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Laboratory for Atmospheric and Space Physics

Instrument

The UVIS instrument

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 the UVIS Science Team’s paper accepted for publication in Space Science Reviews: “The Cassini Ultraviolet Imaging Spectrograph Investigation.”

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.