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Resulting from its unusual optical configuration, the Vera C. Rubin Observatory requires precise top-end assembly (TEA) thermal control. The three-mirror system locates the large camera, the secondary mirror (M2), the secondary mirror hexapod, the camera hexapod/rotator, and associated electronics on the TEA. Escaping heat, or overcooling, crosses the optical path three times potentially significantly degrading the image quality. Most observatories follow a common thermal control strategy. A central refrigeration system, composed of chillers and pumps, supplies non-precision temperature-controlled ethylene glycol/water (EGW) coolant through long pipes, to the observatory’s subsystems including the general ones (Facility Services, Telescope machinery, etc.) and the scientific instrumentation. The refrigeration for the instrumentation is provided by EGW cooled secondary systems. The common strategy is inadequate for this application. For this application, since overcooling is just as detrimental as escaping heat, TEA thermal control is needed to levels impractical with the common strategy. Consequently, a new system was developed to provide superior thermal control. An intermediate cooling stage was added directly under the telescope. Using local chillers, recirculation pumps, and mixing valves, coolant is provided to the TEA at precise temperatures and flow rates. This system itself is cooled by EGW from the central refrigeration system. The location of the Camera, etc. on the TEA, over the main primary tertiary mirror (M1M3), produces a critical leak risk to the optical system. Many glycol/water leaks at different observatories have damaged critical electronics and optics elements. Consequently, less toxic and corrosive Dynalene was chosen, rather than the more common EGW.
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