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VIMS Engineering Technical Write-up

VIMS Engineering Technical Write-up

TL: Dr. Robert H. Brown

VIMS General Description:

The Visual and Infrared Mapping Spectrometer (VIMS) is a pair of imaging grating spectrometers designed to measure reflected and emitted radiation from atmospheres, rings, and surfaces over wavelengths from 0.35 to 5.1 micrometers to determine their compositions, temperatures, and structures.

VIMS Scientific Objectives:

  • To map the temporal behavior of winds, eddies, and other features on Saturn/Titan.
  • To study the composition and distribution of atmospheric and cloud species on S/T.
  • To determine the composition and distribution of the icy satellite surface materials.
  • To determine temperatures, internal structure, and rotation of Saturn's deep atmosphere.
  • To study the structure and composition of Saturn's rings.
  • To search for lightning on Saturn and Titan and for active volcanism on Titan.
  • To observe Titan's surface.

VIMS Sensing Instruments:

  • Visible Channel [VIMS-V] (0.35 to 1.07 µm [96 channels]; 32x32 mrad field of view)
  • Infrared Channel [VIMS-IR] (0.85 to 5.1 µm [256 channels]; 32x32 mrad field of view)

VIMS Instrument Characteristics:

  • Mass (current best estimate) = 37.14 kg
  • Peak Operating Power (current best estimate) = 27.20 W
  • Average Operating Power (current best estimate) = 21.83
  • Peak Data Rate (current best estimate) = 182.784 kilobits/sec
  • Dimensions (approximate) = 78 cm x 76 cm x 55 cm

A spectrometer is an optical instrument that splits the light received from objects into its component wavelengths by means of a prism or -- as in the case of Cassini -- a diffraction grating. The spectrometer then measures the intensities of the individual wavelengths and uses the data to infer the composition and other properties of the objects that emitted the light (e.g., a distant star), that absorbed specific wavelengths of the light as it passed through them (e.g., a planetary atmosphere), or that reflected the light (e.g., a planetary surface). Spectrometers may be sensitive to a variety of wavelengths, from the ultraviolet, through the visible, to the infrared portions of the electromagnetic spectrum.

The Cassini Visible and Infrared Mapping Spectrometer Subsystem (VIMS) will be used to map the surface spatial distribution of the mineral and chemical features of a number of primary and secondary targets. These targets include the Saturnian ring and satellite surfaces, the Saturnian atmosphere, and the atmosphere of Titan.

The VIMS Subsystem is organized into two assemblies: the optical pallet assembly and the main electronics assembly. For more information on these assemblies, click on their names.

(VIMS Links)

The optical pallet assembly consists of the following elements: the infrared channel, the visible channel, the visible channel electronics, and the signal processing electronics. The optical pallet has one mechanical interface with the spacecraft, and all electrical interfaces are via the main VIMS electronics. The pallet maintains all alignments internal to the VIMS instrument relative to the spacecraft mounting surface.

The infrared channel (VIMS-IR) is an opto-mechanical subassembly designed to produce multispectral images in the IR range. It consists of a Cassegrain telescope, a conventionally ruled spectrometer grating, and a 256-element linear array focal plane assembly cooled to its required operating temperature by a passive radiator. The VIMS-IR will be configured as a "whiskbroom" scanning imager, which means that the optical instrument's instantaneous field of view (IFOV) is a single pixel. A two-dimensional image is created by scanning along a row of pixels, dropping down a row, scanning that row, etc., using a two-axis scanning mirror.

The visible channel (VIMS-V) is an opto-mechanical assembly designed to produce multispectral images in the visible range. It consists of a Shafer telescope, a holographic spectrometer grating, and a silicon CCD area array focal plane detector cooled to its required temperature by a passive radiator. The VIMS-V will be configured as a "pushbroom" imager, which means that the optical instrument's IFOV is an entire line of pixels. This is scanned over the scene with a single-axis scanning mirror to produce a series of contiguous rows, which together form a two-dimensional image.

The visible channel electronics (VCE) support the operation of the VIMS-V and the preprocessing of its data for relay to the signal processing electronics. The signal processing electronics (SPE) support the operation of the VIMS-IR and the preprocessing of its data for relay to the main electronics.

The VIMS main electronics (ME) is a single assembly that synchronizes the visible and IR channel mirror scanning mechanisms and controls the acquisition of data according to the selected operational hardware configuration. The ME serves as the only electronic interface between all elements of the VIMS instrument and the spacecraft, including power, data, command, and telemetry.

During flight operations, the ME will serve as the interface between ground operations (via the spacecraft) and the VIMS Subsystem. During the first 180 days after launch, all VIMS decontamination heaters will be on to prevent contamination from outgassing products (i.e., gasses liberated from nearby components). During the cruise phase, VIMS will be turned on once for monitoring of instrument health and performance. At other times, the instrument be off. During these times, decontamination heaters will be used periodically to keep the optics and radiator surfaces clean. At Saturn, VIMS will be used as determined by spacecraft command sequencing to acquire the desired science data.

For additional information, see: