| DESCRIPTION |
Instrument Information
======================
Instrument Id : UVS
Instrument Host Id : GO
Instrument Name : ULTRAVIOLET SPECTROMETER
Instrument Type : ULTRAVIOLET SPECTROMETER
Instrument Description
======================
The Galileo Ultraviolet Spectrometer investigation will use data
obtained by two instruments. The Ultraviolet Spectrometer (UVS)
covers the wavelength range from 113 to 432 nm and was the original
instrument selected for the Galileo Orbiter. The Extreme
Ultraviolet Spectrometer (EUV) was added to the Orbiter payload
after the Challenger accident in 1986. The UVS instrument is
described in this document; the EUV instrument will be discussed in
a separate document.
The UVS instrument consists of a Cassegrain telescope and an
Ebert-Fastie scanning spectrometer. Spectral scanning is
accomplished using a fully programmable diffraction grating drive.
Three separate photomultiplier detectors, located in the exit focal
plane of the spectrometer, are used to cover the entire
ultraviolet-near-visible spectrum from 113 to 432 nm. Spectral
scanning, instrument command and control, data formatting, and
spacecraft interface are all normally controlled by a microprocessor
within the instrument. A hardware-controlled logic circuit, called
Cold Start Mode, controls scanning at power on in the event normal
commanding capability is inadvertently lost. The UVS instrument
components are summarized in Table 1 and detailed in subsequent
sections of this document.
TABLE 1
Summary of Galileo UVS characteristics
---------------------------------------------------------------
Telescope
Focal length 250 mm
Focal ratio f/5
Aperture 50.3 mm x 52.8 mm
Unobscured area 13.89 cm**2
Spectrometer
Focal length 125 mm
Grating
Ruling 2400 lines / mm
Blaze angle 16.75 deg
Detectors
G channel EMR 510G-09 CsI photocathode
F channel EMR 510F-06 CsTe photocathode
N channel EMR 510N-06 KCsSB photocathode
Nominal wavelength range
G channel 113.3 - 192.1 nm second order
F channel 162.0 - 323.1 nm first order
N channel 282.0 - 432.0 nm first order
Nominal resolution
G channel 0.67 nm
F channel 1.36 nm
N channel 1.27 nm
Field of view
G and N channels 0.1 x 1 deg
F channel 0.1 x 0.4 deg
Exit slit solid angle
G and N channels 3.05E-5 steradians
F channel 1.20E-5 steradians
Instrument
Mass 5.2 kg
Power consumption 2.4 W
Heater power consumption 4 W
Instrument Optics
=================
The optical design for the UVS telescope is a Dall-Kirkham
configuration (elliptical primary mirror and spherical secondary
mirror) with an effective focal length of 250 mm and a focal ratio
of f/5. In order to measure accurate limb profiles, the telescope
has been equipped with an external sunshade and an extensive baffle
system for rejection of off-axis scattered light. The field of view
is wavelength-dependent, being limited by the spectrometer entrance
slit to 1 degree by 0.1 degree for two of the detectors (G channel -
113 to 192 nm and N channel - 282 to 432 nm) and by one of the
spectrometer exit slits to 0.4 degree by 0.1 degree for the third
detector (F channel - 162 to 323 nm). A bright object sensor (limb
sensor) with a 1.5 degree full width half maximum (FWHM) field of
view located below the telescope sunshade structure is used to
protect the long wavelength detector during atmospheric limb
observations.
Spectrometer
============
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
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).
Spectral scanning is accomplished by rotating a diffraction
grating. The UVS grating drive uses a moire fringe pattern,
generated by overlaying two radially etched transmission gratings,
to control the angular position of the grating. One of the
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
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
increment steps or 0.0225 degree. Thus a grating step results in a
0.1-mm displacement of the spectrum in the spectrometer focal plane
so that the spectrum is sampled on the average of 4 times per
spectral resolution element.
Three photomultiplier tubes, located behind three separate exit
slits in the focal plane of the spectrometer record the spectrum in
three overlapping wavelength ranges: the far-ultraviolet detector
(G channel) covers the wavelength range 113 to 192 nm, the
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
has its own high voltage power supply and pulse counting
electronics, allowing for independent operation. All three
detectors are mounted in a single mechanical housing along with
their high voltage power supplies and
pulse-amplifier-discriminators. The G and N photomultipliers are
located directly behind their respective exit slits in the
spectrometer housing. Volume constraints require that the F
photomultiplier be mounted above the slit plane and light is
directed to it by a small two mirror periscope located behind the F
channel exit slit.
Instrument Detectors
====================
Three EMR Photoelectric Corp. 510 photomultiplier tubes, located
behind three separate exit slits in the focal plane of the
spectrometer record the spectrum in three overlapping wavelength
ranges. Each detector has its own high voltage power supply and
pulse counting electronics, allowing for independent operation.
Photocathodes and windows for the detectors were chosen to optimize
measurements in narrow spectral ranges. The far-ultraviolet
detector (G channel) is equipped with a magnesium fluoride window
and a cesium iodide photocathode resulting in a solar blind detector
with high sensitivity in the wavelength range 113 to 192 nm. The
middle-ultraviolet detector (F channel) is equipped with a quartz
window to block radiation below 160 nm and a cesium telluride
photocathode to suppress its response to radiation from wavelengths
longer than 350 nm. The near-ultraviolet-visible detector (N
channel) is equipped with a quartz window and a bi-alkali
photocathode and is sensitive to radiation in the wavelength range
160 to 450 nm. The Voyager instruments experienced high radiation
noise, so additional aluminum shielding was added to the UVS
instrument.
Instrument Microprocessor and Electronics
======================================
The UVS uses an RCA 1802 CMOS microprocessor for command parsing,
spacecraft time recognition and synchronization, and instrument
control. In addition, the UVS design incorporates additional
electronics called the Cold Start Logic (CSL) that places it into a
cyclical F-G scan mode until microprocessor control is initiated by
spacecraft command. The instrument receives commands and spacecraft
timing information via the Bus Adaptor and associated Direct Memory
Access (DMA) logic. The Bus Adaptor serves as the bi-directional
interface between the Galileo spacecraft and the UVS. Its circuitry
serves to isolate the UVS electrically from the spacecraft and to
allow for 8-bit information flow to and from the UVS.
Science Objectives
==================
The scientific objectives of the Galileo Ultraviolet Spectrometer
(UVS) investigation include the following:
(1) THE INTERPLANETARY MEDIUM: By carrying out a systematic program
of H and He measurements over the course of the mission, UVS will
improve our knowledge of the interstellar wind (ISW) and of the
processes that affect its passage through the solar system.
(2) VENUS: The geometry of the Galileo flyby permits pole-to-pole
and dawn-to-dusk measurements by the UVS of the abundance of SO2 in
the cloud-top region, and of the abundance of H, O, C, and CO in the
thermosphere.
(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
below 200 nm. A search for a tenuous lunar atmosphere using the
resonance emissions of H, O, and OH will address the question of the
rate of bombardment of the Moon by small bodies, and of the fate of
solar wind protons that strike the surface. The flybys also allow
the Earth-Moon system to be mapped, and these data contain an image
from each encounter of the hydrogen geocorona from a unique sunward
vantage point.
(4) ASTEROIDS: The UVS measured the albedo of the asteroids Gaspra
and Ida during flyby. Spatial resolution on the asteroid surfaces
was not possible, but their scattering properties as a function of
phase angle were measured, and the presence of absorption features
at wavelengths longer than 200 nm was determined. At these and
shorter wavelengths the asteroid's albedo may be directly compared
to that of the Moon measured during the two Earth encounters. The
data returned from Gaspra and Ida were limited to a few spectra.
(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
to examine small scattering angles is restricted by solar protection
considerations, the contribution of UVS will be to determine the
imaginary parts of the aerosols' refractive indices by obtaining the
single-scattering albedo from photometric measurements. It will
sample the lower end of the aerosol size distribution due to its
sensitivity down to 200 nm. The distribution of aerosols with
altitude will be measured in the stratosphere by measuring limb
radiance profiles and in the troposphere by making nadir-to-limb
scans. Temporal variability in the properties of clouds and hazes
will be investigated at time scales ranging from days to the
duration of the mission.
(6) COMPOSITION AND CHEMISTRY OF THE JOVIAN STRATOSPHERE: UVS will
use reflectance spectroscopy during disc and limb scans to compile
and inventory numerous hydrocarbons (such as methane, acetylene, and
ethane) as a function of location and altitude. UVS limb scans will
yield stratospheric temperatures through the scale height of the
signal from Rayleigh-scattered sunlight.
(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
terminator. None of these phenomena have been totally explained. A
careful study of spectral, horizontal, vertical, and diurnal and
other time variations is an important objective for the Galileo UVS
and EUV experiments, with the goal of gaining insight into these
phenomena.
(8) JOVIAN AURORA: Galileo's mostly equatorial orbits mean that
the aurora will be observed near the northern or southern limbs,
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
determination of longitudinal dependencies of the emissions on each
orbit. The possibility of correlations between the aurora and
conditions in the Io torus will be explored. Galileo will also
allow comparison of day-side and night-side auroral emissions.
(9) JOVIAN SATELLITES: While close-range observations of Io by the
UVS will be prevented by the radiation environment, Europa and the
outer two Galilean satellites will be visited a few times in close
encounters. The Galileo UVS will measure and map the UV albedos of
areas of these moons. The measurements will be compared with those
of the Moon and of the asteroids Gaspra and Ida. The rich variety
of surface terrain and materials will greatly expand our knowledge
of the UV scattering properties of satellite surfaces. The UVS will
also look for evidence of tenuous and possibly sporadic atmospheres
that might be produced by sublimation, sputtering by co-rotating
plasma, or even eruptive events.
(10) IO TORUS: In conjunction with the EUV instrument, the UVS will
measure the abundance and distribution of the neutral and ionized
species existing in the Io torus. Midnight/noon comparisons of the
torus plasma will be possible. The surface and atmospheric
composition of Io and the nature and efficiency of escape and
ionization processes, as well as the complex interaction of the
ionized material with the magnetic and gravitational fields of
Jupiter and with the rest of the magnetosphere, will be
investigated. The data are expected to reveal many dynamical
aspects of the torus in addition to its composition.
(11) JOVIAN MAGNETOSPHERE: There are many processes in the
exosphere of Jupiter, on the constantly irradiated satellites, in
the Io torus, and in the magnetosphere in general, that might
provide sources of neutral atoms in the magnetosphere, including H
and even OH in addition to oxygen and sulfur. The UVS will search
for such material at times when the radiation noise in the
instrument is at a minimum.
(12) JOINT INVESTIGATIONS: Collaborative studies are planned with
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
auroral region. Joint investigations with the Photopolarimeter
Radiometer (PPR) experiment will help define the particulate
properties of the Jupiter atmosphere, providing constraints on cloud
particle size, shape, and composition. Complementary UVS and PPR
observations will also provide information about the spatial extent
and altitude distributions of these clouds. Properties of the
satellite surfaces will be measured in cooperation with the Near
Infrared Mapping Spectrometer (NIMS), the Solid State Imaging (SSI)
instrument, and the PPR. Scattering properties as well as
ultraviolet absorbers, e.g., sulfur dioxide, will be measured to add
leverage to our understanding of the Galilean satellites.
(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'
characterized by a brightness temperature near 8000K.
Operational Considerations
==========================
The UVS instrument has operated nominally since launch.
Calibration Description
=======================
The Galileo UVS flight instrument (Unit 0001) and engineering test
instrument (Unit 0000) were calibrated on the ground before launch.
Several documents and data files exist. Inflight calibrations were
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
should contact the UVS team. Calibration documents include these
references:
1. Galileo UVS Functional Requirement Document GLL-625-205,
4-2034, Rev A.
2. 'Galileo UVS Calibration Report, Preliminary Version',
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
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
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
sensitivity.
The several inflight calibrations include cross-calibration
activities between the UVS and EUV instruments during the
Lyman-alpha All Sky mapping, Earth 1 and Earth 2 X-Cal observations,
and several boresight observations of the platform teams during the
cruise, Earth 1 and Earth 2 periods involving several stars. Two
UVS star calibrations are currently planned during the Orbital
period. Target stars have included: Alpha CMa, Eta UMa, Alpha Eri,
Alpha Ori, and Alpha Leo. The Earth's Moon was also used as a
significant calibration observation during Earth 2.
Instrument Modes
================
The Galileo UVS has two operating modes: Cold Start and
Microprocessor-controlled. Microprocessor modes are different
between pre-Jupiter, called Phase 1, operations and post-Jupiter
operations, called Phase 2. Generally there are also cruise and
encounter modes discussed as well within the Phase 1 and Phase 2
categories. The instrument delivers 1008 bps to the Command and
Data System (CDS) Bus in all modes.
Cold Start is actually an automatic, or fail-safe, mode whereby
hardware circuits control the instrument's grating, or scanning,
operation. Two full-wavelength spectral scans are performed using
the F-channel detector in its standard wavelength range (162 to 323
nm) and the G-channel detector over its standard wavelength range
(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)
direction during the first scan of a RIM and moves in the down
(descending wavelength) direction for the second spectrum; 2) 84
zeroes, representing one minor frame of 0.666 seconds, are produced
by the instrument at the BEGINNING of each RIM.
Microprocessor mode describes any time that the UVS microprocessor
program is controlling the high voltage and/or grating operation of
the instrument. Originally designed for both recorded and real-time
transmission operations, the microprocessor program was modified,
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
real-time telemetry stream, with occasional backup to tape.
PHASE 1
-------
The original Phase 1 microprocessor program, Version 5.1, allowed
for full scan modes with one or two detectors being used during a
scan pair, and for mini-scan modes where up to four selectable
wavelength ranges from one detector could be scanned up and down
during the 4.333 second scan period. Two wavelength ranges were the
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
detectors were changed only at RIM boundaries.
During Venus, Earth 1 and Earth 2 the UVS made full rate real-time
and recorded observations of these bodies. They were generally full
wavelength scanning observations. Two mini-scan exceptions were the
Venus observations and the Hydrogen line all-sky maps.
PHASE 2
-------
Microprocessor Version 6.1 is used for all post-Jupiter UVS
observations. The two main distinctions of the Phase 2 UVS program
from the Phase 1 are: a) whether the data are being recorded or
are being summed (over time) by the CDS, and b) the movement of the
grating drive when in mini-scan mode is different between V5.1 and
V6.1 flight software. In Phase 2 a Real Time Science (RTS) CDS
routine was added to sum pairs of UVS spectra into a CDS internal
buffer, called the Summation Buffer, in order to reduce the bits to
ground. There are three summation periods which are dependent on
the downlink telemetry format. The three periods are 29 RIMS, 59
RIMS, and 1 RIM less than 24 hours. In each case, one RIM is used
to transfer and clear the buffer. This RTS data format allows torus
data to be obtained during the tape (cruise) playback periods. In
record mode the full UVS 1008 bps resolution is maintained on the
tape.
The order in which mini-scan wavelengths were sampled changed
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
or fourth different position was commanded then the next mode was
performed in the next spectrum. The next spectrum would contain the
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
spectra always repeat in the up wavelength pattern. The two
position mini-scan becomes 1-2-1-2. There are no third and fourth
wavelengths in Phase 2. This enables the CDS to sum consecutive
pairs of UVS spectra in the Summation Buffer. Phase 2 operations
allow the second mini-scan to be executed with a different
detector.
In all cases, the detector and wavelength motion and direction are
sensed within the UVS housekeeping data in the instrument 'fiducial'
at the start of each spectrum. The last byte of the microprocessor
program contains the software version number (times 10).
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