July 30, 2019 - If shiny things get your attention, prepare to be dazzled by the newly coated LSST Secondary Mirror (M2)! The mirror received its first reflective coating on July 16, 2019, at the LSST summit facility building on Cerro Pachón. The 3.4-meter M2 mirror arrived on the summit in December 2018, after being shipped from Rochester, NY, where it was fabricated by L3Harris (formerly Harris Corporation). Since its arrival the container holding the mirror has been stored inside the LSST observatory building, on the roof of the camera clean room. Until the coating campaign, the mirror hadn't been removed from the box, although careful inspections of all parts of the mirror were conducted to make sure no damage occurred during shipping.
The M2 surrogate (an aluminum structure that stands in for the glass mirror during testing) was used to test all coating parameters, with witness samples placed all around and over the mirror surrogate, first on May of this year and again a week before the glass M2 was placed into the coating chamber. After evaluating the results, the team determined that the performance of the coating plant, the coating parameters, and everything else was ready to coat the glass mirror.
It's critical to remove all dust and debris from the surface of the mirror before applying the coating, so on July 8th the M2 mirror was lifted from its storage container into a dedicated washing station, using the bridge crane installed over the coating area, and cleaned. Then the mirror was lifted into the coating chamber and the vacuum process was initiated. Overnight the chamber was evacuated to a vacuum level of 3 x 10-7 millibars and by the next morning conditions were perfect for coating the mirror.
The LSST Coating Chamber uses magnetron sputtering technology, a method that has proven successful with mirrors for other large telescopes including the twin Gemini Telescopes and the European Southern Observatory Very Large Telescope. There are two rings of magnetrons inside the LSST Coating Chamber: an inner ring that will be used to apply coatings to the M2 mirror and the M3 mirror, and an outer ring that will apply coatings to the 8.4-meter M1 mirror. In order to achieve the overall combination of durability and reflectivity necessary to achieve LSST's science goals over its ten-year survey lifetime, the M2 mirror was coated with protected silver, and the M1M3 mirror will be coated with protected aluminum.
Coating the M2 mirror took just over four hours and, according to LSST Senior Coating Engineer Tomislav Vucina, everything went smoothly throughout the process. The attached video shows the mirror being coated through a small window built into the coating chamber; the mirror is the stationary object in the center.
The coating team ran a series of tests at the conclusion of the coating process and found that reflectivity results, adhesion tests, and pinholes count not only met, but exceeded the requirements. With the M2 coating campaign successfully accomplished, the mirror was returned to its container and lifted back into its storage area, where it will be out of the way of other activities being performed on the maintenance level of the building. The mirror will be inspected every three months, to monitor the reflectivity and clean off dust, until it's time to integrate the glass mirror with the M2 cell assembly in 2021.