The 25.4 m Giant Magellan Telescope (GMT) consists of seven 8.4 m primary mirror (M1) segments with matching segmentation of the Gregorian secondary mirror (M2). The GMT will operate in four basic optical correction modes, Natural Seeing (NS), Ground Layer Adaptive Optics (GLAO), Natural Guide Star Adaptive Optics (NGAO) and Laser Tomography Adaptive Optics (LTAO). Each of these modes must deliver a specified combination of image quality, field of view, and sky coverage over a range of environmental conditions.
With a double segmented mirror configuration, even in the simplest of the correction modes the GMT includes over one thousand controllable degrees of freedom. Exogenous and internal sources of disturbances and noise over these degrees of freedom will limit the image quality. The different ranges of motion and bandwidth of the different degrees of freedom enable a cascade correction of the wavefront error, successively rejecting global to local disturbances. This frequency and spatial separation allows allocating the disturbances in stages, considering the residuals of the low spatial and temporal corrections as the disturbance for the high order corrections.
While a first approach can consider the analysis of systems in isolation in order to allocate coarse budgets, a complex control system such as that of the GMT requires a Dynamic Optics Simulation (DOS) to account for the real interactions between the controlled plant and the controllers. For example, some control loops such as the M1 figure control system will have an update rate of only 0.03 Hz, while the Adaptive Secondary Mirror (ASM) will be updated at 1kHz . The DOS is an end-to-end simulation environment that brings together optics, finite element models (FEM), mechanical motions, surface deformations and control models applied to the GMT main optics. At the center of the DOS there is an optics propagation module with both geometric ray tracing and Fourier propagation capability. The dynamic response of the telescope mount and the large M1 segments has been modeled by applying Craig-Bampton reduction analysis to finite element models. These reductions have been reordered in a second order form, allowing higher computational efficiency than traditional state space models. Each M1 segment is controlled by an array of 330 actuators with realistic precision, noise and discretization errors. The structural dynamics model can be used in time domain simulations that account for all the non-linear effects of actuators and sensors, or in a linear frequency domain model to run more efficiently stochastic analyses.
A high resolution Computational Fluid Dynamics (CFD) model has been developed for simulating unsteady turbulent flow over the optical system. These simulations provide unsteady pressure fluctuations over the main optics and effects of varying index of refraction in the optical path for different operating conditions. These quantities are subsequently used for estimating wind induced image jitter and thermal (dome and mirror) seeing by applying the combined structural, control, and optical models described above.
The DOS allows GMT to understand the sensitivity of image quality to any of the thousands of parameters of our plant and control system., Due to the cascade layers of control loops, DOS allows specifying design parameters without over-constraining the solution space.
The Giant Magellan Telescope (GMT) is going to be a complex and versatile exploration machine, which makes systems engineering GMT challenging. This paper addresses three particularly critical aspects of systems engineering: a general and flexible definition of the observatory, image quality specifications, and compliance assessment for statistical performance requirements. The observatory definition and its high-level flow down is captured in a set of Foundation Documents, from level-1 (stakeholders’ intentions and the objective specifications of science data) through level-2 (engineering specification) to level-3 (architectural design and operational concepts). Image quality requirements for atmospheric resolution modes are balancing observing efficiency considerations and system capabilities enabling exceptional image quality under the best conditions. To address statistical specifications, requirements validation and early design verification is carried out in an integrated modeling framework that takes advantage of sequential Monte- Carlo analysis over the Standard Year, representing our understanding of correlated summit conditions and GMT operational constraints.
The 25.4m Giant Magellan Telescope (GMT) consists of seven 8.4 m primary mirror (M1) segments with matching segmentation of the Gregorian secondary mirror (M2). When operating the GMT in the diffraction-limited Adaptive Optics (AO) modes, using the Adaptive Secondary Mirror (ASM), the M1-M2 pairs of segments must be phased to a small fraction of the observing wavelength. To achieve this level of correction across the scientific field of view (<90” in diameter), the phasing system relies on multiple (up to four) natural guide-star probes deployed across the field of view (from 6’ to 10’ from the center of the field) measuring at slow rates (~0.033 Hz) segment phase piston in the infrared and low-order field-dependent phase aberrations in the visible. This paper describes the overall phasing strategy and requirements when operating in the Natural Guide-star AO (NGAO) and the Laser Tomography AO (LTAO) modes. We will also present a first evaluation of segment piston error induced by wind buffeting on the telescope structure. Wind loads have been computed for different observatory configurations using Computational Fluid Dynamics (CFD) simulations. This analysis showcases the GMT Dynamic Optical Simulation (DOS) environment which integrates the optical and structural dynamic models of the GMT with the Fourier optics models of AO and phasing sensors.
The Giant Magellan Telescope (GMT) is 25 meter diameter extremely large ground based infrared/optical telescope
being built by an international consortium of universities and research institutions. It will be located at the Las
Campanas Observatory in Chile. The GMT primary mirror consists of seven 8.4 meter diameter borosilicate mirror
segments. Two seven segment Gregorian secondary mirror systems will be built; an Adaptive Secondary Mirror (ASM)
to support adaptive optics modes and a Fast-steering Secondary Mirror (FSM) with monolithic segments to support
natural seeing modes when the ASM is being serviced.
Wind excitation results in static deformation and vibration in the telescope structure that affects alignment and image
jitter performance. The telescope mount will reject static and lower frequency windshake, while each of the Faststeering
Secondary Mirror (FSM) segments will be used to compensate for the higher frequency wind-shake, up to 20
Hz. Using a finite element model of the GMT, along with CFD modeling of the wind loading on the telescope structure,
wind excitation scenarios were created to study the performance of the FSM and telescope against wind-induced jitter.
A description of the models, methodology and results of the analyses are presented.
The preliminary design of the 25 m Giant Magellan Telescope (GMT) has been completed. This paper describes the design of the optics, structure and mechanisms, together with the rationales that lead to the current design. Analyses that were conducted to verify structure and optical performance are summarized. Science instruments will be mounted within the telescope structure. A common instrument de-rotator is provided to compensate for field rotation caused by the alt-az tracking of the telescope. The various instrument stations and provisions for mounting instruments are described. Post-PDR development plans for the telescope are presented.