From: ARIES::LEE 8-JUL-1996 21:21:56.36 To: BPER::SIMMONS,ZODIAC::SCHAFFNER CC: LEE Subj: GLL_UVS_INSTR.CAT DRAFT: 7/7/96 PDS_VERSION_ID = PDS3 RECORD_TYPE = FIXED_LENGTH RECORD_BYTES = 80 OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = GLL INSTRUMENT_ID = UVS OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "ULTRAVIOLET SPECTROMETER" INSTRUMENT_TYPE = "ULTRAVIOLET SPECTROMETER" INSTRUMENT_DESC = " Instrument Information ====================== Instrument Id : UVS Instrument Host Id : GLL 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 a 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 photmultiplier 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 was 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 ansae 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_ETAL_1992. 4. "Galileo UVS Calibration Report #2", McClintock, W., May 1993, Internal UVS Team document. HORD_ETAL_1992 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 target 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 UVS observations after the Jupiter orbit insertion period. 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). " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "HORD_ETAL_1992" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END