LSST primary tertiary mirror cell assembly: Integration and functional testing

The Vera C. Rubin Observatory is currently under construction on Cerro Pachon, in Chile. It was designed to conduct a 10-year multi-band survey of the southern sky with frequent re-visits (with both an intra- and extra-night cadence) to identify transient and moving objects. The mirror cell assembly was designed in Tucson, Arizona by the Rubin Observatory engineering department, and was tested twice in Tucson. The first testing campaign was performed at CAID industries, where the mirror cell was fabricated, using a steel mirror surrogate that has the same geometry and mass of the glass mirror2,4. The glass mirror is a single monolith that contains both the primary and tertiary mirrors on a single substrate. The testing results confirmed that the mirror support system was performing within the design specifications, and that it was safe to install the glass mirror on the cell. The second test campaign was performed at the Richard F. Caris Mirror Lab of the University of Arizona using the actual glass mirror16. This test campaign was performed under the test tower, which contains a vibration insensitive interferometer to measure mirror figure. This confirmed the mirror support system could achieve proper optical surface figure control for both primary and tertiary mirrors. After successful test campaigns at CAID, and the mirror Lab, the mirror cell assembly was disassembled, packed and shipped to the Rubin Observatory site at the Cerro Pachon summit in Chile. Upon arrival, the mirror cell has been integrated with the mirror surrogate once again to perform the third test campaign that confirmed the system has arrived safe and operational to the summit. This integrated system will be tested on the telescope mount assembly to verify that it still meets it requirements under the effects of variations in gravitational orientation, and dynamic (slewing) loads.