A detailed Computational Fluid Dynamics (CFD) model for the Giant Magellan Telescope (GMT) telescope has been developed and used to simulated and analyze the aero-optical environment around the observatory. The developed model accounts for the major observatory components such as the primary (M1) and secondary (M2) mirrors, the M2 supporting truss, other subcomponents of the telescope mount, and enclosure building along with the auxiliary and site support buildings on the summit. A large topographical area around the installation site is included. This study evaluates three different lower enclosure designs; a closed soffit, an open soffit and a perforated ring-wall (partially closed soffit). Timevarying CFD simulations provide detailed flow and temperature fields along the optical path, which are subsequently used to compute optical parameters such as Optical Path Difference (OPD) maps and Point Source Sensitivity normalized (PSSn), the GMT Image Quality (IQ) metric. Results show that enclosure-induced turbulent flow patterns and refractive index variations have a greater influence on optical performance compared to flow and thermal behavior external to the enclosure. Instantaneous and mean PSSn values obtained for the three soffit configurations show minor differences, indicating that the lower enclosure design has minimal impact on observatory optical performance for the simulated operating conditions.
Validation of Computational Fluid Dynamics (CFD) solutions using experimental data is critical as CFD simulations are regularly used for site characterization and design analysis of Extremely Large Telescopes (ELT). Site testing data for wind, temperature and optical turbulence are used to validate the GMT CFD model configuration for the construction site at Las Campanas Peak in Northern Chile. CFD simulations, both steady-state and unsteady, combined with the corresponding seeing models are performed and estimates of the Ground Layer (GL) optical turbulence are calculated. Comparisons with wind, temperature and optical turbulence profiles are made that show a good match between simulated and observed data.
The Giant Magellan Telescope (GMT) will be one of the most powerful ground-based telescopes in the world upon commissioning at the Las Campanas Observatory (LCO) in Chile. The GMT enclosure protects the telescope, and its systems, from the external environment and plays a crucial role in delivering high quality celestial imagery. This paper describes the development and application of a 3D finite element model of the GMT enclosure and key internal components. This model was developed by Boeing Research & Technology (BR&T), under contract from the Giant Magellan Telescope Organization (GMTO), to characterize the complex interplay of convective, radiative and conductive heat transfer between components within the GMT enclosure and the surrounding environment. The primary intent of this analysis tool is to provide GMT engineers with input conditions for detailed conjugate heat transfer and aero-optic simulations. These simulations will support GMTO’s optimization of their enclosure design to maximize image quality and daily imaging time with minimal use of active thermal controls.
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 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.