Cooling Flow Requirements for the Honeycomb Cells of the LSST Cast Borosilicate Primary-Tertiary Mirror

摘要

As a result of the relatively large coefficient of thermal expansion, cast borosilicate glass mirrors require extensive thermal control. Historically, this thermal control was accomplished by injecting high velocity ambient temperature air into each honeycomb cell through holes in the mirror's bottom plate at a constant velocity of approximately 8 liters per second. Although this approach was adequate for previous projects that utilize these mirrors, it cannot meet both the mirror seeing requirements and the thermal distortion requirements of the LSST mirror simultaneously. At the beginning of the observing night, when the ambient air temperature is changing rapidly, providing an air cooling rate adequate enough to control mirror seeing would produce excessive thermal distortion of the mirror. This thermal distortion is the result of uneven cooling between the face plate and back plate. The face plate's top surface is cooled by ambient air while its bottom surface and both surfaces of the back plate are cooled by the cooling air flow. By precooling the mirror (～1 ℃) below the expected initial exterior ambient temperature, and reducing the cooling air flow rate (2-3 L/s) both the mirror seeing and distortion requirements can be met.