The Galileo Orbiter carries a 1500-mm (59-inch) focal length narrow-angle telescope inherited from Voyager. Along with an image sensor, filter wheel, focal plane shutter, and electronics, this forms the solid-state imaging subsystem (SSI).
"The design of the SSI was dictated by a combination of goals and constraints," explains SSI science coordinator Ken Klaasen. "The need to study both atmospheric motion and geologic formations dictates a high-resolution camera with as large a field of view as possible, while the need to study the composition of satellite surfaces and the vertical structure of features in Jupiter's atmosphere dictates the use of seven spectral filters within the range 400 to 1100 nanometers plus a 'clear' filter. Accurate mapping and atmospheric velocity measurements require a camera with excellent geometric fidelity, while precise photometric requirements demand a linear detector, stable calibration, and adequate data encoding. Low lighting situations, such as observations of the auroras, lightning, and ring system, require a detector of very high sensitivity and an optical system with low scattered light. Constraints on the design included limitations on the available telemetry rate, potential image smearing caused by residual motions in the scan platform, use of large amounts of shielding, to protect the instrument from Jupiter's harsh radiation environment, limited electrical power and mass, and protection from contamination during launch and from propellant by-products in flight."
Since the SSI's wavelength range extends from the visible into the near-infrared, the experimenters will be able to map variations in the satellites' color and albedo (reflectivity) that show differences in the composition of surface materials.
The SSI's near-infrared filters will allow us to "see" at different levels in the atmosphere to study relationships among vertical structure, color, and morphology.
The imaging instrument is mounted with three other optical instruments on a movable platform on the nonspinning portion of the Orbiter. This scan platform can be slewed up, down, or sideways to point the instruments. The optical axes of the instruments--the SSI, the near-infrared mapping spectrometer, the ultraviolet spectrometer, and the photopolarimeter-radiometer--are aligned so they are looking at the same areas, so that their data can be easily correlated. For example, the use of Voyager's imaging in the visible and infrared data from an infrared spectrometer (IRIS) confirmed the possibility and extent of lava lakes on Io.
"The SSI uses a Cassegrain telescope with a 176.5-mm (7-inch) aperture, and a fixed focal ratio of f/8.5," explains retired instrument manager Maurice Clary. "It is focused on infinity. Light from a scene is collected on an 800 line by 800 column solid-state silicon image sensor array called a charge-coupled device (CCD). Charge is transferred by rapidly cycling the voltage level applied to the 640,000 gates in this integrated circuit. Analog video data from the CCD are converted to digital bits and sent to the spacecraft tape recorder for temporary storage. During the cruise portion of each orbit, the data are played back off the recorder, edited and/or compressed by the spacecraft's computers, and transmitted to Earth. There, the data is relayed from the tracking stations to image reconstruction computer at JPL"
The detector is a virtual phase CCD in which the pixel structure is defined by ionic implants. It is thick and frontside illuminated. It is also packaged and surrounded by a tantalum radiation shield. Out of several thousand attempt only 2 detectors of flight quality where made. The original plan was to fly a thinned 3-phase device - but this was abandoned because under irradiation by high energy particles the applied voltages could not be adequately controlled and the full-well capacities were too small. Hubble ST cameras latter picked up the 3-phase developement (as did the the astronomical ground based community) which was pioneered for this application by the Galileo development project.
"While the SSI technology is 1970-80's vintage, it was pioneered by the Galileo project. The camera has operated flawlessly and the early images indicate that it has now produced the finest planetary images ever taken and with a resolution and wavelength range previously unavailable.
Moreover, there is much more to come!" - Michael J. S. Belton, SSI Team Leader
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Galileo Solid State Imaging Team Leader: Dr. Michael J. S. Belton
The SSI Education and Public Outreach webpages were originally created and managed by Matthew Fishburn and Elizabeth Alvarez with significant assistance from Kelly Bender, Ross Beyer, Detrick Branston, Stephanie Lyons, Eileen Ryan, and Nalin Samarasinha.
Last updated: September 17, 1999, by Matthew Fishburn
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