Rubin's Secondary Mirror is Installed

The construction of the Vera C. Rubin observatory has just crossed a major milestone with the successful installation of its 3.5 meter diameter secondary mirror. The observatory is now one step closer to first light in 2025, when it will begin the Legacy Survey of Space and Time (LSST): a mission to repeatedly image the entire sky, at high resolution, to create a time-lapse record of the Universe.

The construction of the Vera C. Rubin Observatory (https://rubinobservatory.org/) in Chile has crossed a major milestone with the successful installation of the secondary mirror assembly. The 3.5 meter convex mirror is the first permanent optical component to be integrated into the Simonyi Survey Telescope. Construction is expected to be completed by 2025, when it will achieve first light. In its completed state, it will effectively be the largest digital camera in the world, built to perform the Legacy Survey of Space and Time (LSST), a project to create a ten-year time-lapse view of the entire southern sky.

The mirror is made from Corning® ULE® Glass (Ultra-Low Expansion Glass), and was manufactured by Corning Advanced Optics in Canton, New York. After delivery in 2009, it was stored at Harvard University for five years. After this, L3Harris Technologies, in Rochester, New York, got to work finishing and polishing the mirror. They developed new techniques to work the mirror, as it is very technically challenging to work such a large convex mirror to the necessary precise tolerance. They also designed and built the mirror’s cell assembly, which has adaptive optics capabilities. The cell is built on a stiff steel mounting plate and features 72 axial actuators and six tangent actuators. These allow the supporting structure to constantly adjust as the telescope moves, compensating for the distorting effect of its own weight to keep the mirror at exactly the correct shape at all times.

In 2018, the mirror and cell assembly were shipped to the observatory site in Chile. On arrival, it was given its silver coating. Telescope mirrors are usually coated with aluminum, which is hardier and less prone to tarnishing, and so doesn’t need to be renewed as often. But the Simonyi Survey Telescope in the Vera C. Rubin observatory uses silver (for its superior reflectivity) together with a protective coating to seal it away from atmospheric oxygen and extend the life of the coating. After silvering, it was then sealed back into its container to be stored until construction had reached the point where the telescope would be ready for it. Finally, in July 2024, the complete assembly was installed into the telescope, and integrated with its control electronics.

“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.”

The mirrors in observatory telescopes need to be removable, so that they can be cleaned and occasionally resurfaced. But these large mirrors are very heavy, and it would be a disaster if so much glass were to be dropped. That’s why installation and removal is done very carefully, with specialized machinery, following a documented process. To install the secondary mirror assembly, engineers in the summit team loaded it onto a custom-built cart, which rotated the mirror to a vertical position. It was then hoisted up into position on the telescope structure, and carefully bolted into place. Once it was securely attached, the electronics were connected, and the software control system was activated.

“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 at 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.”

The next component to be installed will be the Commissioning Camera (ComCam). This is a temporary camera, meant to be used for testing and integration. At “only” 144 megapixels, it’s only a fraction of the size of the LSST camera. This is not the first time that ComCam has been installed – it is used at various stages of construction to test the various components, ensure that they are properly installed, and that they work together as expected. After ComCam has done its job, the team will get to work on integrating the 8.4 meter primary mirror assembly, followed by the LSST camera.

The Vera C Rubin Observatory was named after the astronomer who first provided convincing evidence of Dark Matter. She and a colleague studied over 60 galaxies to measure how fast they were rotating. In 1978, they found that these galaxies were all spinning too fast: Given the amount of visible matter, they should have not had enough gravity to stop themselves from flying apart. There had to be extra invisible mass, and this work was the first convincing proof of the Dark Matter theory.

The telescope itself is a survey instrument: It is designed to take deep, wide-field images of the sky, very rapidly. The design of the telescope allows it to move very quickly, switching from target to target in short order. With an 8.4 meter primary mirror, it is very sensitive, and can see very faint, distant objects. But it also has a very fast focal ratio of f/1.234, giving it a very wide field of view and allowing it to take much faster exposures.

When the LSST camera is installed, it will capture images covering an area of 9.6 square degrees. Each image will be made from two 15 second exposures, at a resolution of 3.2 gigapixels. At this rate, it will be able to image the entire sky every ten days, and it will repeat this process for ten years. The resulting data will be a ten year time-lapse video of the entire universe, monitoring 20 billion galaxies, 17 billion individually resolved stars, and the orbits of around 6 million objects within our Solar System!

For more information, read the original press release at https://noirlab.edu/public/news/noirlab2419/