Icy Target Thermal Test Apparatus and Calibration of a Planetary Spectrometer
Berisford, Daniel F.
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The Compositional Infrared Imaging Spectrometer (CIRIS) is currently under development at JPL for outer planetary missions, and is undergoing thermal-vacuum environmental testing as part of the TRL progression effort. This involves analyzing icy targets under cryogenic vacuum conditions with the spectrometer and the use of an optical calibration device designed for integration with the test setup. CIRIS was designed for use on a future Europa orbiter, with applications for missions to other worlds including Mars. It has a compact and rugged design due to its spinning refractor, which replaces the moving mirror setup of a traditional Michelson interferometer. We have designed and fabricated a test apparatus to calibrate CIRIS using a black body source and to measure reflectance of ice specimens. The apparatus presented here is validating the performance of CIRIS under Europa-like conditions, including temperature of the spectrometer, temperature of the target, and illumination of the target. A dewar assembly resides inside the vacuum chamber and is filled with liquid nitrogen using a fluid feedthrough. A cold plate mounted directly to the dewar will freeze/ contain the ice or rock targets. An external blackbody light source will illuminate the targets via a mirror/ window assembly, and a second mirror will direct the reflected light into the spectrometer. The mirror configuration can be moved in six degrees of freedom for optical alignment. The calibration source is designed to emit blackbody radiation at known and controllable temperatures between 150 and 350K via interchangeable apertures. To maintain an emissivity as close as possible to that of an ideal blackbody, the calibration source is a conical cavity coated with highly emissive paint within a solid cylinder. The cavity is made of aluminum and contains multiple heaters to ensure isothermal conditions, with ceramic standoffs to create a long thermal path which allows the cavity to be at a high temperature while the spectrometer and other nearby hardware remain cryogenic.