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Cryogenic High-Accuracy Refraction Measuring System (CHARMS): a new facility for cryogenic infrared through far-ultraviolet refractive index measurements

The infrared optics community continually expresses its desire for high accuracy, cryogenic, infrared, refractive index data for a wide variety of materials, as well as knowledge of index variability for each species. The Near-Infrared Camera (NIRCam) for the James Webb Space Telescope (JWST) will not only provide astronomical scientific imaging in the 0.6 to 5 micron wavelength range but it will also serve as the observatory’s wavefront control sensor. The provision of accurate, infrared (IR) cryogenic index data for optimizing the refractive optical design of NIRCam will be an important component in mission risk reduction. To this end, we have built the Cryogenic, High-Accuracy Refraction Measuring System (CHARMS). This paper is the fourth covering the design and construction of CHARMS and highlights the significant design changes culminating in the finished refractometer. Aspects which make CHARMS a high accuracy system, one which will measure refractive index more accurately than previous refractometers, are presented along with sub-system performance data. Finally, we present our "first light," cryogenic, infrared refractive index data for lithium fluoride (LiF) for temperatures down to 25 K, relating it to previously published ambient data for this material.