Large aperture telescopes require active control to maintain focus, collimation, and correct figure errors in the Primary Mirror (M1) due to gravity and thermal deformations. The Giant Magellan Telescope (GMT) M1 active optics subsystem consists of the hardware and software that controls the shape, position, and thermal state of each mirror segment. Pneumatic force actuators support the weight and control the surface figure while linear position actuators control the six solid-body degrees of freedom of each mirror segment. A forced convection system comprised of fan-heat exchanger units control the mean temperature and thermal gradient of each mirror segment. The M1 Subsystem design leverages existing technology and employs innovations driven by more demanding requirements compared to heritage systems. These differences led to the identification of three key GMT project risks: determining if the vibration environment induced by the fan-heat exchanger units and the error in the applied influence functions required to shape the mirror are within image quality budget allocations. The third risk is incorporating damping to the force actuators to meet the seismic requirements. GMT is currently mitigating these risks by integrating a fully functional off-axis M1 Test Cell at the University of Arizona’s Richard F. Caris Mirror Lab. This paper summarizes our requirements and design presented at the M1 Subsystem Preliminary Design Review in June 2019, describes our risk burn-down strategy for the M1 Subsystem, and presents our integration and test progress of the M1 Test Cell.
The Giant Magellan Telescope project is proceeding with design, fabrication, and site construction. The first two 8.4m primary mirror segments have been completed and placed in storage, three segments are in various stages of grinding and polishing, the sixth segment is in the initial stages of casting, and glass is in hand to cast the seventh segment. An industry contract is in place to complete the design and proceed with fabrication of the telescope structure. Residence buildings and other facilities at the Las Campanas site in Chile are complete. Hard rock excavation of the foundations for the enclosure and telescope pier is complete. Preliminary design of the enclosure has been completed and final design is underway. Seismic isolation system bearings have been tested. A primary mirror segment test cell that will be used to qualify control system components and software is being fabricated. Prototyping continues in several areas, including on-telescope wavefront sensing and control elements, telescope laser metrology, and a subscale Adaptive Secondary Mirror (ASM). Adaptive optics and phasing testbeds are under development. Construction activities were delayed by the global coronavirus pandemic, but work has now resumed.
The Primary Mirror Device Control System (M1 DCS) is one of the many Device Control Systems (DCS) included in the Giant Magellan Telescope (GMT) control system and is responsible for the overall control and operation of the GMT primary mirror segments. The primary mirror is composed of seven 8.4m diameter segments, six off-axis and one in the center. The active support system of each segment comprises 170 support actuators for the off-axis segments and 154 actuators for the center segment to control the mirror figure, and 6 hardpoints to control the six degrees of freedom of rigid body motion. The software design follows a component model-based architecture, implemented using the GMT core software frameworks. Software components of the M1 DCS are specified using a custom Domain Specific Language (DSL) and inherit all key features of the core components such as communication ports, default behaviors, telemetry, logs, alarms, faults, state machines and engineering user-interface without the need of a separate implementation. The communication between the real time software and the controlled devices is implemented by an EtherCAT Fieldbus in a ring topology. This master-slave standard protocol enables the control system to reach 100 Hz closed loop rate for active support control. This paper describes the software of the M1 DCS, the tests performed with different software and hardware simulators, and the strategy to ensure software readiness with the final optical mirror.
This paper describes the design, status, and test program for the Giant Magellan Telescope (GMT) Primary Mirror Subsystem (M1). It consists of the mirror cells, positioning system, support systems, and thermal control system. The seven 8.4m mirror segments are excluded from this paper because they are considered a separate subsystem of the M1 System.
The M1 Subsystem leverages heritage design of similar telescope systems; for example, the Magellan telescopes and the Large Binocular Telescope. The M1 Subsystem incorporates pneumatic force actuators, hardpoints, and a thermal control ventilation system.
Design developments have been introduced to address the challenging levels of performance and unique requirements needed by the GMT telescope. Imaging goals necessitate an increase in mirror support performance, figure control, and higher-levels of thermal control. Additionally, there are challenges associated with matching and tracking the relative position of the seven mirror segments for mirror phasing. The design of the static support system needs to protect the mirrors from loads transmitted through the structure during an earthquake. Finally, the telescope design with interchangeable off-axis mirror cells necessitate mirror cells and support components that function under any range of gravitational vector orientations
. A full-scale Test Cell prototype is being constructed including production versions of mirror cell components to test and validate the M1 subsystem design. A Mirror Simulator will be used with the Test Cell to validate the M1 Control System. Later, a primary mirror segment will be used with the Test Cell to perform optical tests at the University of Arizona.