GMT ASM co-phasing numerical simulation and experimental results

One of the most challenging aspects of the Giant Magellan Telescope (GMT) is the need to co-phase the seven segments for the Laser Tomographic Adaptive Optics (LTAO) mode of operation. Low and high frequency co-phasing systems are used that overlap at 1/30 Hz. The high frequency co-phasing of the seven Adaptive Secondary Mirrors (ASMs) is obtained using 48 interferometric distance measuring systems mounted on the Reference Bodies outer edges. The goal is to keep optically co-phased for 120 seconds, this period being related to the bandwidth of the optical phasing sensor that will be available at the telescope. A complex control system computes the co-phasing correction based on commands from the Observatory Control Systems and measurements from the interferometric sensors. The correction command is distributed to the deformable mirror actuators. The disturbances to be compensated are quasi static ones, including gravity and thermal, together with dynamic contributions, in particular wind and telescope vibrations. A comprehensive numerical simulation has been developed and used to predict the overall system behavior, including the telescope optical loop. We report the control architecture and the results of the numerical simulation, which includes the calibration procedure with its error propagation. Moreover, we present the test results of the sensor's stability and noise levels obtained with a dedicated breadboard mounting three pairs of sensors.