George Z. Angeli

Characterization of wind loading of telescopes

Ground-based telescopes operate in a turbulent atmosphere that affects the optical path across the aperture by changing both the mirror positions (wind induced vibrations) and the air refraction index. Although the characteristics of the atmosphere are well understood in the inertial range, the validity of the homogeneous, isotropic field assumption is questionable inside the enclosure and in the close vicinity of the structure. To understand the effect of wind on an actual telescope, we conducted extensive wind measurements at the Gemini South Telescope. Simultaneous measurements were made of pressures at multiple points on the mirror surface, as well as wind velocity and direction at several locations inside and outside the dome. During the test we varied the dome position relative to the wind, the telescope elevation angle, the position of windscreens in the observing slit, and the size of the openings in the ventilation gates. The data sets have been processed to provide the temporal and spatial characteristics of the pressure variations on the primary mirror in comparison to the theory of atmospheric turbulence. Our investigation is part of an effort leading to the development of a scalable wind model for large telescope simulations, which describes the forces due to air turbulence on the primary mirror and telescope structure reasonably well even inside an enclosure.

High-resolution optical modeling of the Thirty Meter Telescope for systematic performance trades

We consider high-resolution optical modeling of the Thirty Meter Telescope for the purpose of error budget and instrumentation trades utilizing the Modeling and Analysis for Controlled Optical Systems tool. Using this ray-trace and diffraction model we have simulated the TMT optical errors related to multiple effects including segment alignment and phasing, segment surface figures, temperature, and gravity. We have then modeled the effects of each TMT optical error in terms of the Point Source Sensitivity (a multiplicative image plane metric) for a seeing limited case and an adaptive optics corrected case (for the NFIRAOS). This modeling provides the information necessary to rapidly conduct design trades with respect to the planned telescope instrumentation and to optimize the telescope error budget.

Thermal Performance Prediction of the TMT Optics

Thermal analysis for the Thirty Meter Telescope (TMT) optics (the primary mirror segment, the secondary mirror, and the tertiary mirror) was performed using finite element analysis in ANSYS and I-DEAS. In the thermal analysis, each of the optical assemblies (mirror, mirror supports, cell) was modeled for various thermal conditions including air convections, conductions, heat flux loadings, and radiations. The thermal time constant of each mirror was estimated and the temperature distributions of the mirror assemblies were calculated under the various thermal loading conditions. The thermo-elastic analysis was made to obtain the thermal deformation based on the resulting temperature distributions. The optical performance of the TMT optics was evaluated from the thermally induced mirror deformations. The goal of this thermal analysis is to establish thermal models by the FEA programs to simulate for an adequate thermal environment. These thermal models can be utilized for estimating the thermal responses of the TMT optics. In order to demonstrate the thermal responses, various sample time-dependent thermal loadings were modeled to synthesize the operational environment. Thermal responses of the optics were discussed and the optical consequences were evaluated.

Active optics challenges of a thirty-meter segmented mirror telescopy

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.

Thermal modeling of the TMT Telescope

Thermal modeling of the Thirty Meter Telescope (TMT) was conducted for evaluations of thermal performances by finite element (FE) and optical analysis tools. The thermal FE models consist of the telescope optical assembly systems, instruments, laser facility, control and electronic equipments, and telescope structural members. A three-consecutive-day thermal environment data was implemented for the thermal boundary created by Computational Fluid Dynamics (CFD) based on the environment conditions of the TMT site. Temporal and spatial temperature distributions of the optical assembly systems and the telescope structure were calculated under the environmental thermal conditions including air convections, conductions, heat flux loadings, and radiations. With the calculated temperature distributions, the thermo-elastic analysis was performed to predict thermal deformations of the telescope structure and the optical systems. The line of sight calculation was made using the thermally induced deformations of the optics and structures. Merit function routines (MFR) were utilized to calculate the Optical Path Difference (OPD) maps after repositioning the optics based on a best fit of M1 segment deformations. The goal of this thermal modeling is to integrate the mechanical and optical deformations in order to simulate the thermal effects with the TMT site environment data from CFD.

Integrated modeling and systems engineering for the Thirty Meter Telescope

Modeling is an integral part of systems engineering. It is utilized in requirement validation, system verification, as well as for supporting design trade studies. Modeling highly complex systems poses particular challenges, including the definition and interpretation of system performance, and the combined evaluation of physical processes spanning a wide range of time frames. Our solution is based on statistical interpretation of system performance and a unique image quality metric developed by TMT. The Stochastic Framework and Point Source Sensitivity allow us to properly estimate and combine the optical effects of various disturbances and telescope imperfections.

Integrated modeling tools for large ground based optical telescopes

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.

GMT aerothermal modeling validation through site measurements

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.

Systems engineering for the Giant Magellan Telescope

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 Giant Magellan Telescope integrated modeling and performance (Conference Presentation)

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. nWith 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. nWhile 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.nA 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.nThe 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.