The team at the NSF-DOE Vera C. Rubin Observatory in Chile has completed the installation of the 3.5-meter secondary mirror assembly on the Simonyi Survey Telescope. This milestone signifies the first permanent component of the telescope's optical system, which also includes an 8.4-meter primary/tertiary mirror and the LSST Camera - the world's largest digital camera. Funded by the U.S. National Science Foundation and the U.S. Department of Energy's Office of Science, the Rubin Observatory is set to begin the 10-year Legacy Survey of Space and Time (LSST) in 2025, ushering in a new era of astronomy and astrophysics.

The mirror assembly installation marks the result of years of meticulous planning and effort by teams in the U.S. and Chile. After the mirror was fabricated and polished, it and the mirror cell assembly components were shipped to Chile in 2018 and stored at the observatory. The secondary mirror was coated with protected silver at Rubin Observatory in 2019, integrated with the mirror cell in early July 2024, and then installed on the telescope.

"Working with the mirror again after five years is extremely exciting because it really feels like we're in the home stretch," said Sandrine Thomas, Deputy Director for Rubin Observatory Construction, "Now we have glass on the telescope, which brings us a thrilling step closer to revolutionary science with Rubin."

At nearly four meters in diameter, Rubin's secondary mirror is one of the largest convex mirrors ever constructed. The 10-centimeter-thick monolithic mirror blank was produced by Corning Advanced Optics in Canton, New York, using Corning ULE Glass (Ultra-Low Expansion Glass).

"Corning is proud of our nearly 20-year collaboration with the Rubin Observatory team," said Claude Echahamian, VP and GM Corning Advanced Optics. "As a result, Corning's cutting-edge ULE mirror blank for the Simonyi Survey Telescope will help to enable crystal clear views of deep space, revealing millions of previously unknown Solar System objects with more detail than ever before."

After its delivery in 2009, the mirror blank was stored at Harvard University in Cambridge, MA for five years before being polished and completed at L3Harris Technologies in Rochester, New York. L3Harris employed innovative measurement techniques in the polishing process to manage the large precision convex surface. The company also designed and built the secondary mirror cell assembly, including a steel mounting plate, 72 axial and six tangent actuators for shape control, electronics and sensors, a thermal control system, and the mirror control system.

"Our 55-year legacy of designing and constructing high-end optical systems for space and ground continues with the world's largest active secondary mirror system built for Rubin Observatory," said Charles Clarkson, Vice President and General Manager, Imaging Systems, Space and Airborne Systems, L3Harris. "With this milestone, we are closer to pushing scientific frontiers and charting the Universe like never before, and we look forward to the science that will be discovered."

To install the mirror assembly on the telescope mount, the Rubin summit team used a specialized cart to position the assembly vertically, lifted it onto the mount, and ensured the system was controlled to prevent stress on the glass. Once the assembly was bolted in place, the team connected the mirror cell to the electronics cabinet on the telescope mount and reactivated the mirror's software control system.

In the coming weeks, the Rubin team will re-install the Commissioning Camera, a smaller version of the LSST Camera, to conduct various test campaigns on the optical system. They will also focus on preparing the primary mirror assembly for integration in August and the LSST Camera for installation later this year.

Rubin Observatory is a program of NSF NOIRLab and will be jointly operated with DOE's SLAC National Accelerator Laboratory.

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NSF-DOE Vera C. Rubin Observatory
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