Development of GMT fast steering secondary mirror assembly

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.

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.

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.

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.

A detailed Computational Fluid Dynamics (CFD) model for the Giant Magellan Telescope (GMT) telescope has been developed and used to simulated and analyze the aero-optical environment around the observatory. The developed model accounts for the major observatory components such as the primary (M1) and secondary (M2) mirrors, the M2 supporting truss, other subcomponents of the telescope mount, and enclosure building along with the auxiliary and site support buildings on the summit. A large topographical area around the installation site is included. This study evaluates three different lower enclosure designs; a closed soffit, an open soffit and a perforated ring-wall (partially closed soffit). Timevarying CFD simulations provide detailed flow and temperature fields along the optical path, which are subsequently used to compute optical parameters such as Optical Path Difference (OPD) maps and Point Source Sensitivity normalized (PSSn), the GMT Image Quality (IQ) metric. Results show that enclosure-induced turbulent flow patterns and refractive index variations have a greater influence on optical performance compared to flow and thermal behavior external to the enclosure. Instantaneous and mean PSSn values obtained for the three soffit configurations show minor differences, indicating that the lower enclosure design has minimal impact on observatory optical performance for the simulated operating conditions.

A Prototype of Fast-steering Secondary Mirror (FSMP) for the Giant Magellan Telescope (GMT) has been developed by the consortium consisting of institutes in Korea and the US. In 2014 the FSMP development was finalized by combining the two major sub-systems, the mirror fabricated and the mirror cell with the tip-tilt control parts. We have developed an assembly procedure in which potential difficulties, such as handling without contacting mirror surface, and optimizing bonding process, have been resolved. Supporting jigs were produced, and optimized bonding techniques have been developed. The assembled FSMP system was installed in a test tower, and stability of the system were checked. Performance of the FSMP system will be evaluated in static and dynamic environments for the validation of the FSMP system operation as the future works.

In this paper the mechanical structure and the operating principle of a fast steering mirror (FSM) are introduced, and a novel FSM based on magnetostrictive actuators is designed. The modal analysis of the FSM is carried out with the help of the finite element method, and relative experiments are also conducted. The experimental results and that of simulation are given and in good qualitative agreement with each other. This paper pays attention to ameliorating the FSM structure for higher mechanical natural frequency and perfect control performance. The experimental results would be helpful for the construction design of the FSM.

The Fast Steering Mirror (FSM) of the GMT is going to be the first-light secondary mirror system. An FSM prototype has been developed which consists of 1 m off-axis aspheric mirror and tip-tilt test-beds. The successful development results are presented.

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.

The Giant Magellan Telescope (GMT)¹ is a 25 m telescope composed of seven 8.4 m "unit telescopes", on a common mount. Each primary and conjugated secondary mirror segment will feed a common instrument interface, their focal planes co-aligned and co-phased. During telescope operation, the alignment of the optical components will deflect due to variations in thermal environment and gravity induced structural flexure of the mount. The ultimate co-alignment and co-phasing of the telescope is achieved by a combination of the Acquisition Guiding and Wavefront Sensing system (AGWS) and two segment-edge-sensing systems². An analysis of the capture range of the AGWS indicates that it is unlikely that that system will operate efficiently or reliably with initial mirror positions provided by open-loop corrections alone³. Since 2016 GMT have been developing a telescope metrology system, that is intended to close the gap between openloop modelling and AGWS operations. A prototyping campaign was initiated soon after receipt of laser metrology hardware in 2017. This campaign is being conducted in collaboration with the Large Binocular Telescope Observatory (LBTO), and hardware was first deployed on the LBT in August 2017. Since that time the system had been run and developed over some hundreds of hours on-sky. It has been shown to be capable of reliably measuring the relative positions of the main optics over ~ 10 m to a repeatability of ~ 1-2 microns RMS. This paper will describe the prototyping campaign to date, the basic design of the system, lessons learned and results achieved. It will conclude with a discussion of future prototyping efforts.

The paper presents a newly developed helium-cooled spectrometer and radiometer with a diffraction limited telescope. The instrument is designed for ir-measurements aboard spacecraft. The instrument uses the the well-known limb technique to detect the spatially integrated spectral emission of the atmosphere and the atmospheric structure as well. The optical layout of the telescope is optimized for high straylight rejection of the earth s radiation and for good image quality. The telescope has an off-axis Herschel type arrangement. The focus of the primary mirror is reimaged with an elliptical (secondary) mirror into the instrument s focal plane. The suppression of the earth s straylight is achieved by means of an integrated Lyot optics, which are composed of field stop, secondary mirror and Lyot stop. This technique and high quality low BRDF mirrors give the off-axis rejection necessary for limb observations. The nominal focus of the telescope coinsides with the entrance slit of a spectrometer of Ebert-Fastie type. The detectors for the spectrometer cover the wavelenghts region between 2.5 μm to 25 μm in different grating orders, which are separated by means of wavelength selecting filters. The second focal plane instrument is a detector array specially designed for the scientific requirements. This array measures the ir-radiation in 10 spectral channels. The channel selection is performed with a filter wheel near the focus of the primary mirror. As the filters are not illuminated with parallel light the filter thickness and the cryogenic refractive indices of the filters have to be carefully taken into account.

We present laboratory results of the closed-loop performance of the Magellan\nAdaptive Optics (AO) Adaptive Secondary Mirror (ASM), pyramid wavefront sensor\n(PWFS), and VisAO visible adaptive optics camera. The Magellan AO system is a\n585-actuator low-emissivity high-throughput system scheduled for first light on\nthe 6.5 meter Magellan Clay telescope in November 2012. Using a dichroic\nbeamsplitter near the telescope focal plane, the AO system will be able to\nsimultaneously perform visible (500-1000 nm) AO science with our VisAO camera\nand either 10 micron or 3-5 micron science using either the BLINC/MIRAC4 or\nCLIO cameras, respectively. The ASM, PWS, and VisAO camera have undergone final\nsystem tests in the solar test tower at the Arcetri Institute in Florence,\nItaly, reaching Strehls of 37% in i'-band with 400 modes and simulated\nturbulence of 14 cm ro at v-band. We present images and test results of the\nassembled VisAO system, which includes our prototype advanced Atmospheric\nDispersion Corrector (ADC), prototype calcite Wollaston prisms for SDI imaging,\nand a suite of beamsplitters, filters, and other optics. Our advanced ADC\nperforms in the lab as designed and is a 58% improvement over conventional ADC\ndesigns. We also present images and results of our unique Calibration Return\nOptic (CRO) test system and the ASM, which has successfully run in closed- loop\nat 1kHz. The CRO test is a retro reflecting optical test that allows us to test\nthe ASM off-sky in close-loop using an artificial star formed by a fiber\nsource.\n

Design and wide-bandwidth control of large aperture fast steering mirror with integrated-sensing unit