N channel - 282 to 432 nm) and by one of the
            GS-3            2                3.84
      spectrometer exit slits to 0.4 degree by 0.1 degree for the third
            CR-1            3                12
      detector (F channel - 162 to 323 nm).  A bright object sensor (limb
            CR-2            4                48
      sensor) with a 1.5 degree full width half maximum (FWHM) field of
            CR-4            6                192
      view located below the telescope sunshade structure is used to
            CR-6            8                720
      protect the long wavelength detector during atmospheric limb
            CR-5T           9                240
            UV-5A           10               3.84
      A description of the UVS investigation is given by
      [BROADFOOTETAL1977].  Performance and analysis techniques are
      described by [BROADFOOTETAL1981].
      The spectrometer is a standard, 125 mm focal length, Ebert-Fastie
      design which uses a single spherical mirror as both collimator and
      camera and a plane diffraction grating.  A ruling density of 2400
    Scientific Objectives
      grooves per mm provides a first-order dispersion of 23.9 nm per mm
      and an average spectral resolution of 200 for a 0.43-mm-wide
      entrance slit (0.1 degree telescope field of view).
      The primary goal of the UVS is to study the composition and
      structure of the atmospheres of the outer planets and their
      satellites.  Secondary goals include the study of
      Spectral scanning is accomplished by rotating a diffraction
      grating.  The UVS grating drive uses a moire fringe pattern,
      magnetospheric particle populations, magnetosphere-atmosphere
      interactions, the composition and distribution of the
      generated by overlaying two radially etched transmission gratings,
      interplanetary wind, determinations of the solar flux, and
      to control the angular position of the grating.  One of the
      stellar astronomy.
      transmission gratings is fixed, and the other rotates with the
      diffraction grating housing.  The transmission gratings have a
      ruling of 1500 lines per 360 degree rotation resulting in a single
    Operational Considerations
      cycle of 0.024 degree and a single phase increment step size of
      0.00375 degree.  Each grating step for the UVS is a sum of six phase
      The Voyager UVS instruments have operated nearly continuously
      increment steps or 0.0225 degree.  Thus a grating step results in a
      since launch in 1977.  With the singular exception of a
      0.1-mm displacement of the spectrum in the spectrometer focal plane
      decrease in the Voyager 1 microchannel plates (MCP) gain, due
      so that the spectrum is sampled on the average of 4 times per
      spectral resolution element.
      to a high radiation-induced count rate during passage through
      the inner Jovian magnetosphere, both instruments have remained
      photometrically stable at a better than 3% level since 1977.
      Three photomultiplier tubes, located behind three separate exit
      In-flight performance of the UVS from launch through the 1979
      slits in the focal plane of the spectrometer record the spectrum in
      Jupiter encounters is reviewed in [BROADFOOTETAL1981] and an up
      three overlapping wavelength ranges:  the far-ultraviolet detector
      (G channel) covers the wavelength range 113 to 192 nm,  the
      to date description of astronomical observations is contained
      in [HOLBERG1990] and [LINICK&HOLBERG1991].
      middle-ultraviolet detector (F channel) covers the wavelength range
      162 to 323 nm, and the near-ultraviolet-visible detector (N
      channel) covers the wavelength range 160 to 450 nm.  Each detector
    Calibration Description
      has its own high voltage power supply and pulse counting
      electronics, allowing for independent operation.  All three
      Laboratory calibration of the UVS included measurements of:
      detectors are mounted in a single mechanical housing along with
      their high voltage power supplies and
          1) sensitivity at a number of wavelengths throughout the
      pulse-amplifier-discriminators.  The G and N photomultipliers are
             spectral range,
      located directly behind their respective exit slits in the
          2) response to scattered light,
      spectrometer housing.  Volume constraints require that the F
          3) off-axis response, including collimator transmission
      photomultiplier be mounted above the slit plane and light is
             function, and
      directed to it by a small two mirror periscope located behind the F
      channel exit slit.
          4) intrinsic dark count rate.
      In-flight calibration has included assessments of absolute
    Instrument Detectors
      sensitivity by comparisons with stars, and measurements of the
      FOV response profile using stars.
      Three EMR Photoelectric Corp. 510 photomultiplier tubes, located
      behind three separate exit slits in the focal plane of the
      Before the absolute calibration can be applied to a measured
      spectrum, three or four spectral analysis steps are required.
      spectrometer record the spectrum in three overlapping wavelength
      These are flat field correction, dark count subtraction, and
      ranges.  Each detector has its own high voltage power supply and
      descattering, and (sometimes) sky background removal.
      pulse counting electronics, allowing for independent operation.
      Photocathodes and windows for the detectors were chosen to optimize
      Channel-to-channel variations in sensitivity result from
      measurements in narrow spectral ranges.  The far-ultraviolet
      variations in effective count threshold among channels.
      detector (G channel) is equipped with a magnesium fluoride window
      Applying a 'fixed pattern noise' (FPN) correction adjusts the
      and a cesium iodide photocathode resulting in a solar blind detector
      signal levels to their equivalents for a common threshold in
      with high sensitivity in the wavelength range 113 to 192 nm.  The
      all channels.  The FPN correction involves a channel-by-channel
      middle-ultraviolet detector (F channel) is equipped with a quartz
      multiplication by a correction spectrum.  The correction
      window to block radiation below 160 nm and a cesium telluride
      spectra are different for Voyager 1 and Voyager 2.  In fact,
      photocathode to suppress its response to radiation from wavelengths
      two spectra are in use for Voyager 1.  The first is used for
      longer than 350 nm.  The near-ultraviolet-visible detector (N
      channel) is equipped with a quartz window and a bi-alkali
      data acquired before Jupiter encounter and the second for data
      after encounter.  The two differ to account for changes in the
      photocathode and is sensitive to radiation in the wavelength range
      response of the Voyager 1 UVS induced by its operation in the
      160 to 450 nm.  The Voyager instruments experienced high radiation
      intense Jovian radiation environment.
      noise, so additional aluminum shielding was added to the UVS
      Channels 3 and 4 have large FPN corrections, i.e.  they are
      less sensitive than the others.  Therefore the statistical
    Instrument Microprocessor and Electronics
      accuracy of the signal in these channels is lower than in the
      other channels.
      The UVS uses an RCA 1802 CMOS microprocessor for command parsing,
      spacecraft time recognition and synchronization, and instrument
      Dark Counts
      control.  In addition, the UVS design incorporates additional
      electronics called the Cold Start Logic (CSL) that places it into a
        In interplanetary space, detector dark counts arise mainly
      cyclical F-G scan mode until microprocessor control is initiated by
        from effects of gamma radiation from the radioisotope
      spacecraft command.  The instrument receives commands and spacecraft
        thermoelectric generators that power the spacecraft.  The
      timing information via the Bus Adaptor and associated Direct Memory
        count rate is about 0.02 counts per channel per second.  The
      Access (DMA) logic.  The Bus Adaptor serves as the bi-directional
        shape is approximately flat in wavelength, with a step near
      interface between the Galileo spacecraft and the UVS.  Its circuitry
        the edge of the filter in the detector.  The shape is
      serves to isolate the UVS electrically from the spacecraft and to
      allow for 8-bit information flow to and from the UVS.
        accurately known from long observations of the calibration
        plate mounted on the spacecraft.  Almost no photon signal
        (except for a weak reflection of sky-background H Lyman
    Science Objectives
        alpha) is recorded during these observations.  The absolute
        level varies slightly with scan platform position, because
        rotating the scan platform changes the shielding mass between
      The scientific objectives of the Galileo Ultraviolet Spectrometer
      (UVS) investigation include the following:
        the UVS detector and the generators.  For data acquired
        outside a planetary magnetosphere, subtracting a scaled dark
        spectrum from a calibration plate observation is usually a
      (1)  THE INTERPLANETARY MEDIUM: By carrying out a systematic program
        satisfactory correction for dark counting.
      of H and He measurements over the course of the mission, UVS will
      improve our knowledge of the interstellar wind (ISW) and of the
        Within a planetary magnetosphere, the dark count rate can
      processes that affect its passage through the solar system.
        include contributions from high-energy particles.  For lower
        levels, scaling a calibration plate spectrum is again
      (2)  VENUS:  The geometry of the Galileo flyby permits pole-to-pole
        satisfactory, but for higher levels the shape of the dark
      and dawn-to-dusk measurements by the UVS of the abundance of SO2 in
        spectrum changes and another method must be used to
      the cloud-top region, and of the abundance of H, O, C, and CO in the
        estimate dark levels.  The best alternative is to use a
        spectrum taken at nearly the same time with no significant
        source in the FOV.  Satellite observations often fill this
      (3)  EARTH AND MOON:  The post-encounter passage near the subsolar
      point at long range allows the near-simultaneous measurement of
      pole-to-pole and dawn-to-dusk variations in the UV airglow and in
      reflected sunlight, allowing investigation of the global O/N2 ratio
      and the distribution of ozone.  It is also of interest to establish
      the Earth's UV albedo in the Schumann-Runge band region near and
        Light scattered within the instrument illuminates channels
      below 200 nm.  A search for a tenuous lunar atmosphere using the
        outside the ideal transmission function of the collimator.
      resonance emissions of H, O, and OH will address the question of the
        The effects of scattering are removed by a process called
      rate of bombardment of the Moon by small bodies, and of the fate of
        descattering.  Descattering is accomplished through the use
      solar wind protons that strike the surface.  The flybys also allow
        of a matrix operator, a 126x126 element matrix which
      the Earth-Moon system to be mapped, and these data contain an image
        describes the response of detector channel 'j' to the
      from each encounter of the hydrogen geocorona from a unique sunward
      vantage point.
        measured signal at channel 'i'.  This scattering matrix is
        completely empirical, having been determined from laboratory
        measurements of 50 individual emission lines covering the
      (4)  ASTEROIDS:  The UVS measured the albedo of the asteroids Gaspra
        entire passband.  Descattering is a linear operation.  Dark
      and Ida during flyby.  Spatial resolution on the asteroid surfaces
        counts must be subtracted prior to descattering as the
      was not possible, but their scattering properties as a function of
        descattering algorithm is based on the assumption that only
      phase angle were measured, and the presence of absorption features
        photon events are present.  Descattering will also correct
      at wavelengths longer than 200 nm was determined.  At these and
        for second order response.  Therefore, if the spectrum to be
      shorter wavelengths the asteroid's albedo may be directly compared
        descattered contains artifacts, such as anomalously high or
      to that of the Moon measured during the two Earth encounters.  The
        low counts in channels 3 or 4, a corresponding error will be
      data returned from Gaspra and Ida were limited to a few spectra.
        introduced in the vicinity of both the first and second order
      (5)  JOVIAN CLOUDS AND HAZES:  The Galileo orbiting mission offers
      the opportunity to observe Jupiter's clouds and hazes repeatedly
      over a wide range of phase angle and wavelength.  Since its ability
      Sky Background Subtraction
      to examine small scattering angles is restricted by solar protection
      considerations, the contribution of UVS will be to determine the
        When the UVS slit is not completely filled by the disk of a
      imaginary parts of the aerosols' refractive indices by obtaining the
        planet, the portion off the planet sees the sky background.
      single-scattering albedo from photometric measurements.  It will
        Fortunately, in the far UV the sky is generally quite dark
      sample the lower end of the aerosol size distribution due to its
      sensitivity down to 200 nm.  The distribution of aerosols with
        and diffuse starlight is seldom significant.  However, bright
        emissions at H Lyman alpha, Lyman beta, and He 0.0584 microns
      altitude will be measured in the stratosphere by measuring limb
        from the interplanetary medium (IPM) often must be taken into
      radiance profiles and in the troposphere by making nadir-to-limb
        account.  These lines arise from strong solar chromospheric
      scans.  Temporal variability in the properties of clouds and hazes
        emission lines that are scattered from neutral H and He of
      will be investigated at time scales ranging from days to the
      duration of the mission.
        the local interstellar medium.  The physics of this
        'interstellar wind' is complex and leads to emission which is
        inhomogeneous in space and variable in time.  The IPM
        responds to active regions of the solar chromosphere as the
      use reflectance spectroscopy during disc and limb scans to compile
        sun rotates.  This means that the sky brightness as seen by
      and inventory numerous hydrocarbons (such as methane, acetylene, and
        the UVS can change noticeably on time scale of days.  As with
      ethane) as a function of location and altitude.  UVS limb scans will
        instrumental dark counts, there are two standard means of
      yield stratospheric temperatures through the scale height of the
      signal from Rayleigh-scattered sunlight.
        removing sky background: direct subtraction of an adjacent
        sky background suitably scaled, if available, and
        construction of a synthetic sky background spectrum.
      (7)  JOVIAN THERMOSPHERE:  The thermosphere of Jupiter is
      characterized by unexpectedly high temperatures (of order 1100 K in
      the upper thermosphere) and by unexpectedly bright UV emissions from
      molecular hydrogen.  Lyman-alpha  emission from H shows an
      equatorial bulge that sometimes extends across the morning
        Calibrating the spectra converts them from count units to
      terminator.  None of these phenomena have been totally explained.  A
        absolute brightness units. This step has not been included in
      careful study of spectral, horizontal, vertical, and diurnal and
        the data processing because the correct procedure depends on
      other time variations is an important objective for the Galileo UVS
        the type of source viewed. The data spectra represent count
      and EUV experiments, with the goal of gaining insight into these
        rates after correction for fixed pattern noise, a background
        subtraction, and descattering. Multiplying these spectra by
        one of the calibration spectra converts it to brightness
      (8)  JOVIAN AURORA:  Galileo's mostly equatorial orbits mean that
        units. There is a calibration spectrum corresponding to each
      the aurora will be observed near the northern or southern limbs,
        of the two source types, namely point sources and extended
      allowing excellent longitudinal resolution at the cost of lesser
      latitude resolution.  The spectral effects of atmospheric absorption
      will be enhanced.  Jupiter's rapid rotation will facilitate the
        Point Source: Multiplying a data spectrum by the calibration
      determination of longitudinal dependencies of the emissions on each
      orbit.  The possibility of correlations between the aurora and
        spectrum VxPTCAL.TAB (x=1 for Voyager 1 and x=2 for Voyager 2)
        converts the spectrum from counts/(channel) to
      conditions in the Io torus will be explored.  Galileo will also
        photons/(cm**2-Angstrom-time), where time is the integration
      allow comparison of day-side and night-side auroral emissions.
        time of the spectrum.
      (9)  JOVIAN SATELLITES:  While close-range observations of Io by the
        Extended Source, continuum emission: Multiplying a data
      UVS will be prevented by the radiation environment, Europa and the
        spectrum that has been normalized to an integration time of 1
      outer two Galilean satellites will be visited a few times in close
        second by the calibration spectrum VxFLCAL.TAB (x=1 for
      encounters.  The Galileo UVS will measure and map the UV albedos of
        Voyager 1 and 2 for Voyager 2) converts the spectrum from
      areas of these moons.  The measurements will be compared with those
        counts/(channel-second) to spectral brightness in units of
      of the Moon and of the asteroids Gaspra and Ida.  The rich variety
        Rayleighs/Angstrom for a source that fills the field of view.
      of surface terrain and materials will greatly expand our knowledge
      of the UV scattering properties of satellite surfaces.  The UVS will
        Extended Source, monochromatic emission: The finite spectral
      also look for evidence of tenuous and possibly sporadic atmospheres
        resolution (about 35 A) of the spectrograph must be considered
      that might be produced by sublimation, sputtering by co-rotating
      plasma, or even eruptive events.
        in this case. For isolated lines (those that are not strongly
        blended with emissions at nearby wavelengths) it is sufficient
        to sum the channels that include light from the emission of
      (10) IO TORUS:  In conjunction with the EUV instrument, the UVS will
        interest (about 7 channels) and multiply by the appropriate
      measure the abundance and distribution of the neutral and ionized
        calibration factor. This factor is the product of the
      species existing in the Io torus.  Midnight/noon comparisons of the
        dispersion (9.26 Angstroms/channel) and the value in
      torus plasma will be possible.  The surface and atmospheric
        VxFLCAL.TAB corresponding to the center channel of the
      composition of Io and the nature and efficiency of escape and
        wavelength of interest. The resulting quantity is the
      ionization processes, as well as the complex interaction of the
      ionized material with the magnetic and gravitational fields of
        brightness of a monochromatic emission that fills the field of
      Jupiter and with the rest of the magnetosphere, will be
        view. For more complex spectra that include blended emissions,
      investigated.  The data are expected to reveal many dynamical
        the most accurate approach is spectral analysis by generating
      aspects of the torus in addition to its composition.
        synthetic spectra. This technique uses an iterative approach
        to adjust an estimated brightness spectrum until the model
      (11)  JOVIAN MAGNETOSPHERE:  There are many processes in the
        spectrum computed from it matches the observed spectrum. The
        model can be fairly simple, but must include the triangular
      exosphere of Jupiter, on the constantly irradiated satellites, in
        transmission profile of the collimator and the instrument
      the Io torus, and in the magnetosphere in general, that might
        sensitivity (calibration). The calibration factor described
      provide sources of neutral atoms in the magnetosphere, including H
        earlier in this paragraph is the correct one to use for this
      and even OH in addition to oxygen and sulfur.  The UVS will search
        kind of synthesis.
      for such material at times when the radiation noise in the
      instrument is at a minimum.
        Calibration and spectral analysis issues are discussed by
      (12)  JOINT INVESTIGATIONS:  Collaborative studies are planned with
        [HOLBERGETAL1982] and [HOLBERG1986].
      the fields and particles investigators, with the goal to improve our
      understanding of the transportation of sulfur and oxygen ions from
      the Io plasma torus to their ultimate precipitation in the Jupiter
    Platform Mounting Descriptions
      auroral region.  Joint investigations with the Photopolarimeter
      Radiometer (PPR) experiment will help define the particulate
      The UVS is mounted on the scan platform.  The instrument is
      properties of the Jupiter atmosphere, providing constraints on cloud
      approximately bore-sighted with the wide and narrow angle
      particle size, shape, and composition.  Complementary UVS and PPR
      television cameras, and with the PPS and IRIS instruments.  The
      observations will also provide information about the spatial extent
      alignment of the fields is not perfect; the following table
      and altitude distributions of these clouds.  Properties of the
      gives offsets of the centers of the UVS fields relative to the
      satellite surfaces will be measured in cooperation with the Near
      centers of the ISS Narrow Angle Camera fields of view.
      Infrared Mapping Spectrometer (NIMS), the Solid State Imaging (SSI)
      instrument, and the PPR.  Scattering properties as well as
      Elevation is positive to the right within the imaging field of
      view, and cross-elevation is positive downward.  The narrow axis
      ultraviolet absorbers, e.g., sulfur dioxide, will be measured to add
      of the UVS slit is aligned with the elevation direction.
      leverage to our understanding of the Galilean satellites.
          Instrument        Elevation        Cross-Elevation
      (13) SHOEMAKER-LEVY 9:  The UVS obtained a unique 292 nm data set
      during the impact of the SL-9 fragment G, showing a brief 'flash'
           Voyager 1         +0.010 deg         -0.030 deg
      characterized by a brightness temperature near 8000K.
                            +18.9 pixels       -56.6 pixels
           Voyager 2          0.0 deg           +0.08 deg
    Operational Considerations
                              0.0 pixels      +150.9 pixels
      Cone Offset Angle              : UNK
      The UVS instrument has operated nominally since launch.
      Cross Cone Offset Angle        : UNK
      Twist Offset Angle             : UNK
    Calibration Description
    Principal Investigator
      The Galileo UVS flight instrument  (Unit 0001) and engineering test
      instrument (Unit 0000) were calibrated on the ground before launch.
      The Principal Investigator for the ultraviolet spectrometer
      Several documents and data files exist.  Inflight calibrations were
      instrument is A. L. Broadfoot.
      also obtained.  The Principal Investigator requests that, until the
      end of the Galileo mission (EOM), any data users who wish
      calibration information beyond that provided in the literature
    Section 'UVS'
      should contact the UVS team.  Calibration documents include these
      Total Fovs                     : 2
      Data Rate                      : VARIABLE
        1.  Galileo UVS Functional Requirement Document GLL-625-205,
            4-2034, Rev A.
      Sample Bits                    : 16
        2.  'Galileo UVS Calibration Report, Preliminary Version',
      'UVS' Detectors
            McClintock, W., March, 1989, Internal UVS Team document.
        3.  UVS/EUV instrument paper, Hord et al. (1992) [HORDETAL1992].
        4.  'Galileo UVS Calibration Report #2', McClintock, W., May
      'UVS' Electronics
             1993, Internal UVS Team document.
      [HORDETAL1992] lists the types of calibrations done before launch.
      These include: instrument absolute sensitivity, telescope off-axis
      light rejection, spectrometer scattered light, instrument
      'UVS' Section Optic IDs
      polarization, and spectral line shape and wavelength scale.
      Spacecraft interface measurements were also performed;  these
      included the limb sensor sensitivity and field of view and the
      alignment of the UVS optic axis.  Other calibrations included
      component calibration, such as the detector spatial response and
      In modes
      The several inflight calibrations include cross-calibration
      activities between the UVS and EUV instruments during the
        HIGH VOLTAGE 0
      Lyman-alpha All Sky mapping, Earth 1 and Earth 2 X-Cal observations,
        HIGH VOLTAGE 1
      and several boresight observations of the platform teams during the
        HIGH VOLTAGE 2
      cruise, Earth 1 and Earth 2 periods involving several stars.  Two
        HIGH VOLTAGE 3
      UVS star calibrations are currently planned during the Orbital
        HIGH VOLTAGE 4
      period.  Target stars have included: Alpha CMa, Eta UMa, Alpha Eri,
        HIGH VOLTAGE 5
      Alpha Ori, and Alpha Leo.  The Earth's Moon was also used as a
        HIGH VOLTAGE 6
      significant calibration observation during Earth 2.
        HIGH VOLTAGE 7
    Instrument Modes
      'UVS' Section FOV Shape 'RECTANGULAR'
      The Galileo UVS has two operating modes: Cold Start and
        The occultation field is offset from the airglow field by a
      Microprocessor-controlled.  Microprocessor modes are different
        small mirror.  The offset is toward lower elevation.  The
      between pre-Jupiter, called Phase 1, operations and post-Jupiter
        elevation offsets are:
      operations, called Phase 2.  Generally there are also cruise and
      encounter modes discussed as well within the Phase 1 and Phase 2
                  Voyager 1       -19.53  deg
      categories.  The instrument delivers 1008 bps to the Command and
      Data System (CDS) Bus in all modes.
                  Voyager 2       -19.296 deg
        Section Id                     : UVS
      Cold Start is actually an automatic, or fail-safe, mode whereby
        Fovs                           : 1
      hardware circuits control the instrument's grating, or scanning,
        Horizontal Pixel Fov           : N/A
      operation.  Two full-wavelength spectral scans are performed using
        Vertical Pixel Fov             : N/A
      the F-channel detector in its standard wavelength range (162 to 323
        Horizontal Fov                 : 0.86
      nm) and the G-channel detector over its standard wavelength range
        Vertical Fov                   : 0.25
      (113 to 192 nm).  One RIM of time, the standard Galileo 'frame',
      consists of fourteen spectra taken over 60.666 seconds.  Note two
      factors: 1) the grating moves in the up (ascending wavelength)
      'UVS' Section Parameter 'SURFACE BRIGHTNESS'
      direction during the first scan of a RIM and moves in the down
      (descending wavelength) direction for the second spectrum; 2) 84
        Sampling Parameter Name        : TIME
      zeroes, representing one minor frame of 0.666 seconds, are produced
        Section Id                     : UVS
      by the instrument at the BEGINNING of each RIM.
        Instrument Parameter Unit      : RAYLEIGHS
        Minimum Instrument Parameter   : 0.000000
      Microprocessor mode describes any time that the UVS microprocessor
        Maximum Instrument Parameter   : 0.000000
      program is controlling the high voltage and/or grating operation of
        Minimum Sampling Parameter     : 0.32
      the instrument.  Originally designed for both recorded and real-time
        Maximum Sampling Parameter     : 720
      transmission operations, the microprocessor program was modified,
        Sampling Parameter Unit        : SECOND
      slightly, for Phase 2 operations:  the major change for phase 2
      includes the use of a CDS buffer to sum pairs of spectra for various
      durations and then to dump the contents of the buffer to the
      'UVS' Section Parameter 'FLUX'
      real-time telemetry stream, with occasional backup to tape.
      PHASE 1
        Sampling Parameter Name        : TIME
        Section Id                     : UVS
        Instrument Parameter Unit      : PHOTONS CM**-2 SEC**-1
      The original Phase 1 microprocessor program, Version 5.1, allowed
        Minimum Instrument Parameter   : N/A
      for full scan modes with one or two detectors being used during a
        Maximum Instrument Parameter   : N/A
      scan pair, and for mini-scan modes where up to four selectable
        Minimum Sampling Parameter     : 0.32
      wavelength ranges from one detector could be scanned up and down
        Maximum Sampling Parameter     : 720
      during the 4.333 second scan period.  Two wavelength ranges were the
        Sampling Parameter Unit        : SECOND
      maximum ever used in this mini-scan operation mode, however.  As
      noted above, the grating moves up and then down, even in mini-scan
      mode.  If two detectors were used in mini-scan mode then the
    Instrument Detector 'SPECTROMETER'
      detectors were changed only at RIM boundaries.
      The windowless, photoevent-counting detector consists of an
      During Venus, Earth 1 and Earth 2 the UVS made full rate real-time
      electron multiplier, a pair of microchannel plates (MCP) in
      and recorded observations of these bodies.  They were generally full
      series, and a 128-element linear self-scanned readout array.
      wavelength scanning observations.  Two mini-scan exceptions were the
      Venus observations and the Hydrogen line all-sky maps.
      Photoelectrons ejected at the front of the MCP stack are
      amplified by a factor of about 1E6, and the resulting charge
      PHASE 2
      pulse is collected by the anode array.  The 128 narrow aluminum
      anodes, each 3 mm long, are deposited on 0.1-mm centers for a
      collecting length (in the dispersion direction) of 13 mm.
      Microprocessor Version 6.1 is used for all post-Jupiter UVS
      observations.  The two main distinctions of the Phase 2 UVS program
      The anodes are accessed sequentially by a shift register and
      from the Phase 1 are:  a)  whether the data are being recorded or
      FET switches contained on the single integrated circuit.  The
      are being summed (over time) by the CDS, and b) the movement of the
      scanning circuitry discharges each anode into a charge
      grating drive when in mini-scan mode is different between V5.1 and
      sensitive preamplifier.  The charge pulse is digitized and the
      V6.1 flight software.  In Phase 2 a Real Time Science (RTS) CDS
      information added into a shift register memory consisting of
      routine was added to sum pairs of UVS spectra into a CDS internal
      128 16-bit words.  The 128-anode array consists of two separate
      buffer, called the Summation Buffer, in order to reduce the bits to
      interdigitated 64-anode arrays scanned by two shift registers.
      ground.  There are three summation periods which are dependent on
      The shift registers and memory are driven by a 200 kHz clock,
      the downlink telemetry format.  The three periods are 29 RIMS, 59
      so that an individual anode is accessed every 320 microsecond.
      RIMS, and 1 RIM less than 24 hours.  In each case, one RIM is used
      The detector scan rate is therefore about 3125 Hz.
      to transfer and clear the buffer.  This RTS data format allows torus
      data to be obtained during the tape (cruise) playback periods.  In
      Wavelengths shorter than about 0.1250 micron strike the MCP
      record mode the full UVS 1008 bps resolution is maintained on the
      directly.  Longer wavelengths first pass through a MgF2 filter
      with a semi-transparent photocathode of CuI.  This serves to
      The order in which mini-scan wavelengths were sampled changed
      boost the quantum efficiency at long wavelengths and to reduce
      the response to second-order light.
      between Phase 1 and Phase 2.  In Phase 1 mini-scan mode, each
      mini-scan mode was performed for one spectrum and if a second, third
      The detector is heavily shielded to reduce its response to
      or fourth different position was commanded then the next mode was
      trapped particle radiation.  A description of the detector may
      performed in the next spectrum.  The next spectrum would contain the
      be found in [BROADFOOT&SANDEL1977].
      third and the next spectrum the fourth.  This 1-2-3-4 pattern would
      then repeat with the down wavelength pattern of 4-3-2-1.  A two
      position mini-scan would repeat 1-2-2-1.  In Phase 2, pairs of
      Detector Type                  : MICROCHANNEL PLATES WITH ANODE
      spectra always repeat in the up wavelength pattern.  The two
      Detector Aspect Ratio          : N/A
      position mini-scan becomes 1-2-1-2.  There are no third and fourth
      Minimum Wavelength             : 0.0535
      wavelengths in Phase 2.  This enables the CDS to sum consecutive
      Maximum Wavelength             : 0.1702
      pairs of UVS spectra in the Summation Buffer.  Phase 2 operations
      Nominal Operating Temperature  : 250
      allow the second mini-scan to be executed with a different
    Instrument Electronics 'UVS'
      In all cases, the detector and wavelength motion and direction are
      sensed within the UVS housekeeping data in the instrument 'fiducial'
      The UVS electronics is housed in an enclosure integral with the
      at the start of each spectrum.  The last byte of the microprocessor
      program contains the software version number (times 10).
      optical section of instrument.  Most of the electronics is in
      the base of the instrument, but clock drive generators for the
      anode array and the first stage of charge sensitive
      preamplification of the analog signal processing electronics
      are mounted in the detector housing so that they are near the
      anode array.  Elements of the electronics complement include:
           (1)  Low voltage power supply
           (2)  High voltage power supply
           (3)  Clock drive generator for the anode array
           (4)  Analog signal processing electronics including A/D
           (5)  128x16 bit accumulation memory for spectrum
           (6)  FDS interface
      The FDS interface sends data to the FDS on demand and accepts
      mode commands from the FDS.  The mode commands set the level of
      the high voltage applied to the MCPs of the detector and set
      the mode of analog signal processing (pulse counting or
      Radiation-hard electronics components were used where possible,
      and spot radiation shielding was used to reduce the fluence at
      certain critical elements.
    Instrument Optics 'UVS'
      The optical system consists mainly of the mechanical collimator
      and concave diffraction grating.  The 13 aperture plates of the
      collimator establish a field of 0.1x0.87 degree for the airglow
      field and 0.25x0.87 degree for the occultation field.  The 0.1
      and 0.25 degree dimensions are in the dispersion direction, and
      the 0.87 degree dimension is in the cross-dispersion direction.
      The collimator provides separate light paths for the airglow
      and occultation ports, and a small mirror diverts the
      occultation field by 20 degrees from the airglow field.
      The concave diffraction grating is a platinum coated replica,
      ruled at 540 lines/mm, blazed at 0.0800 microns and having a
      spherical radius of curvature of 400.1 mm.  Dispersion in the
      image plane is 0.00926 microns/mm, or 0.000926 microns/channel.
      The grating substrate is a 4x6-cm rectangle, and the useful
      ruled area is 21 square cm.
      Telescope Diameter             : 0.06
      Telescope F Number             : 4
      Telescope Focal Length         : 0.20
      Telescope Resolution           : UNK
      Telescope Serial Number        : UNK
      Telescope T Number             : UNK
      Telescope T Number Error       : UNK
      Telescope Transmittance        : UNK
    Instrument Mode 'PULSE COUNTING'
      Two modes of electrical operation allow the detector to operate
      in a photon-counting mode for low source intensities, or in an
      integration mode for high source intensities.  In the pulse
      counting mode, the number in the corresponding memory location
      is incremented by one if a charge above a fixed threshold is
      detected on an anode.  The access time of 320 microsec implies
      that single random photoevents can be recorded on any one of
      the anodes at a rate of about 300 Hz with a coincidence of 10%.
      The pulse-counting mode is used for all measurements except
      solar occultations.
      Data Path Type                 : N/A
      Gain Mode Id                   : N/A
      Instrument Power Consumption   : 3.2
      In sections
    Instrument Mode 'PULSE INTEGRATION'
      In the pulse integration mode a 3-bit A-to-D converter is
      introduced ahead of the adder.  In this case the charge on each
      anode is coarsely digitized and added to the previously
      accumulated signal in memory.  The statistics of sampling these
      events is complicated by the logarithmic pulse height
      distribution of the events.  There is also a logarithmic
      current limit function of the MCPs at the high event rates that
      normally obtain when this mode is used.  Because both these
      characteristics lead to non-linear response, modeling of the
      detector response is needed to restore linearity.  The
      integration mode is used for observing solar occultations.
      Data Path Type                 : N/A
      Gain Mode Id                   : N/A
      Instrument Power Consumption   : 3.2
      In sections
    Instrument Mode 'HIGH VOLTAGE 0'
      The gain of the electron multiplier can be adjusted by setting
      the potential drop across the microchannel plates.  The high
      voltage level is commanded by the FDS.  Level 0 corresponds to
      high voltage off.
      Data Path Type                 : N/A
      Gain Mode Id                   : N/A
      Instrument Power Consumption   : 2.8
      In sections
    Instrument Mode 'HIGH VOLTAGE 1'
      The gain of the electron multiplier can be adjusted by setting
      the potential drop across the microchannel plates.  The high
      voltage level is commanded by the FDS.  Level 1 is used for
      occultation and solar observations.
      Data Path Type                 : N/A
      Gain Mode Id                   : N/A
      Instrument Power Consumption   : 3.2
      In sections
    Instrument Mode 'HIGH VOLTAGE 2'
      The gain of the electron multiplier can be adjusted by setting
      the potential drop across the microchannel plates.  The high
      voltage level is commanded by the FDS.  Level 2 is used for
      occultation and solar observations.
      Data Path Type                 : N/A
      Gain Mode Id                   : N/A
      Instrument Power Consumption   : 3.2
      In sections
    Instrument Mode 'HIGH VOLTAGE 3'
      The gain of the electron multiplier can be adjusted by setting
      the potential drop across the microchannel plates.  The high
      voltage level is commanded by the FDS.  Level 3 is used for
      airglow observations and some occultation observations.
      Data Path Type                 : N/A
      Gain Mode Id                   : N/A
      Instrument Power Consumption   : 3.2
      In sections
    Instrument Mode 'HIGH VOLTAGE 4'
      The gain of the electron multiplier can be adjusted by setting
      the potential drop across the microchannel plates.  The high
      voltage level is commanded by the FDS.  Level 4 is intended to
      recoup losses in supply output that could have occurred due to
      radiation damage.  It is not normally used for observations.
      Data Path Type                 : N/A
      Gain Mode Id                   : N/A
      Instrument Power Consumption   : 3.2
      In sections
    Instrument Mode 'HIGH VOLTAGE 5'
      The gain of the electron multiplier can be adjusted by setting
      the potential drop across the microchannel plates.  The high
      voltage level is commanded by the FDS.  Level 5 is intended to
      recoup losses in supply output that could have occurred due to
      radiation damage.  It is not normally used for observations.
      Data Path Type                 : N/A
      Gain Mode Id                   : N/A
      Instrument Power Consumption   : 3.2
      In sections
    Instrument Mode 'HIGH VOLTAGE 6'
      The gain of the electron multiplier can be adjusted by setting
      the potential drop across the microchannel plates.  The high
      voltage level is commanded by the FDS.  Level 6 is intended to
      recoup losses in supply output that could have occurred due to
      radiation damage.  It is not normally used for observations.
      Data Path Type                 : N/A
      Gain Mode Id                   : N/A
      Instrument Power Consumption   : 3.2
      In sections
    Instrument Mode 'HIGH VOLTAGE 7'
      The gain of the electron multiplier can be adjusted by setting
      the potential drop across the microchannel plates.  The high
      voltage level is commanded by the FDS.  Level 7 is intended to
      recoup losses in supply output that could have occurred due to
      radiation damage.  It is not normally used for observations.
      Data Path Type                 : N/A
      Gain Mode Id                   : N/A
      Instrument Power Consumption   : 3.2
      In sections
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Holberg, J.B., Far-ultraviolet background observations at high galacticlatitude: II. Diffuse emission, Astrophysical Journal, 311, 969-978, 1986.

Holberg, J.B., Extreme and far ultraviolet astronomy from Voyagers 1 and2, in Observatories in Earth Orbit and Beyond, edited by Y. Kondo, pp.49-57, Kluwer Academic Publishers, Boston,1990.

Holberg, J.B., W.T. Forrester, and Jack J. Lissauer, Identification ofresonance features within the rings of Saturn, Nature, 297, 115-120, 1982.

Hord, C.W., W.E. McClintock, A.I.F. Stewart, C.A. Barth, L.W. Esposito,G.E. Thomas, B.R. Sandel, D.M. Hunten, A.L. Broadfoot, D.E. Shemansky, J.M.Ajello, A.L. Lane, R.A. West, Galileo Ultraviolet Spectrometer Experiment,Space Science Reviews 253, 503-530, 1992.

Linick, S.H., and J.B. Holberg, The Voyager ultravioletspectrometers-astrophysical observations from the outer solar system,Journal of the British Interplanetary Society, 44, 513-520, 1991.(replaces LINICKHOLBERG1991)