Andrew T. Sarawit

The Giant Magellan Telescope (GMT): structure design update

The Giant Magellan Telescope (GMT) is a 21.5-meter equivalent aperture optical-infrared ELT to be located in Chile. It is being designed and constructed by a group of U.S. and international universities and research institutions1. The concept design of the telescope structure was summarized in an earlier SPIE paper2 and described in greater detail in the GMT Conceptual Design Review document3. The structure design has matured during the current Design Development Phase. Important among design improvements has been optimization of the secondary truss with the goal of significantly reducing telescope pointing errors due to wind loading. Three detailed structural changes have resulted in calculated pointing error reductions of ~30%. The changes and their contributions to the improved performance as well as other tested features are discussed. Additional refinements to the structure include the instrument mounting system, with a stationary folded-instrument platform plus Gregorian Instrument Rotator utilizing hydrostatic bearings. More detailed features, such as revised C-ring bracing to improve instrument access, are described.

RF-mechanical performance for the Haystack radio telescope

The Haystack radio telescope is being upgraded to support imaging radar applications at 96 GHz. The Cassegrain antenna includes a 37 m diameter primary reflector comprising 432 reflector panels and a 2.84 m diameter hexapod mounted subreflector. Top-level antenna performance is based on meeting diffraction-limited performance over an elevation range of 10 - 40° resulting in a maximum RF half pathlength error requirement of 100 μm RMS. RF-mechanical performance analyses were conducted that allocated subsystem requirements for fabrication, alignment, and environmental effects. Key contributors to system level performance are discussed. The environmental allocations include the effects of gravity, thermal gradients, and diurnal thermal variations which are the dominant error source. Finite element methods and integrated optomechanical models were employed to estimate the environmental performance of the antenna and provide insight into thermal management strategies and subreflector compensation. Fabrication and alignment errors include the manufacturing of the reflector surface panels and assembly of overall reflector surface.

Lessons learned from design, analysis, and rehabilitation of wheel and track systems (Conference Presentation)

Many radio telescopes, radar antennas, and enclosures for optical telescopes use wheel-on-track systems for their azimuth rotations. Design of such systems requires properly understanding of the contact behavior between the wheel and the azimuth track as the wheel rolls forward. Unless the azimuth track has a continuous running surface, one needs to understand how the track joints will affect the contact stress. In the case of multi-layer track systems where the track segments consist of wear plates mounted on base plates, finite element analyses are needed to capture friction and slip, and opening and closing of gaps at the interfaces between the wheel, wear plate, and the base plate. The paper presents lessons learned from the design, analysis, and rehabilitation of a few wheel-on-track systems. The paper discusses (1) how geometry of the wheel, geometry of the track, stiffness of the wheel bogie, and alignment will affect the contact stress, (2) how to evaluate possible impact load at track joints, (3) what design criteria should be used for strength and fatigue at the wheel/track contact, and (4) how wear between wear plate and base plate can be evaluated.

Conceptual Design of the Aluminum Reflector Antenna for DATE5

DATE5, a 5 m telescope for terahertz exploration, was proposed for acquiring observations at Dome A, Antarctica. In order to observe the terahertz spectrum, it is necessary to maintain high surface accuracy in the the antenna when it is exposed to Antarctic weather conditions. Structural analysis shows that both machined aluminum and carbon fiber reinforced plastic (CFRP) panels can meet surface accuracy requirements. In this paper, one design concept based on aluminum panels is introduced. This includes panel layout, details on panel support, design of a CFRP backup structure, and detailed finite element analysis. Modal, gravity and thermal analysis are all performed and surface deformations of the main reflector are evaluated for all load cases. At the end of the paper, the manufacture of a prototype panel is also described. Based on these results, we found that using smaller aluminum reflector panels has the potential to meet the surface requirements in the harsh Dome A environment.

Concept design for seismic upgrade of Keck telescopes

On 15 October 2006, a large earthquake damaged both telescopes at W. M. Keck Observatory resulting in weeks of observing downtime. A significant portion of the downtime was attributed to recovery efforts repairing damage to telescope bearing journals, radial pad support structures, and encoder subsystems. To reduce the risk of damage and loss of observing time in future seismic events, we developed a conceptual design for the seismic upgrade of the twin Keck Telescopes. The paper covers the design requirements and constraints for the seismic upgrade, the evaluation method used to check the safety of sensitive components, and the trade-off study used to compare different options and to select the best design. Various design options such as base isolating the structure, strengthening seismic restraints, adding dampers, adding break-away mechanisms, and combinations of these design options are considered in this study. Nonlinear time history analyses are performed to evaluate the performance of the design concepts.

Finite element analyses of CCAT preliminary design

This paper describes the development of the CCAT telescope finite element model (FEM) and the analyses performed to support the preliminary design work. CCAT will be a 25 m diameter telescope operating in the 0.2 to 2 mm wavelength range. It will be located at an elevation of 5600 m on Cerro Chajnantor in Northern Chile, near ALMA. The telescope will be equipped with wide-field cameras and spectrometers mounted at the two Nasmyth foci. The telescope will be inside an enclosure to protect it from wind buffeting, direct solar heating, and bad weather. The main structures of the telescope include a steel Mount and a carbon-fiber-reinforced-plastic (CFRP) primary truss. The finite element model developed in this study was used to perform modal, frequency response, seismic response spectrum, stress, and deflection analyses of telescope. Modal analyses of telescope were performed to compute the structure natural frequencies and mode shapes and to obtain reduced order modal output at selected locations in the telescope structure to support the design of the Mount control system. Modal frequency response analyses were also performed to compute transfer functions at these selected locations. Seismic response spectrum analyses of the telescope subject to the Maximum Likely Earthquake were performed to compute peak accelerations and seismic demand stresses. Stress analyses were performed for gravity load to obtain gravity demand stresses. Deflection analyses for gravity load, thermal load, and differential elevation drive torque were performed so that the CCAT Observatory can verify that the structures meet the stringent telescope surface and pointing error requirements.

New Finite Element Models and Seismic Analyses of the Telescopes at W.M. Keck Observatory

On 15 October 2006 a large earthquake damaged both telescopes at Keck observatory resulting in weeks of observing downtime. A significant portion of the downtime was attributed to recovery efforts repairing damage to telescope bearing journals, radial pad support structures and encoder subsystems. Inadequate damping and strength in the seismic restraint design and the lack of break-away features on the azimuth radial pads are key design deficiencies. In May, 2011 a feasibility study was conducted to review several options to enhance the protection of the telescopes with the goal to minimize the time to bring the telescopes back into operation after a large seismic event. At that time it was determined that new finite element models of the telescope structures were required to better understand the telescope responses to design earthquakes required by local governing building codes and the USGS seismic data collected at the site on 15 October 2006. These models were verified by comparing the calculated natural frequencies from the models to the measured frequencies obtained from the servo identification study and comparing the time history responses of the telescopes to the October 2006 seismic data to the actual observed damages. The results of two finite element methods, response spectrum analysis and time history analysis, used to determine seismic demand forces and seismic response of each telescope to the design earthquakes were compared. These models can be used to evaluate alternate seismic restraint design options for both Keck telescopes.