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The Galileo Orbiter will carry a 1500-mm (60-inch) focal length narrow-angle telescope inherited from Voyager. Along with an image sensor and electronics, this forms the solid-state imaging subsystem (SSI).
"Galileo's 22-month tour of the Jovian system will allow long-term studies of Io's active volcanoes and Jupiter's atmosphere. The satellites will be mapped at a wide range of angles and lighting conditions and at very high resolution, utilizing satellite flybys that are as much as 20 to 100 times closer than ever achieved before," notes imaging team leader Michael Belton of the National Optical Astronomy Observatories.
"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 large-format camera, while the need to study the composition of satellite surfaces and the vertical structure of features in Jupiter's atmosphere dictates the use of several spectral filters within the range 400 to 1100 nanometers. Accurate mapping and atmospheric velocity measurements require a camera with excellent geometric fidelity, while precise photometric requirements necessitate 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."
For the Galilean satellites Io, Europa, Ganymede, and Callisto, the imaging investigators hope to determine the form and structure of the satellite surfaces at a scale of 1 kilometer (0.6 miles) or better. In many images, features as small as 100 meters (330 feet) will be distinguishable. In the very best pictures, the smallest distinguishable features will be 20 meters (66 feet). Volcanic Io is of prime-interest and, in addition to geological studies, the imaging team will try to detect Io's atmosphere and map the source of sodium emissions that connect it with Io Torus.
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.
As opportunities arise, the camera will also turn toward Jupiter's smaller and more distant satellites to obtain information on the form and structure of their surfaces, colors, and albedos. This information will aid in determining the origin of these small satellites that were captured by Jupiter sometime after its formation. New small satellites may be found in or near Jupiter's rings.
Images of features such as the Great Red Spot, "barges," and white ovals will yield new information on their physical structure and will aid in distinguishing between mass motion and wave motion. Relative motion among clouds at various altitudes will be tracked to learn how local wind flows maintain themselves. 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 bolted to 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, and their data will be correlated. For example, the use of Voyager's imaging and infrared data 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 relative aperture of f/8.5," explains 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 bit stream is relayed from the tracking stations to image reconstruction equipment at JPL"
An eight-position filter wheel is stepped on command to obtain images of scenes through several different filters, which may then be combined electronically at Earth to produce color images. There are 28 selectable exposure times between 0.004 and 51.2 seconds. Galileo's spectral range is three times that of Voyager, and its field of view is 8.13 x 8.13 milliradians. The resolution is about 34 line pairs (of the 800 x 800 sensor array) per millimeter.
Since high levels of neutrons emitted by Galileo's onboard power sources could degrade the image quality, the camera's CCD is cooled to 163 K (-166 deg F) to eliminate the problem. The CCD is protected from Jupiter's natural radiation by a 1-cm-thick tantalum shield. While transient radiation-induced effects may be seen when the spacecraft is in the heaviest radiation environment near Jupiter, no damage is expected up to a total radiation dose of 1000 J/kg (100,000 rads).
The SSI was designed and assembled at JPL. It weighs 29.7 kilograms (65 pounds) and draws 15 watts. Texas Instruments provided the virtual phase CCD. The SSI uses RCA 1802 microprocessors and contains 600 integrated circuits. The telescope, shutter, and filters were inherited from Voyager, but have been improved to better reject off-axis scattered light. The collimator used for prelaunch testing was inherited from Mariner 10. The electronics chassis was fabricated on a numerically-controlled machine at JPL.
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