Joeleff Fitzsimmons

Wind loads on ground-based telescopes

One of the factors that can influence the performance of large optical telescopes is the vibration of the telescope structure due to unsteady wind inside the telescope enclosure. Estimating the resulting degradation in image quality has been difficult because of the relatively poor understanding of the flow characteristics. Significant progress has recently been made, informed by measurements in existing observatories, wind-tunnel tests, and computational fluid dynamic analyses. We combine the information from these sources to summarize the relevant wind characteristics and enable a model of the dynamic wind loads on a telescope structure within an enclosure. The amplitude, temporal spectrum, and spatial distribution of wind disturbances are defined as a function of relevant design parameters, providing a significant improvement in our understanding of an important design issue.

First light adaptive optics systems and components for the Thirty Meter Telescope

Adaptive optics (AO) is essential for many elements of the science case for the Thirty Meter Telescope (TMT). The initial requirements for the observatorys facility AO system include diffraction-limited performance in the near IR, with 50 per cent sky coverage at the galactic pole. Point spread function uniformity and stability over a 30 arc sec field-ofview are also required for precision photometry and astrometry. These capabilities will be achieved via an order 60×60 multi-conjugate AO system (NFIRAOS) with two deformable mirrors, six laser guide star wavefront sensors, and three low-order, IR, natural guide star wavefront sensors within each client instrument. The associated laser guide star facility (LGSF) will employ 150W of laser power at a wavelength of 589 nm to generate the six laser guide stars. We provide an update on the progress in designing, modeling, and validating these systems and their components over the last two years. This includes work on the layouts and detailed designs of NFIRAOS and the LGSF; fabrication and test of a full-scale prototype tip/tilt stage (TTS); Conceptual Designs Studies for the real time controller (RTC) hardware and algorithms; fabrication and test of the detectors for the laser- and natural-guide star wavefront sensors; AO system modeling and performance optimization; lab tests of wavefront sensing algorithms for use with elongated laser guide stars; and high resolution LIDAR measurements of the mesospheric sodium layer. Further details may be found in specific papers on each of these topics.

Parametric modeling and control of telescope wind-induced vibration

A parametric model of the dynamic performance of an optical telescope due to wind-buffeting is presented. The model is being developed to support the design of next generation segmented-mirror optical telescopes through enabling rapid design iterations and allowing a more thorough exploration of the design space. A realistic performance assessment requires parametric descriptions of the wind, the structural dynamics, active control of the structure, and the optical response. The current model and its assumptions are presented, with the primary emphasis being on the parameterization of the wind forces. Understanding the temporal spectrum and spatial distribution of wind disturbances inside the telescope enclosure is one of the most challenging aspects in developing the overall parametric model. This involves integrating information from wind tunnel tests, computational fluid dynamics, and measurements at existing observatories. The potential and limitations of control to mitigate the response are also discussed, with realistic constraints on the control bandwidth obtained from the detailed structural model of a particular point design. Finally, initial results are presented on performance trends with a few key parameter variations.

Steady-state analysis of the multi-tethered aerostat platform for the Large Adaptive Reflector telescope

The Large Adaptive Reflector (LAR), currently being developed at the National Research Council Canada, is a low-cost, large- aperture, wide-band, cm-wave radio telescope designed for implementation in the Square Kilometer Array (SKA). The LAR consists of a 200 m diameter, actuated-surface, parabolic reflector with a feed located at a 500 m focal length. Since the feed must be positioned on a 500 m hemisphere centered about the reflector and between a zenith angle of 0 degree(s) to 60 degree(s), an innovative method for feed positioning is required. This feed positioning will be achieved using a high- tension structure consisting of a 4100 m3 helium aerostat supporting an array of tethers. The length of each tether can be controlled through the use of winches, resulting in accurate control of the feed position. The feasibility of the tethered aerostat feed-positioning system is of critical importance to the success of the LAR. Extensive steady-state analyses of the multi-tethered aerostat have been completed and provide strong evidence that this feed-positioning system will operate reliably in moderate weather conditions (10 m/s constant wind velocity with 2.5 m/s wind gusts). The framework of these analyses and the corresponding results will be presented.

Integrated modeling of the Canadian Very Large Optical Telescope

We describe the VLOT integrated model, which simulates the telescope optical performance under the influence of external disturbances including wind. Details of the implementation in the MATLAB/SIMULINK environment are given, and the data structures are described. The structural to optical interface is detailed, including a discussion of coordinate transformations. The optical model includes both an interface with ZEMAX to perform raytracing analysis and an efficient Linear Optics Model for producing telescope optical path differences from within MATLAB. An extensive set of optical analysis routines has been developed for use with the integrated model. The telescope finite element model, state-space formulation and the high fidelity 1500 mode modal state-space structural dynamics model are presented. Control systems and wind models are described. We present preliminary results, showing the delivered image quality under the influence of wind on the primary mirror, with and without primary mirror control.

Predicting the aerodynamic performance of the Canadian Very Large Optical Telescope

A variety of aerodynamic studies have been completed to assist in the development of an integrated model for the Thirty Meter Telescope. These studies investigated the characteristics of wind loading on the Canadian Very Large Optical Telescope (VLOT) and produced preliminary data for input into the VLOT integrated model. We describe the details of, and present the results from, the computational fluid dynamic (CFD) analyses and wind tunnel (WT) tests. The validity of the CFD results is assessed through correlation studies that compare the CFD and WT results. Through extensive comparison of the mean and RMS coefficients of pressure and the power spectral density plots of the pressures within the enclosure, excellent correlations between the experimental and computational results are shown.

The large adaptive reflector: a 200-m diameter wideband centimeter- to meter-wave radio telescope

The Large Adaptive Reflector (LAR) is a concept for a low- cost, large aperture, wideband, radio telescope, designed to operate over the wavelength range from 2 m to 1.4 cm. It consists of a 200-m diameter actuated-surface parabolic reflector with a focal length of 500 m, mounted flat on the ground. The feed is held in place by a tension-structure, consisting of three or more tethers tensioned by the lift of a large, helium-filled aerostat -- a stiff structure that effectively resists wind forces. The telescope is steered by simultaneously changing the lengths of the tethers with winches (thus the position of the feed) and by modifying the shape of the reflector. At all times the reflector configuration is that of an offset parabolic antenna, with the capability to point anywhere in the sky above approximately 15 degree Elevation Angle. At mid-range wavelengths, the feed is a multi-beam prime-focus phased array, about 5 m diameter; at meter wavelengths, it is a single-beam phased array of up to 10 m diameter. Simulations have shown that in operating wind conditions (10 m/s average speed with 2.5 m/s gusts), the position of the feed platform can be stabilized to within a few cm over time scales of approximately 20 s. Research indicates that the telescope concept is feasible and that an order of magnitude improvement in cost per m2 of collecting area over traditional designs of large parabolic antennas can be achieved.

TMT adaptive optics program status report

We provide an update on the development of the first light adaptive optics systems for the Thirty Meter Telescope (TMT) over the past two years. The first light AO facility for TMT consists of the Narrow Field Infra-Red AO System (NFIRAOS) and the associated Laser Guide Star Facility (LGSF). This order 60 × 60 laser guide star (LGS) multi-conjugate AO (MCAO) architecture will provide uniform, diffraction-limited performance in the J, H, and K bands over 17-30 arc sec diameter fields with 50 per cent sky coverage at the galactic pole, as is required to support TMT science cases. Both NFIRAOS and the LGSF have successfully completed design reviews during the last twelve months. We also report on recent progress in AO component prototyping, control algorithm development, and system performance analysis.

Validation and verification of integrated model simulations of a thirty meter telescope

The Herzberg Institute of Astrophysics has developed Integrated Modeling tools for the Thirty Meter Telescope (TMT) project. This simulation software, implemented in MATLAB, models the telescope optical system, structural dynamics, and the segmented primary mirror and secondary mirror active optical control systems. For the TMT project, the integrated model was used to assess the effect of wind loading on the telescope in terms of delivered image quality. The simulation includes a state-space model of the telescope structural dynamics derived from an ANSYS finite element model, a linear optics model derived from a ZEMAX prescription, and wind loading forces derived from PowerFLOW computational dynamics software. The overall complexity of the model necessitates rigorous validation and verification procedures to ensure that the simulation data structures properly represent the original design, and that the calculations performed by the system are reliable. In this paper we discuss the validation and verification of the model data structures, structural configuration, optical configuration, coordinate system transforms, linear optics model, Zernike calculations, and wind loading model. As a case study, we present the verification, validation, and simulation results of the Thirty Meter Telescope Reference Design.

Design and analysis of flexure mounts for precision optics

The increasing demand within the astronomy community for direct detection of extrasolar Jovian planets is driving the development of the next generation of ground-based, precision instruments like the Gemini Planet Imager (GPI). The precision and stability of the opto-mechanical components within GPI needed to achieve the required 10-7 contrast will challenge the limits of design and material properties. This study examines the challenge of mounting small, high-precision mirrors (~1nm RMS WFE) for all gravity orientations, for 30°C temperature fluctuations and for vibration conditions typical of a cassegrain-mounted instrument on the Gemini telescopes. Various flexure-based mounting schemes, typically used for small optics (12 mm to 50 mm diameter), are considered in the context of the GPI opto-mechanical requirements. Through this study several candidate designs are selected for detailed investigation. The further design, analysis, and optimization of these candidate flexure designs are presented and evaluated against the relevant requirements. Special consideration and discussion is devoted to finite element analysis techniques and optimization procedures. Finally, the chosen flexure configuration is prescribed for the range of mirror sizes within GPI.