The Ultraviolet Imaging Spectrograph Subsystem (UVIS) is a set of telescopes
used to measure ultraviolet light from the Saturn system's atmospheres, rings,
and surfaces. The UVIS will also observe the fluctuations of starlight and
sunlight as the sun and stars move behind the rings and the atmospheres of
Titan and Saturn, and it will determine the atmospheric concentrations of
hydrogen and deuterium.
 
The following is a brief description of the components of the UVIS. For a
more detailed description, see [ESPOSITOETAL2005] and contained in the
DOCUMENT directory of this archive (pending permission).
 
The UVIS has two spectrographic channels: the extreme ultraviolet channel
and the far ultraviolet channel. The ultraviolet channels are built into
weight-relieved aluminum cases, and each contains a reflecting telescope, a
concave grating spectrometer, and an imaging, pulse-counting detector. The
UVIS also includes a high-speed photometer channel, a hydrogen-deuterium
absorption cell channel, and an electronic and control subassembly.
 
The extreme ultraviolet channel (EUV) will be used for imaging
spectroscopy and spectroscopic measurements of the structure and composition
of the atmospheres of Titan and Saturn. The EUV consists of a telescope with
a three-position slit changer, a baffle system, and a spectrograph with a
CODACON microchannel plate detector and associated electronics. The telescope
consists of an off-axis parabolic section with a focal length of 100 mm, a 22
mm by 30 mm aperture, and a baffle with a field of view of 3.67 degrees by
0.34 degrees. A precision mechanism positions one of the three entrance slits
at the focal plane of the telescope, each translating to a different spectral
resolution.
 
The spectrograph uses an aberration-corrected toroidal grating that
focuses the spectrum onto an imaging microchannel plate detector to achieve
both high sensitivity and spatial resolution along the entrance slit. The
microchannel plate detector electronics consist of a low-voltage power
supply, a programmable high-voltage power supply, charge-sensitive
amplifiers, and associated logic.
 
The EUV channel also contains a solar occultation mechanism to allow solar
flux to enter the telescope when the sun is still 20 degrees off-axis from
the primary telescope.
 
The far ultraviolet channel (FUV) will be used for imaging spectroscopy
and spectroscopic measurements of the structure and composition of the
atmospheres of Titan and Saturn and of the rings. The FUV is similar to the
EUV channel except for the grating ruling density, optical coatings, and
detector details.  The FUV electronics are similar to those for the EUV
except for the addition of a high-voltage power supply for the ion pump.
 
The high-speed photometer channel (HSP) will perform stellar occultation
measurements of the structure and density of material in the rings. The HSP
resides in its own module and measures undispersed (zero-order) light from
its own parabolic mirror with a photomultiplier tube detector. The
electronics consist of a pulse-amplifier-discriminator and a fixed-level
high-voltage power supply.
 
The hydrogen-deuterium absorption cell channel (HDAC) will be used to
measure hydrogen and deuterium in the Saturn system using a hydrogen cell,
a deuterium cell, and a channel electron multiplier (CEM) detector to record
photons not absorbed in the cells. The hydrogen and deuterium cells are
resonance absorption cells filled with pure molecular hydrogen and deuterium,
respectively. They are located between an objective lens and a detector. Both
cells are made of stainless steel coated with teflon and are sealed at each
end with MgF2 windows.  The electronics consist of a pulse-amplifier-
discriminator, a fixed-level high-voltage power supply, and two filament
current controllers.
 
The UVIS microprocessor electronics and control subassembly consists of
input-output elements, power conditioning, science data and housekeeping data
collection electronics, and microprocessor control elements.