April 22, 2022 - Washing and drying an 8.4-meter mirror is no simple task, but with specialized equipment and procedures that are now being developed, the team on the summit will be able to keep both of Rubin Observatory’s mirrors clean and optimally reflective throughout the ten-year Legacy Survey of Space and Time (LSST). The video below shows the washing station in action with the mirror surrogate, as the team works on defining the washing and drying parameters that will be used to clean the real Primary/Tertiary Mirror (M1M3) in the future.
The 8.4-meter mirror surrogate isn’t really a mirror at all; this steel structure was designed to test many different procedures and its white surface was intended to receive a large amount of water during the washing tests. In current images you’ll notice silver-colored sections along with the white areas—these are places where we’re testing different silver and aluminum coating recipes to determine which one will be best, in terms of reflectivity and durability, for the M1M3 mirror. Rubin Observatory’s Secondary Mirror (M2) was coated with protected silver in July 2019.
After functionality tests for the washing station determined that it worked as expected, the team shifted focus to testing how it will be used. One goal, for example, was to define the mirror drying parameters. The washing station has an air blower that can operate at anywhere from 20% to 100% of its capacity, and two air knives (perpendicular to each other) with variable rotation and radial speeds. A series of trial-and-error tests were performed to find the combination that would get all the water off the mirror, and not take so long that water dried on its own. In this case, the final parameters were defined with the air blower operating at 100% of capacity, and the air knives working with a rotation speed of .3rpm and a radial speed of 150mm/sec. Using this configuration dries the mirror in approximately 30 minutes.
During operations, the reflectivity of the M1M3 mirror will be measured weekly. When it has degraded below the requirements, the mirror will either be washed in place or removed for a full washing and recoating. Previous experience indicates that a wet, in-situ wash of the M1M3 mirror would be necessary every six months, and a full re-coating would be necessary after two years. Data is being recorded onsite using coated glass samples to validate these values.
The real M1M3 mirror, which arrived on the summit in May 2019, is still packed safely in its transport crate. It is scheduled to be unpacked and installed on the M1M3 cell (the steel structure that supports the mirror) after dynamic testing of the M1M3 mirror support system on the Telescope Mount Assembly (TMA) later this year. After integration and testing with the cell, the mirror will be washed and coated with either bare-aluminum, protected aluminum or protected silver—the final decision, based on the results of the coating sample tests, will be made in a few months.
Read more about the coating chamber, which will be used to coat the M1M3 mirror, here.
More information about the history of the M1M3 mirror can be found at this link.