August 27, 2018 – The LSST 8.4-meter M1M3 Mirror Cell Assembly is a busy place. It includes the M1M3 monolithic steel surrogate mirror, the mirror support systems, the thermal control system, a laser tracker interface, the supporting steel structure, various mirror sensors, and the M1M3 control system. All of these components will work together to ensure the mirror holds its shape precisely as the telescope works through the night, pointing to various locations in the sky. The accompanying 15-second video demonstrates the response of the mirror support system to “transient human forces” – that is, in response to a person walking on the surface of the surrogate mirror. Each “dot” in the video represents the force applied at the location of one of the 156 actuators, ready to respond in situations where help is needed to hold the mirror shape. In the video, as the individual walks counter-clockwise around the surrogate mirror, you see green dots change to blue, representing the forces applied with the force balance system to counter the moving weight and keep M1M3 positioned accurately within the mirror cell. This is essential to counter forces such as those from changing gravity loads, slew inertial loads, and wind loads. Senior engineer Ed Hileman explains that, “to achieve specified image quality, the force actuators and Hardpoint Force Balance Control System must work together in concert. Force actuator testing, both individually in our instrumented test stand, and as a controlled composite group within the mirror cell, demonstrates that the accuracy of the support system is within the budgeted error allocation, meaning that we expect to meet image quality requirements if all other budget contributors meet their error allocations also.” Further testing continues with the surrogate mirror and mirror cell positioned at an elevation of 15 degrees off-zenith, allowing us to verify support system performance under significant lateral loads. Surrogate testing is happening now at CAID industries, which will be followed by optical testing at the U of A Mirror Lab facility, leading to final verification and acceptance of the mirror cell later this year.
