Development of a FSMP mirror assembly

The Giant Magellan Telescope (GMT) is one of Extremely large telescopes, which is 25m in diameter featured with two Gregorian secondary mirrors, an adaptive secondary mirror (ASM) and a fast-steering secondary mirror (FSM). The FSM is 3.2 m in diameter and built as seven 1.1 m diameter circular segments conjugated 1:1 to the seven 8.4m segments of the primary. The guiding philosophy in the design of the FSM segment mirror is to minimize development and fabrication risks ensuring a set of secondary mirrors are available on schedule for telescope commissioning and early operations in a seeing limited mode. Each FSM segment contains a tip-tilt capability for fine co-alignment of the telescope subapertures and fast guiding to attenuate telescope wind shake and mount control jitter, thus optimizing the seeing limited performance of the telescope. The final design of the FSM mirror and support system configuration was optimized using finite element analyses and optical performance analyses. The optical surface deformations, image qualities, and structure functions for the gravity print-through cases, thermal gradient effects, and dynamic performances were evaluated. The results indicated that the GMT FSM mirror and its support system will favorably meet the optical performance goals for residual surface error and the FSM surface figure accuracy requirement defined by encircled energy (EE80) in the focal plane. The mirror cell assembly analysis indicated an excellent dynamic stiffness which will support the goal of tip-tilt operation.

This paper develops a tip-tilt motion controller of fast steering mirror (FSM) in the Giant Magellan telescope (GMT). A mathematical model of tip-tilt motion system of FSM is derived, and then based on this model, stability analysis is carried out. A heuristic adaptive controller is designed for the tip-tilt motion control with modeling error. The heuristic controller consists of two different adaptations such as initial adaptive control and adaptation at steady state errors. Through numerical simulations, the validity of the controller is illustrated. After that this paper addresses several practical issues like disturbance from wind, actuator saturation and resonance frequency from mechanical structure in implementing the controller to the actual system.

The Giant Magellan Telescope (GMT) will be a 25m class telescope which is one of the extremely large telescope projects in the design and development phase. The GMT will have two Gregorian secondary mirrors, an adaptive secondary mirror (ASM) and a fast-steering secondary mirror (FSM). Both secondary mirrors are 3.2 m in diameter and built as seven 1.1 m diameter circular segments conjugated 1:1 to the seven 8.4m segments of the primary. The FSM has a tip-tilt feature to compensate image motions from the telescope structure jitters and the wind buffeting. The support system of the lightweight mirror consists of three axial actuators, one lateral support at the center, and a vacuum system. A parametric study and optimization of the FSM mirror blank and central lateral flexure design were performed. This paper reports the results of the trade study. The optical image qualities and structure functions for the axial and lateral gravity print-through cases, thermal gradient effects, and dynamic performances will be discussed for the case of a lightweighted segment with a center thickness of 140 mm weighing approximately 105 kg.

The Fast-Steering secondary mirror (FSM) of Giant Magellan Telescope (GMT) consists of seven 1.1m diameter segments with effective diameter of 3.2m. Each of the seven segments is held by three axial supports and a central lateral support with a vacuum system to mirror gravity compensations. The mirror assembly has a tip-tilt feature capable of guiding to attenuate telescope wind shake and mount control jitter. Both on-axis and off-axis mirror segments were optimized under various design considerations using finite element analyses and optical performance analyses. An integrated model of the mirror cell assembly was developed including sub-assembly parts consisting of axial supports, lateral support, breakaway mechanism, seismic restraints, and pressure seal. In this paper for the optimized FSM mirror and mirror cell assembly, the design considerations are addressed, and performance prediction results are discussed in detail with respect to the specifications.

The Giant Magellan Telescope (GMT) will feature two Gregorian secondary mirrors, an adaptive secondary mirror (ASM) and a fast-steering secondary mirror (FSM). The FSM has an effective diameter of 3.2 m and consists of seven 1.1 m diameter circular segments, which are conjugated 1: 1 to the seven 8.4m segments of the primary. Each FSM segment contains a tip-tilt capability for fast guiding to attenuate telescope wind shake and mount control jitter. This tiptilt capability thus enhances performance of the telescope in the seeing limited observation mode. The tip-tilt motion of the mirror is produced by three piezo actuators. In this paper we present a simulation model of the tip-tilt system which focuses on the piezo-actuators. The model includes hysteresis effects in the piezo elements and the position feedback control loop.

Giant Magellan Telescope (GMT) is one of the largest next-generation ground telescopes whose diameter is 25.4m. It has been developed since 2003, and is expected to be finished by 2018. Korea officially joined the GMT project in 2009, and actively participates in the development. KASI (Korea Astronomy & Space Science Institute) and Korean astronomical society are planning several activities to enhance the capabilities in science research. Building a secondary mirror system is another activity Korea wants to develop. A prototype of the GMT secondary is being developed. Developing a spectrograph is another one. In this talk, I will present the plan of Korea's works for the GMT.

The fast steering mirror (FSM) is a key element in astronomical telescopes to provide real-time angular correction of line-of-sight error due to telescope jitter and wind-induced disturbance. The Giant Magellan Telescope (GMT) will utilize a FSM as secondary mirror under unfavorable wind conditions that excites the telescope at the lowest resonance frequency around 8Hz. A flexure in the center of the mirror constrains lateral displacements, while still allowing tip-tilt motion to steer. Proper design of this central flexure is challenging to meet lateral loading capability as well as angular and axial flexibility to minimize optical surface distortion forced by redundant constraints at the flexure. We have designed the lateral flexure and estimated its performance from a variety of design case studies in a finite element analysis tool. A carefully designed finite element model at the sub-system level including the flexure, lightweight mirror and 3 point axial supports allows evaluating whether the designed flexure is qualified within specifications. In addition, distorted surface maps can be achieved as a function of forces that could be induced in telescope operation or due to misalignment errors during assembling. We have also built a test set-up to validate the finite element analysis results. Optical quality was measured by a phase shifting interferometer in various loading conditions and the measurements were decomposed by standard Zernike polynomials to concentrate specific surface shapes and to exclude low order shapes as measurement uncertainties.

The primary mirror of the Giant Magellan Telescope (GMT) consists of seven 8.4-meter diameter borosilicate segments. While this design has optical advantages, the unmitigated seismic risk is significant. Recently added design features meet this challenge. A 2D seismic isolation system decouples the telescope from horizontal ground motion and dampers at the interface with the primary mirror segments attenuate their motion. A detailed analysis framework has been developed to justify and refine the design. Discrete analyses, in which the models are exercised at discrete seismic intensity levels, are used to generate requirements throughout the telescope via a seismic response spectrum approach. Risk-based analyses utilize a broad characterization of uncertainty to assess probability of survival of the most critical components. These tools support a general design strategy that effectively trades the cost of designing and constructing the telescope against the seismic risk.

The Primary Mirror Device Control System (M1 DCS) is one of the many Device Control Systems (DCS) included in the Giant Magellan Telescope (GMT) control system and is responsible for the overall control and operation of the GMT primary mirror segments. The primary mirror is composed of seven 8.4m diameter segments, six off-axis and one in the center. The active support system of each segment comprises 170 support actuators for the off-axis segments and 154 actuators for the center segment to control the mirror figure, and 6 hardpoints to control the six degrees of freedom of rigid body motion. The software design follows a component model-based architecture, implemented using the GMT core software frameworks. Software components of the M1 DCS are specified using a custom Domain Specific Language (DSL) and inherit all key features of the core components such as communication ports, default behaviors, telemetry, logs, alarms, faults, state machines and engineering user-interface without the need of a separate implementation. The communication between the real time software and the controlled devices is implemented by an EtherCAT Fieldbus in a ring topology. This master-slave standard protocol enables the control system to reach 100 Hz closed loop rate for active support control. This paper describes the software of the M1 DCS, the tests performed with different software and hardware simulators, and the strategy to ensure software readiness with the final optical mirror.

The Giant Magellan Telescope (GMT) is a 24.5m diameter optical/infrared telescope. Its seven 8.4m primary mirrors give it a collecting area equivalent to a 21.4m filled aperture. The ten GMT partners are constructing the telescope at the Las Campanas Observatory in Chile with first light planned for the end of 2018. In this paper, we describe the plans for the first-generation focal plane instrumentation for the telescope. The GMTO Corporation has solicited studies for instruments capable of carrying out the broad range of objectives outlined in the GMT Science Case. Six instruments have been selected for 14 month long conceptual design studies. We briefly describe the features of these instruments and give examples of the major science questions that they can address.

We present expected performance for the Giant Magellan Telescope (GMT) adaptive optics system. Using a custom adaptive optics modeling code developed at the University of Arizona, we are able to simulate the various aspects of atmospheric turbulence, wavefront sensing, and adaptive correction for the specific geometry of the GMT AO system. This allows us to explore the design space of the GMT and determine the performance of the system for various adaptive optics modes, including Ground-Layer AO. We present results showing the effect of the segmented aperture on AO performance, and discuss strategies for sensing and correction with the GMT aperture.

Laser Tomography Adaptive Optics (LTAO) is one of adaptive optics systems planned for the Giant Magellan Telescope (GMT). End-to-end simulation tools that are able to cope with the complexity and computational burden of the AO systems to be installed on the extremely large telescopes such as GMT prove to be an integral part of the GMT LTAO system development endeavors. SL95, the Fortran 95 Simulation Library, is one of the software tools successfully used for the LTAO system end-to-end simulations. The goal of SL95 project is to provide a complete set of generic, richly parameterized mathematical models for key elements of the segmented telescope wavefront control systems including both active and adaptive optics as well as the models for atmospheric turbulence, extended light sources like Laser Guide Stars (LGS), light propagation engines and closed-loop controllers. The library is implemented as a hierarchical collection of classes capable of mutual interaction, which allows one to assemble complex wavefront control system configurations with multiple interacting control channels. In this paper we demonstrate the SL95 capabilities by building an integrated end-to-end model of the GMT LTAO system with 7 control channels: LGS tomography with Adaptive Secondary and on-instrument deformable mirrors, tip-tilt and vibration control, LGS stabilization, LGS focus control, truth sensor-based dynamic noncommon path aberration rejection, pupil position control, SLODAR-like embedded turbulence profiler. The rich parameterization of the SL95 classes allows to build detailed error budgets propagating through the system multiple errors and perturbations such as turbulence-, telescope-, telescope misalignment-, segment phasing error-, non-common path-induced aberrations, sensor noises, deformable mirror-to-sensor mis-registration, vibration, temporal errors, etc. We will present a short description of the SL95 architecture, as well as the sample GMT LTAO system simulation results.

The Giant Magellan Telescope (GMT) is a 25-meter optical/infrared extremely large telescope that is being built by an international consortium of universities and research institutions. It will be located at the Las Campanas Observatory, Chile. The GMT primary mirror consists of seven 8.4-m borosilicate honeycomb mirror segments made at the Steward Observatory Mirror Lab (SOML). Six identical off-axis segments and one on-axis segment are arranged on a single nearly-paraboloidal parent surface having an overall focal ratio of f/0.7. The fabrication, testing and verification procedures required to produce the closely-matched off-axis mirror segments were developed during the production of the first mirror. Production of the second and third off-axis segments is underway. GMT incorporates a seven-segment Gregorian adaptive secondary to implement three modes of adaptive-optics operation: natural-guide star AO, laser-tomography AO, and ground-layer AO. A wide-field corrector/ADC is available for use in seeing-limited mode over a 20-arcmin diameter field of view. Up to seven instruments can be mounted simultaneously on the telescope in a large Gregorian Instrument Rotator. Conceptual design studies were completed for six AO and seeing-limited instruments, plus a multi-object fiber feed, and a roadmap for phased deployment of the GMT instrument suite is being developed. The partner institutions have made firm commitments for approximately 45% of the funds required to build the telescope. Project Office efforts are currently focused on advancing the telescope and enclosure design in preparation for subsystem- and system-level preliminary design reviews which are scheduled to be completed in the first half of 2013.

In the era of extremely large telescopes (ELTs), with telescope apertures growing in size and tighter image quality requirements, maintaining a controlled observation environment is critical. Image quality is directly influenced by thermal gradients, the level of turbulence in the incoming air flow and the wind forces acting on the telescope. Thus any ELT enclosure must be able to modulate the speed and direction of the incoming air and limit the inflow of disturbed ground-layer air. However, gaining an a priori understanding of the wind environment's impacts on a proposed telescope is complicated by the fact that telescopes are usually located in remote, mountainous areas, which often do not have high quality historic records of the wind conditions, and can be subjected to highly complex flow patterns that may not be well represented by the traditional analytic approaches used in typical building design. As part of the design process for the Giant Magellan Telescope at Cerro Las Campanas, Chile; the authors conducted a parametric design study using computational fluid dynamics which assessed how the telescope's position on the mesa, its ventilation configuration and the design of the enclosure and windscreens could be optimized to minimize the infiltration of ground-layer air. These simulations yielded an understanding of how the enclosure and the natural wind flows at the site could best work together to provide a consistent, well controlled observation environment. Future work will seek to quantify the aerothermal environment in terms of image quality.

The fast steering mirror is a commonly used image motion compensation device in space cameras. The mechanical properties of the support structure will directly affect the response speed and closed loop bandwidth of the fast steering mirror system. A fast steering mirror driven by voice coil motor in a low earth orbit space camera was taken as the research object, and a two-axis flexible support structure based on cross-shaped flexible hinges was designed, which had the advantages of compact structure and small center drift. According to the approximate differential equation of cantilever beam deflection, the mathematical model of the rotational stiffness of the two-axis flexible support was established. The main structural parameters of the flexible hinge were selected for the resonant frequency requirements of the fast steering mirror mechanism, and a modal analysis of the mechanism was performed by using the finite element software MSC.Patran. The analysis results show that the resonant frequency of the fast steering mirror mechanism in the two working directions is 18.6 Hz and 18.7 Hz, while the resonant frequencies in other non-working directions are above 292.2 Hz, which proves that the choice of flexible hinge structural parameters is reasonable. In order to test the correctness of the theoretical model, a prototype of the fast steering mirror was processed and the experimental platform was built. The rotational stiffness and resonant frequency of the mechanism were tested. The results of the test and the theoretical analysis are consistent within the allowable error range.