Modeling extremely large ground based optical telescopes is growing in
complexity as the level of design detail is increasing. Our model of a 30m telescope includes a modal system of differential equations that represent the telescope structure, an edge controller to monitor and correct the gaps between mirrors of the hexagonally segmented primary mirror, an optical engine that propagates light through the telescope and calculates performance metrics at the exit pupil of the telescope, and a course optical control that can correct the wavefront for tip and tilt. Simulating one minute of telescope observing time with wind and atmospheric disturbances is a computationally intense proposition. The software tools in our arsenal include MATLAB (The MathWorks, Inc.) and OSLO (Lambda Research Corporation). Descriptions of the simulation components, and
the results of telescope simulations are presented.
Proc. SPIE. 5497, Modeling and Systems Engineering for Astronomy
KEYWORDS: Computational fluid dynamics, Telescopes, Mirrors, Thirty Meter Telescope, Integrated modeling, Space telescopes, Finite element methods, Integrated optics, Large telescopes, Systems modeling
Wind loading is one of the critical parameters influencing the performance of large telescopes, with potentially more dramatic consequences for proposed future giant telescopes. This study describes a strategy for modeling the effects of wind loading on extremely large telescopes such as the Thirty Meter Telescope (TMT). The optical performance of the telescope is estimated by an integrated model, which incorporates the telescope structure, optics, and control. To model the dynamic force variation on the telescope, a Finite Element Analysis (FEA) model of the telescope is created along with an unsteady Computational Fluid Dynamics (CFD) model of the airflow around the enclosure-telescope configuration, which should have a suitable level of geometric fidelity. Numerical simulations using the CFD model are performed for a chosen wind speed and telescope orientation (azimuth, zenith), through which the dynamic force pattern on the primary and secondary mirrors as well as on the secondary support structure can be determined. Finally the force pattern is applied to the FEA model. This can be achieved either by applying temporally and spatially filtered white noise forces with random distribution deducted from the CFD analysis, or by considering the dynamic force pattern itself from the unsteady CFD calculations. Since the FEA and CFD models usually have different resolution requirements and consequently different, non-uniform spatial sampling grids, a key part of the interface is the conversions necessary to transfer the forces from CFD surface cells to structural nodes.
A sound system engineering approach and the appropriate tools to support it are essential in achieving the scientific and financial objectives of the Thirty Meter Telescope project. Major elements of the required tool set are those providing estimates for the performance of the telescope. During the last couple of years, the partners in the consortium developed a wide range of modeling and simulation tools with various levels of fidelity and flexibility. There are models available for time domain and frequency domain simulations and analysis, as well as for lower fidelity, parametric investigations of design trade-offs and for high fidelity, integrated modeling of structure, optics and control. Presented are characteristic simulation results using the existing preliminary point designs of the TMT, with emphasis on the telescope performance degradation due to wind buffeting. Under the conditions modeled, the wind induced image jitter and image quality degradation was found comparable to good atmospheric seeing.
Ground-based telescopes operate in a turbulent atmosphere that affects the optical path across the aperture by changing both the mirror positions (wind seeing) and the air refraction index in the light path (atmospheric seeing). In wide field observations, when adaptive optics is not feasible, active optics are the only means of minimizing the effects of wind buffeting. An integrated, dynamic model of wind buffeting, telescope structure, and optical performance was devleoped to investigate wind energy propagation into primary mirror modes and secondary mirror rigid body motion.Although the rsults showed that the current level of wind modeling was not appropriate to decisively settle the need for optical feedback loops in active optics, the simulations strongly indicated the capability of a limited bandwidth edge sensor loop to maintain the continuity of the primary mirror inside the preliminary error budget. It was also found that the largest contributor to the wind seeing is image jitter, i.e. OPD tip/tilt.
Diffraction-limited performance of 30-m class telescopes requires the integration of structural, optical and control systems to sense and counteract real time disturbances to the telescope. Accurate simulation of an integrated telescope model is essential for optical performance estimation and design validation. Our approach to integrated time domain modeling of large telescopes is to interface commercially available structural,optical and control modeling software packages. The model architecture, data structures, and the interfacing tools of the simulation environment are presented. Preliminary simulation results of a 30-m class telescope subject to
wind load and a ground layer phase screen are presented.