Near Infrared Mapping Spectrometer (NIMS)

Description

The objectives of the Galileo Near Infrared Mapping Spectrometer (NIMS) experiment were to: (1) map the distribution of surface minerals on the Galilean satellites at spatial resolutions of 5-30 km; (2) identify the phases and mixtures present thereon; (3) correlate observed mineralogical distributions with the geomorphology observed with the solid-state imaging (SSI) system; and, (4) determine cloud morphology of the Jovian atmosphere and its structure over a wide range of phase angles. In addition to these primary goals, similar observations were made during Galileo's encounters with Venus, Earth, the Moon, Gaspra, and Ida.

The instrument consisted of a 22.8 cm diameter, f/3.5 Ritchey-Chretien telescope with one dimension of spatial scanning (via a moving secondary mirror) and a diffraction grating spectrometer. The secondary mirror motion was performed in steps of twenty equal increments of 0.5 mrad each. The field stop of the telescope also defined a 0.5 mrad field-of-view, normal to the mirror scan direction and parallel to the plane of dispersion of the spectrometer.

An InGaAs light-emitting diode, mounted on the telescope spider, was used for in-flight wavelength verification of the spectrometer. The spectrometer itself was a plane grating (39 lines/mm) illuminated by a 400 mm focal length (f/3.5) Dall-Kirkham collimator backed by a wide-angle (200 mm focal length, f/1.8), flat-field camera which focussed the entrance slit (the telescope field stop) onto its detectors. There were a total of seventeen individual detectors (15 InSb and 2 Si) in the focal plane along with their associated spectral filters and electronics. Each of the photodiode detectors had a photo-active area of 0.2 x 0.2 mm and was anti-reflection coated for a specific spectral region. The quantum efficiencies of the coated photodiodes were measured at 70% or better prior to launch.

Shielding from high-energy particles was provided by a hermetically-sealed tantalum case with a sapphire window for optical input. Cryogenic temperatures (required for the InSb detectors) were achieved through the use of a single-stage passive radiative cooler. The cold stage contained the photodiode array and radiated energy to space, cooling the detectors to as low as 64 K. The radiator was mounted at an angle of 62.5 degrees to the optical axis of the telescope and in the rotational plane of the scan platform. Pre-launch and/or in-flight spectral, radiometric, and spatial calibrations of the instrument were performed. The instrument could operate in eleven different modes, six mapping and five spectrometer.