Abstract: This paper describes the design, analysis, and testing of a rapidly cooled beryllium mirror. The mirror is the primary mirror in a four element optical system for the Long Wavelength Infrared Advanced Technology Seeker (LATS). The seeker has a requirement to have its optical elements; primary mirror, secondary mirror and two germanium lenses, cooled from 300 K to less than 120 K within five minutes. At the operating temperature of less than 120 K, the optical components must be in alignment and also maintain their optical figure. The cooling strategy for the mirror is to use a single stage argon Joule-Thomson refrigerator to generate cryogenic cooling fluid and circulate the coolant through flow channels incorporated in an integral heat exchanger in the body of the mirror. During the rapid cooling process methane, a phase change material (PCM) with a triple point of 90.7 K, is injected into the mirror cavities. These cavities are integral in the body of the mirror. The methane is condensed and subsequently solidified in the cavities as the mirror is cooled to less than 120 K. The mirror will be maintained at its operating temperature by utilizing the heat of sublimation of the solid methane to absorb any parasitic heat load during the operation of the seeker which is about fifteen minutes. The final product is two matched sets of beryllium mirrors, consisting of a 12.5 cm diameter parabolic primary and a 4.1 cm diameter ellipsoid secondary. While the beryllium mirrors were being fabricated, two matched sets of aluminum mirrors were diamond turned and used in the seeker for testing. A single beryllium proof of concept (POC) spherical mirror with a 12.5 cm diameter was also made and tested off-line prior to the fabrication of the off axis beryllium mirror sets. All of these were tested in a vacuum chamber with numerous cycles of cooling to 80 K using liquid nitrogen as the alternate laboratory cryogenic cooling fluid. The results of the testing and the comparisons with the thermal analysis are presented. The impact design analysis and development testing will be discussed in this paper. !1
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