Solid-State Imaging (SSI)

Description

The Solid-State Imaging (SSI) experiment was intended to study Jupiter and its satellites using a multi-spectral, high-resolution, charge-coupled device (CCD) camera. The primary science objectives addressed by the experiment were to: (1) investigate the structure of the Jovian atmosphere and clouds; (2) examine the dynamics of the Jovian atmosphere through synoptic imaging of cloud structures; (3) measure the sizes and shapes of the Galilean satellites and determine their librations; (4) determine the geologic processes which formed the surfaces of the Galilean satellites by mapping them at a spatial resolution of less than 1 km and over a range of viewing and lighting angles; (5) identify and map the distribution of ices and minerals on the various satellite surfaces; (6) search for auroral or other atmospheric emission phenomena on the night side of Jupiter, its satellites, and/or the region around Jupiter; and, (7) seek opportunities for imaging the irregular Jovian satellites.

The optical system used was a modified flight spare of the narrow-angle telescope flown on Voyager and was similar in its basic design to the telescopes flown on Mariner 10. The telescope was a 1500 nm focal length (f/8.5), all-spherical, catadioptric telescope, 90 cm in length and 25 cm in diameter. The field of view of the telescope was 0.46 degrees with an angular resolution of 10.16 microradians/pixel.

The camera was operated in eight filtered band passes from 350-1100 nm. Because of the anti-reflective coatings used on the optics of the SSI, the short wavelength sensitivity was more limited than the camera on Voyager, thus there was no analog for the ultraviolet passband on the SSI. Otherwise, the spectral coverage was superior, both in total bandwidth and resolution, to the Voyager narrow-angle camera. The eight-position filter wheel, also inherited from Voyager, consisted of three broad-band filters: violet (404 nm), green (559 nm), and red (671 nm). These broad-band filters allowed for the reconstruction of visible color photographs and were compatible with the Voyager passbands. Four of the filters were chosen to optimize performance of the SSI in the near-infrared: two for methane absorption bands (727 nm and 889 nm), one for continuum measurements (756 nm), and one to proved spectral overlap with the near-infrared mapping spectrometer (986 nm). The final filter was a clear filter (611 nm) with a very broad (440 nm) passband.

Modifications to the Voyager design also included improved baffling (to further suppress off-axis scattered light), changes in the thermal coatings (to allow for the greater spectral range of the detector), the addition of a front aperture cover (to protect the exterior optical surfaces from contamination during the early phases of the mission), improved shielding (to protect the detector from high-energy radiation), and the addition of a pre-flash system (to prepare the detector for an exposure and ensure linearity in the images). The aperture cover was kept in place until just prior to Galileo's flyby of Gaspra. The camera shutter (also a Voyager flight spare) allowed for a minimum exposure time of 4.167 ms and a maximum exposure time of 51.2 s.

The detector was a virtual phase, buried channel, thick, frontside illuminated, 800 x 800 line CCD, 12.19 x 12.19 mm in size. The use of a CCD permitted the SSI to have an image geometry which was independent of brightness gradients, a linear photometric response to light, greater sensitivity to incident photons, and a wider spectral range than any camera previously flown on a planetary mission. Because pre-flight testing found that the CCD retained a residual image after an exposure, a pre-flash system was designed to bathe the CCD in near-infrared light (~930 nm) several times and then have the chip read out several times at high speed.

In addition to imaging Jupiter and its satellites, the SSI was used to obtain images of Venus, the Earth, the Moon, two asteroids, Gaspra and Ida, and a small satellite of Ida, Dactyl.