The Giant Magellan Telescope (GMT) primary mirror subsystem (M1S) consists of seven 8.4m diameter borosilicate primary mirrors that must be maintained at the ambient nighttime air temperature as it changes throughout the observing night to prevent seeing effects at the mirror surface. Additionally, thermal gradients internal to the mirrors must be minimized to prevent figure errors caused by distortions of the mirror due to the non-zero thermal expansion coefficient of the glass. To address these requirements, the GMT M1S team is fabricating a prototype thermal control system design that consists of a sub-critical refrigeration system utilizing high pressure (~30 to ~60 bar) CO2 (R744) refrigerant. This paper describes the design and status of the M1 Subsystem Thermal Control (M1STC).
GMTO has developed a full-scale prototype of the cell that can house an 8.4-meter borosilicate mirror. This test cell is populated with all the active support control hardware and a mass simulator that simulates the mirror weight and the moment of inertia. GMTO has implemented the control software with all the core features needed to operate the active support system. A series of tests have been carried out to verify the functions, performance, and safety of the active support control system. The tests were carried out at several different orientations of the cell to demonstrate that the active support system works with the changing zenith angle and location of the mirror on the telescope mount. This paper describes the results of important safety and dynamic response tests of the active support system.
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 M1 active optics and thermal control systems called the M1 Subsystem (M1S) consists of the hardware and software that controls the shape, position, and thermal state of each mirror segment. A full-scale off-axis M1S prototype called the Test Cell is being fabricated and tested. The primary objective of the Test Cell is to mitigate risk by verifying that the mirror figure and position can be controlled within the image quality error budget and that the thermal control system vibration is within its system level allocation. The M1S components for the active optics support system have been fabricated, assembled, tested at the component level, and integrated into the Test Cell. The team completed the Test Readiness Review and started system level testing with the M1 Device Control Software. Lessons learned throughout the component and integrated system testing of the Test Cell will be incorporated into the M1S design for the production phase. This paper will summarize the progress of the Test Cell and results presented at the Test Readiness Review.
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.
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