Near Infrared Mapping Spectrometer (NIMS)

NSSDC ID: 89-084B-01
Mission Name: Galileo Orbiter
Principal Investigator: Carlson

Experiment mass: 18.00 kg
Average experiment power: 12.00 W
Average experiment bit rate: 11.52 kbps


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.