UVSTAR Overview


UVSTAR is a pair of telescopes with imaging spectrographs that are sensitive to EUV and FUV wavelengths. From the Space Shuttle, UVSTAR will form spectrally resolved images of extended emission regions such as the Io plasma torus. It is being prepared for flight jointly by the Lunar and Planetary Laboratory (University of Arizona) and the Center for Advanced Research in Space Optics (University of Trieste, Italy). Observing time will be shared to achieve objectives in both planetary science and stellar astronomy. UVSTAR offers an important advantage for torus research, namely the capability to form simultaneous images of the torus in each of its brightest emission lines. The spectral dispersion is great enough to separate images of most of the important emissions, leading to easy analysis without deconvolution.

Schedule and Flight Opportunities. The electronics and mechanical structure for UVSTAR are complete and will be delivered to GSFC in January 1995. The electronics package is the same as that of the University of Arizona's GLO instrument which was sponsored by the USAF/Phillips Laboratory. Fabrication of the mechanical structure was contributed by our Italian partners. UVSTAR is manifested for its first flight on STS-69 in July 1995, and for four reflights at 13-month intervals.

Plasma Torus Studies

The EUV spectrum of the plasma torus is rich in information about the ion composition, density, and electron temperature of the plasma. Spectral and morphological studies bear directly on such fundamental questions as the energy and mass budgets of the torus. Azimuth al asymmetries in the torus relate not only to localized torus-magnetosphere interactions, such as the Io-related brightness enhancement, but also to large-scale magnetospheric phenomena involving plasma flow in Jupiter's magnetotail. Although ground-based studies of torus emissions at visual wavelengths have added much to our knowledge of this plasma, measure ments in the EUV offer a more direct means of studying certain processes in the torus. Recent EUV measurements (the only since those of the Voyager UVS) by the Hopkins Ultraviolet Telescope and the Extreme Ultraviolet Explorer have emphasized the importance of this wavelength range. However, the former instrument lacks imaging capability, and the latter has substantially lower spatial resolution and is less sensitive to the most diagnostic wavelengths than UVSTAR.

Studies of the plasma torus have demonstrated that
Observations in the EUV range are essential,
Full spectral coverage of this range is required, and
Extensive morphological studies, i.e. imaging of the entire system is needed.
UVSTAR provides all these capabilities. It will complement and extend observations by Galileo. Galileo's orbital geometry limits its opportunities for viewing the plasma torus with the UV instruments, and especially with the EUV spectrograph. UVSTAR will provide observations of the Jovian system at improved spectral resolution.

The imaging spectrographs will measure the intensity of several important emission lines of S II, S III, S IV, and O II as a function of position and time in the torus, measure H2 bands and H Lya from Jupiter's dayglow and aurora, and possibly help to clarify the coupling between the torus and the aurora. All these emissions and full spatial coverage will be recorded simultaneously.

The spectrographs, when operating with wide slits, form two-dimensional images of a spatially-extended target within the slit. Images at different emission lines are displaced in the dispersion direction. The dispersion is great enough to separate monochromatic images of the torus at a number of wavelengths. During one orbit UVSTAR will record torus emissions for about 30 minutes, giving an image that can be analyzed to determine the morphology of several of the strong emission lines. The spatial resolution over the disk of the planet (~0.15 Rj) will separate polar and equatorial emissions of H2 and H Lya.


UVSTAR Hardware


The UVSTAR optics section consists of two nearly identical telescopes and imaging spectrographs mounted on an articulated platform. The telescope mirrors are off-axis paraboloids having diameters of 30 cm and focal lengths of 1.4 m. They are coated with silicon carbide for good EUV reflectivity, and their mounting is thermally compensated to maintain focus. The spectrographs are similar to units successfully flown on several University of Arizona Shuttle instruments. They have slits of length 0.25 and selectable slit widths summarized in the table below.


Narrow ----3 pixels ----- 9 arcsec ------1 : Best spectral resolution
Medium ---13 pixels -----39 arcsec -----4 : Global images of Jupiter
Wide ----- 33 pixels ----100 arcsec ----12 : Encompasses entire torus

The spectrographs cover wavelength ranges of 500-850 and 800-1250 . Their holographical ly fabricated concave gratings are aberration corrected, and image on a flat focal surface over the area of the detector.

Detectors.

The detectors are intensified CCDs similar to those used in earlier University of Arizona Shuttle instruments. They consist of proximity-focussed windowless intensifiers fiber- optically coupled to CCDs for readout. These 2-D imaging detectors are sensitive to single photoevents. The CCDs are 1152 298 pixel arrays (dispersion spatial dimensions), and the readout format can be controlled electronically.

Finding and Tracking.

UVSTAR includes capabilities for independent target acquisition and tracking. Rough pointing will be provided by orienting the Orbiter. The finder telescope will locate the target and move the optical axis to it. Pointing control will then be transferred to the tracker telescope. A computer-controlled digital feedback system will hold the UVSTAR on target by cancelling the Orbiter's attitude control motions. The internal gimbals allow 3 motion about each of two axes.

Telemetry.

Command uplink and real-time data downlink telemetry permit good control of the experiment. Ground commands are transmitted over a 1200-baud uplink, and the data downlink consists of two medium-rate channels operating at 60 kBytes/sec for real-time data and 120 Kbytes/sec for playback of recorded data. UVSTAR stores records data on its optical disk during periods when the data downlink is not available.



Bill R. Sandel
A. Lyle Broadfoot
16 December 1994
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