University of Colorado at Boulder University of Colorado CU Home Search A to Z Index Map
Laboratory for Atmospheric and Space Physics


The UVIS instrument

The Ultraviolet Imaging Spectrograph Subsystem (UVIS) was a set of telescopes used to measure ultraviolet light from the Saturn system’s atmospheres, rings, and surfaces. The UVIS also observed the fluctuations of starlight and sunlight as the sun and stars moved behind the rings and the atmospheres of Titan and Saturn, and it determined 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 had two spectrographic channels: the extreme ultraviolet channel and the far ultraviolet channel. The ultraviolet channels were built into weight-relieved aluminum cases, and each contained a reflecting telescope, a concave grating spectrometer, and an imaging, pulse-counting detector. The UVIS also included a high-speed photometer channel, a hydrogen-deuterium absorption cell channel, and an electronic and control subassembly.

The extreme ultraviolet channel (EUV) was used for imaging spectroscopy and spectroscopic measurements of the structure and composition of the atmospheres of Titan and Saturn. The EUV consisted 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 consisted 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 positioned one of the three entrance slits at the focal plane of the telescope, each translating to a different spectral resolution.

The spectrograph used an aberration-corrected toroidal grating that focused 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 consisted of a low-voltage power supply, a programmable high-voltage power supply, charge-sensitive amplifiers, and associated logic. The EUV channel also contained 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) was 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 was similar to the EUV channel except for the grating ruling density, optical coatings, and detector details. The FUV electronics were 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) performed stellar occultation measurements of the structure and density of material in the rings. The HSP resided in its own module and measured undispersed (zero-order) light from its own parabolic mirror with a photomultiplier tube detector. The electronics consisted of a pulse-amplifier-discriminator and a fixed-level high- voltage power supply. The hydrogen-deuterium absorption cell channel (HDAC) was 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 were resonance absorption cells filled with pure molecular hydrogen and deuterium, respectively. They were located between an objective lens and a detector. Both cells were made of stainless steel coated with teflon and were sealed at each end with MgF2 windows. The electronics consisted of a pulse- amplifier-discriminator, a fixed-level high-voltage power supply, and two filament current controllers.

The UVIS microprocessor electronics and control subassembly consisted of input-output elements, power conditioning, science data and housekeeping data collection electronics, and microprocessor control elements.