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SSI - Solid State Imaging

MEASUREMENT OBJECTIVES

• Map the Galilean satellites at a spatial resolution of less than 1 km.

• Investigate the Jovian atmosphere and cloud structure.

• Investigate Jovian atmospheric dynamics.

• Determine the shapes and sizes of the Galilean satellites and determine their spin axis within +/- 3 km.

• Map the Jovian ring structure and look for associated small satellites.

• Map and characterize auroral phenomena in the Jovian atmosphere and on the satellites.

• Characterize the state of the atmosphere at the time and place of the Probe descent.

SUMMARY

The other smaller satellites will be studied throughout the orbital tour. Studies will also be made of Jupiter's rings. Small, new satellites may be found in or near the rings.

The SSI will be used to determine structure, motions, and radiative properties of the atmosphere of Jupiter. It will measure wind profiles by tracking how fast clouds move at various altitudes. Radiative properties of the atmosphere, which are important for understanding energy management, will be determined by measuring the scattering of light from specific features at various wavelengths and at various angles of illumination. Observations of auroral phenomena will be correlated with fields and particles measurements done with other instruments.

The SSI is an 800- by 800-pixel solid-state camera consisting of an array of silicon sensors called a "charge-coupled device" (CCD). The optical portion of the camera is built as a Cassegrain (reflecting) telescope. Light is collected by the primary mirror and directed to a smaller secondary mirror that channels it through a hole in the center of the primary mirror and onto the CCD. The CCD sensor is shielded from radiation, a particular problem within the harsh Jovian magnetosphere. The shielding is accomplished by means of a 1-centimeter-thick layer of tantalum that surrounds the CCD except, of course, where the light enters the system.

An eight-position filter wheel is used to obtain images of scenes through different filters. The images may then be combined electronically on Earth to produce color images.

The spectral response of the SSI ranges from about 0.4 to 1.1 micrometers. (A micrometer is one millionth of a meter.) Visible light has a wavelength covering the band of 0.4 to 0.7 micrometers.

The SSI weighs 29.7-kilograms and consumes, on average, 15 watts of power.

SSI DESCRIPTION

• The SSI is a catadioptric telescope with a 150-cm focal length (f/8.5).

• The SSI telescope has a field of view of 8.1 mrad and an angular resolution of 10.16 urad/pixel.

• The SSI optical system is a modified Voyager narrow-angle telescope flight spare.

• The SSI camera shutter and eight-position filter wheel are inherited from Voyager.

DESIGN DETAILS

• The filter wheel has eight filters, of the interference type, centered at 611, 404, 559, 671, 734, 756, 887, and 986 nm.

• The SSI detector is a virtual phase, buried channel, thick, frontside illuminated, 800 x 800 line CCD.

• The SSI has a pre-flash system to eliminate residual images after each exposure. This system bathes the CCD in near-infrared light (~930 nm) several times and reads the CCD several times at high speed.

• The SSI has five transmissive elements plus two mirrors.

• All SSI optical surfaces have anti-reflective coatings with a transmission factor of 62% at 576 nm.

• The front optical element is coated to minimize radiated heat loss and heaters provide thermal stability across the front aperture and within the body of the telescope.

• The long wavelength sensitivity limit (~1100 nm) is set by the detector and the short wavelength limit (~375 nm) by the anti-reflection coatings.

INSTRUMENT PARAMETERS

• Instrument Mass: 28 Kg
• Power Consumption:
  • Instrument - 23.0 W
• Microprocessor: 1802 type
• ROM/RAM: 3 Kb/3 Kb
• Fields of View: 8.1 mrad circular
• Size: 90 x 25 x 30 cm
• Thermal Range:'
  • Operating
    • Electronics: -10 to 18 Deg. C
    • Rear Optics: -10 to 18 Deg. C
    • Detector: -113 to -107 Deg. C
  • Non-operating
    • Electronics -10 to 30 Deg. C
    • Rear Optics -10 to 30 Deg. C
    • Detector -130 to 50 Deg. C

RELATED HOME PAGES

• Galileo Solid State Imaging Team
• Galileo Images