<title>Cryogenic Optical Alignment Stability Test (COAST) of the Composite Infrared Spectrometer (CIRS) engineering unit for the Cassini mission to Saturn</title>

Abstract

The composite infrared spectrometer (CIRS) of the Cassini mission to Saturn has two interferometers covering the far- IR (FIR) and mid-IR (MIR) wavelength region. The instrument is aligned at ambient temperature, but operates at 170 Kelvin and has challenging interferometric alignment tolerances. Interferometric alignment sensitivity tests of the CIRS FIR breadboard indicated that the instrument was sensitive to alignment perturbations in the few arc second regime; therefore, a cryogenic alignment stability test was designed and implemented to determine the stability of the CIRS optics module. Test beamsplitters were installed in the instrument to allow transmission of HeNe laser beams through both channels of the instrument onto test focal planes consisting of position sensing photodiodes to measure the actual shear and boresight change in the focal planes. Cryogenic vacuum compatible shutters were designed and fabricated to allow separate measurements of the reflected and transmitted components of the test beam. The test determined that the optics bench was distorting an unacceptable amount between ambient and operating temperature, but that the distortion was very repeatable, opening the possibility of performing an interferometric alignment at cryogenic temperature.