KEYWORDS: Mirrors, Telescopes, Actuators, Space telescopes, Off axis mirrors, Integrated modeling, Interfaces, Optical instrument design, Phase transfer function, Control systems
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 segment is held by three axial supports and a central lateral support with a vacuum system for pressure compensation. Both on-axis and off-axis mirror segments are optimized under various design considerations. Each FSM segment contains a tip-tilt capability for guiding to attenuate telescope wind shake and mount control jitter. The design of the FSM mirror and support system configuration was optimized using finite element analyses and optical performance analyses. The design of the mirror cell assembly will be performed including sub-assembly parts consisting of axial supports, lateral support, breakaway mechanism, seismic restraints, and pressure seal. . In this paper, the mechanical results and optical performance results are addressed 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 Giant Magellan Telescope (GMT) wavefront control system will provide active optics control and optimized atmospheric turbulence correction to every instrument on the 25.4 m diameter GMT. All subsystems of the GMT wavefront control system have passed their preliminary design reviews, and several are now in the detailed design and prototyping phases. Significant progress has been made developing optimized control algorithms, and simulating observatory performance over a wide range of environmental conditions. We review the wavefront control strategy in each observing mode, and the associated image quality error budgets. We also describe recent d and prototyping progress, and our plans to complete the wavefront control system development and prepare for first light.
The Fast-steering Secondary Mirror (FSM) of Giant Magellan Telescope (GMT) consists of seven 1.1 m diameter circular segments with an effective diameter of 3.2 m, which are conjugated 1:1 to the seven 8.4 m segments of the primary. Each FSM segment contains a tip-tilt capability for fast guiding to attenuate telescope wind shake and mount control jitter by adapting axial support actuators. Breakaway System (BAS) is installed for protecting FSM from seismic overload or other unknown shocks in the axial support. When an earthquake or other unknown shocks come in, the springs in the BAS should limit the force along the axial support axis not to damage the mirror. We tested a single BAS in the lab by changing the input force to the BAS in a resolution of 10 N and measuring the displacement of the system. In this paper, we present experimental results from changing the input force gradually. We will discuss the detailed characteristics of the BAS in this report.
The Giant Magellan Telescope (GMT) will be equipped with two Gregorian secondary mirrors: a fast-steering mirror (FSM) system for seeing-limited operations and an adaptive secondary mirror (ASM) for adaptive optics observing modes. The FSM has an effective diameter of 3.2 m and is comprised 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 has a tip-tilt capability for fast guiding to attenuate telescope wind shake and jitter. To verify the tip-tilt performance at various orientations, we performed tiptilt tests using a conceptual prototype of the FSM (FSMP) which was developed at KASI for R&D of key technologies for FSM. In this paper, we present configuration, methodology, results, and lessons from the FSMP test which will be considered in the development of FSM.
The Fast Steering Secondary Mirror (FSM) for the Giant Magellan Telescope (GMT) will have seven 1.05 m diameter circular segments and rapid tip-tilt capability to stabilize images under wind loading. In this paper, we report on the assembly, integration, and test (AIT) plan for this complex opto-mechanical system. Each fast-steering mirror segment has optical, mechanical, and electrical components that support tip-tilt capability for fine coalignment and fast guiding to attenuate wind shake and jitter. The components include polished and lightweighted mirror, lateral support, axial support assembly, seismic restraints, and mirror cell. All components will be assembled, integrated and tested to the required mechanical and optical tolerances following a concrete plan. Prior to assembly, fiducial references on all components and subassemblies will be located by three-dimensional coordinate measurement machines to assist with assembly and initial alignment. All electronics components are also installed at designed locations. We will integrate subassemblies within the required tolerances using precision tooling and jigs. Performance tests of both static and dynamic properties will be conducted in different orientations, including facing down, horizontal pointing, and intermediate angles using custom tools. In addition, the FSM must be capable of being easily and safely removed from the top-end assemble and recoated during maintenance. In this paper, we describe preliminary AIT plan including our test approach, equipment list, and test configuration for the FSM segments.
The Giant Magellan Telescope (GMT) will be equipped with two Gregorian secondary mirrors; a fast-steering secondary mirror (FSM) for seeing-limited operations and an adaptive secondary mirror (ASM) for adaptive optics observing modes. The FSM has an effective diameter of 3.2 m and is comprised of seven 1.1 m diameter circular segments, which are conjugated 1:1 to the seven 8.4m segments of the primary mirror. Each FSM segment has a tip-tilt capability for fast guiding to attenuate telescope wind shake and jitter. The FSM is mounted on a two-stage positioning system; a macro-cell that positions the entire FSM segments as an assembly and seven hexapod actuators that position and drive the individual FSM segments. In this paper, we present a technical overview of the FSM development status. More details in each area of development will be presented in other papers by the FSM team.
KEYWORDS: Mirrors, Telescopes, Optical fabrication, Telescopes, Control systems, Prototyping, Control systems design, Space telescopes, Surface finishing, Manufacturing, Profilometers
The Giant Magellan Telescope (GMT) will be featured with 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 built as 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 fine co-alignment of the telescope sub-apertures and fast guiding to attenuate telescope wind shake and mount control jitter. This tip-tilt capability thus enhances performance of the telescope in the seeing limited observation mode. As the first stage of the FSM development, Phase 0 study was conducted to develop a program plan detailing the design and manufacturing process for the seven FSM segments. The FSM development plan has been matured through an internal review by the GMTO-KASI team in May 2016 and fully assessed by an external review in June 2016. In this paper, we present the technical aspects of the FSM development plan.
During 2009, Tinsley finished most of the Configuration 1 pre-cryo test Computer Controlled Optical Surfacing (CCOS)
operations on the James Webb Space Telescope primary mirror segments and in mid-2009 we began the Configuration 2
post-cryo test CCOS operations. After completing the grinding and polishing operations, including final figuring to a
cryo-null target, we delivered the finished Engineering Development Unit (EDU) to Ball Aerospace Technology
Corporation on 4 December 2009. Achieving fabrication and metrology conditions to meet the specifications for this off-axis
~1.5 m hexagonal point-to-point segmented mirror required special methods. Achieving repeatable and accurate
interferometric alignment of the off-axis aspherical mirror surface and stable thermal gradient control of the beryllium
substructure during tests required rigorous component and system-level validation. Final optical wavefront
measurements over the various spatial frequency ranges have demonstrated that all of the requirements are met. This
success has validated our processes of fabrication and metrology and allows us to proceed with the production of the 18
flight mirror segments. The first finished flight mirror, the Tertiary Mirror, was shipped to BATC on 24 February, 2010.
Performance of that mirror is reported here also.
JWST optical component in-process optical testing and cryogenic requirement compliance certification, verification &
validation is probably the most difficult metrology job of our generation in astronomical optics. But, the challenge has
been met: by the hard work of dozens of optical metrologists; the development and qualification of multiple custom test
setups; and several new inventions, including 4D PhaseCam and Leica Absolute Distance Meter. This paper summarizes
the metrology tools, test setups and processes used to characterize the JWST optical components.
Tinsley, under JWST funding, has led the team that has developed a novel and highly versatile piece of ground support equipment for optical surface testing of JWST beryllium mirror segments during optical fabrication. The infrared Scanning Shack Hartmann System (SSHS) offers the advantage of being able to characterize mid-to-high spatial frequency structure on a mirror from early stages of fabrication when slopes may be high and surface irregular, eliminating the need for an extra polishing step before metrology. Working at 9.3μm, the system will accept and measure a wide dynamic range of surface characteristics, including roll-off near the edge of the segment. Knowledge of these surface features at the early grinding stage is imperative if characteristics such as mirror edge roll-off are to be minimized. WaveFront Sciences, producer of commercial COAS and Columbus Shack Hartmann systems, has provided systems engineering and component support for the SSHS system.
The SSHS system is based around a special Long Wave Infrared (LWIR) wavefront sensor developed by WaveFront Sciences that is scanned over the mirror surface, making sub-aperture measurements. The smaller, high-resolution measurements are then stitched together to provide high-resolution measurement of the entire mirror surface, even though the surface is in a rough ground state.
The system leverages technology from smaller visible instrumentation produced by Wavefront Sciences, especially those for surface sub-aperture measurements of semiconductor wafers. This paper will describe the implementation of the first infrared scanning Shack Hartmann system at Tinsley to address optical fabrication optimization of the JWST Primary Mirror Segments.
The context, preparation, and facilitization of Tinsley to produce the 18 JWST primary mirror segments are described,
and an overview of the Project at Tinsley is presented. The mirror segments are aggressively lightweighted,
approximately hexagonal, and approximately 1.32m flat-to-flat. While the optical finishing approach is strongly seated
in Tinsley's Computer Controlled Optical Surfacing (CCOSTM) technology, extensions have been implemented to
address safe and efficient nearly simultaneous flow of the high value mirror segments through numerous cycles of
optical finishing, processing and metrology steps. JWST will operate at cryogenic temperatures, and Tinsley will do
final figuring from a "hit map" made during cryogenic testing at the NASA MSFC X-Ray Calibration Facility (XRCF).
A formal beryllium safety protocol has been established throughout. Extensive handling fixtures assure that the mirrors
are moved from station to station experiencing low accelerations. A rigorous qualification process is applied to each
new fixture, machine and instrument. Special problems of cryo figuring, and co-finishing the segments to stringent
specifications are described.
Dispersed fringe sensor (DFS) using broadband point source can unambiguously estimate piston to several hundred microns. We demonstrate a rapid technique to analyze the data from a DFS. The technique is less susceptible to higher order aberrations and returns the average phase difference between two aperture elements.
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