OPTICAL DESIGN OF THE ULTRAVIOLET IMAGING SPECTROGRAPH FOR THE CASSINI MISSION TO SATURN

When the Cassini spacecraft arrives at Saturn early in the next centur y it will carry an Ultraviolet imaging Spectrograph (UVIS) designed an d built by the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado. Observations made with the UVIS will suppo rt a broad set of scientific investigations including spectroscopy, im aging, and occultations. The UVIS consists of two spectroscopic channe ls covering the wavelength ranges 56 to 118 and 110 to 190 nm. Each ch annel has an off-axis parabolic telescope followed by a toroidal grati ng spectrograph and an imaging microchannel plate-CODACON detector. Mi rror coatings and detector photocathode materials optimize the sensiti vity of each channel for its particular wavelength range. Spectrograph entrance slit mechanisms provide three independent spectral and spati al resolution modes for each of the three channels. A third optical tr ain consisting of a parabolic telescope and solar blind photomultiplie r tube with a Csl photocathode provides a high-sensitivity photometer mode within the UVIS. The UVIS configuration was selected as a balance d solution to a large number of engineering and scientific constraints . We describe these constraints, the optical design, and the anticipat ed performance of the instrument.