Ho-Sang Kim

Prototype Development for the GMT FSM Secondary - Off-axis Aspheric Mirror Fabrication -

【A prototype of the GMT FSM has been developed to acquire and to enhance the key technology - mirror fabrication and tip-tilt actuation. The ellipsoidal off-axis mirror has been designed, analyzed, and fabricated from light-weighting to grinding, polishing, and figuring of the mirror surface. The mirror was tested by using an interferometer together with CGHs, which revealed the surface error of 13.7 nm rms in the diameter of 1030 mm. The SCOTS test was employed to independently validate the test results. It measured the surface error to be 17.4 nm rms in the diameter of 1010 mm. Both tests show the optical surface of the FSMP mirror within the required value of 20 nm rms surface error.】

Flexure design development for a fast steering mirror

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.

Development status of the prototype of the GMT fast steering mirror

FSM is a secondary mirror of the Giant Magellan Telescope (GMT), which is 3.2 m in diameter. It consists of seven segments, each of which is 1m in diameter. The surrounding six segments are off-axis mirrors. FSM compensates image degradations caused by wind turbulence and structure jitter by using a tip-tilt mechanism. Korea Astronomy and Space Science Institute is developing a prototype of Fast Steering Mirror (FSM) together with four other institutions. The prototype is a full-size FSM segment, and it has two features; an off-axis mirror and a testbed for tip-tilt actuation. The off-axis mirror has the diameter of 1.06 m and fast focal ratio of 0.65. At present, lightweighting is successfully finished, and polishing of the front surface is proceeding. As for the tip-tilt, several test-beds have been assembled and the functions of tip-tilt have been checked. The third testbed is being integrated with a dummy aluminum mirror, three axial supports, a lateral support, and a test-bed frame. It is activated by a vacuum system. The test-bed will examine and verify the tip-tilt parameters. The FSM prototype is expected to be completed by 2012. In this paper, system engineering and progress of the prototype development are to be presented.

Development of a FSMP mirror assembly

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.

Control modeling of the fast-steering secondary mirror for GMT

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.

Development status of the fast-steering secondary mirror of GMT

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.

Tip-tilt performance test of FSMP for development of the GMT FSM

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.

Preliminary AIT plan for the FSM segments of GMT

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.

Experimental study of breakaway system for the fast-steering secondary mirror prototype of GMT

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.

Design of the fast steering secondary mirror assembly for the Giant Magellan Telescope

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