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 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.
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.
