Development of the primary mirror segment support assemblies for the Thirty Meter Telescope

The Thirty Meter Telescope (TMT) Primary Mirror (M1) is composed of 492 hexagonal aspheric segments. In order to compensate for residual polishing errors, installation errors, gravity effects and parasitic forces in the whiffletree support, each segment is equipped with 21 Warping Harness (WH) mechanisms, which allow low order corrections to the optical surface. The primary mirror segment aberrations after shape corrections with warping harness have been identified as the single largest error term in the Thirty Meter Telescope (TMT) image quality error budget [1]. The WH mechanism consists of a Linear Actuator, Ball Link and a Leaf Spring with strain gauge sensor. The Warping Harness key requirements are: high positioning accuracy, 50 years operational life time with minimum maintenance, low cost, high reliability, high resolution, low hysteresis, high stability in unpowered state, large operational temperature range, low power dissipation, vacuum compatibility and survival of accidental condensing conditions. Smooth motor has developed a robust, low cost linear actuator for the TMT M1 Warping Harness. A WH Accelerated Life Cycle Test with increased temperature extremes was successfully completed. Acceleration factors were calculated with the Coffin-Manson model for temperature. Compliance with the technical requirements for the WH system has been demonstrated.

Unsteady wind loads due to turbulence inside the telescope enclosure result in image jitter and higher-order image degradation due to M1 segment motion. Advances in computational fluid dynamics (CFD) allow unsteady simulations of the flow around realistic telescope geometry, in order to compute the unsteady forces due to wind turbulence. These simulations can then be used to understand the characteristics of the wind loads. Previous estimates used a parametric model based on a number of assumptions about the wind characteristics, such as a von Karman spectrum and frozen-flow turbulence across M1, and relied on CFD only to estimate parameters such as mean wind speed and turbulent kinetic energy. Using the CFD-computed forces avoids the need for assumptions regarding the flow. We discuss here both the loads on the telescope that lead to image jitter, and the spatially-varying force distribution across the primary mirror, using simulations with the Thirty Meter Telescope (TMT) geometry. The amplitude, temporal spectrum, and spatial distribution of wind disturbances are all estimated; these are then used to compute the resulting image motion and degradation. There are several key differences relative to our earlier parametric model. First, the TMT enclosure provides sufficient wind reduction at the top end (near M2) to render the larger cross-sectional structural areas further inside the enclosure (including M1) significant in determining the overall image jitter. Second, the temporal spectrum is not von Karman as the turbulence is not fully developed; this applies both in predicting image jitter and M1 segment motion. And third, for loads on M1, the spatial characteristics are not consistent with propagating a frozen-flow turbulence screen across the mirror: Frozen flow would result in a relationship between temporal frequency content and spatial frequency content that does not hold in the CFD predictions. Incorporating the new estimates of wind load characteristics into TMT response predictions leads to revised estimates of the response of TMT to wind turbulence, and validates the aerodynamic design of the enclosure.

TMT primary mirror consists of hexagon shape (1.44 m in diagonal diameter and 45 mm in thickness) segment mirrors. This article describes the fabrication process developed for these segment mirrors. Stressed mirror polishing (SMP) method is adopted taking the advantages of its process speed and attained smoothness in higher spatial frequency figure. Segment substrate bending fixtures enables versatile aspheric polishing of various types of TMT segments by adjusting the bending force distribution. Mass production of segment polishing has started and 22 segments were successfully polished to comply with the specifications. Detailed studies for hexagonal cutting and mounting of the polished mirror onto the segment support assembly to complete segment mirror assemblies are under way.

Lightweight, high stability, and high-temperature adaptability are the primary considerations when designing the primary mirror of a micro/nano satellite remote sensing camera. In this paper, the optimized design and experimental verification of the large-aperture primary mirror of the space camera with a diameter of Φ610 mm is carried out. First, the design performance index of the primary mirror was determined according to the coaxial tri-reflective optical imaging system. Then, SiC, with excellent comprehensive performance, was selected as the primary mirror material. The initial structural parameters of the primary mirror were obtained using the traditional empirical design method. Due to the improvement of SiC material casting complex structure reflector technology level, the initial structure of the primary mirror was improved by integrating the flange with the primary mirror body design. The support force acts directly on the flange, changing the transmission path of the traditional back plate support force, and has the advantage that the primary mirror surface shape accuracy can be maintained for a long time when subjected to shock, vibration, and temperature changes. Then, a parametric optimization algorithm based on the mathematical method of compromise programming was used to optimize the design of the initial structural parameters of the improved primary mirror and the flexible hinge, and finite element simulation was conducted on the optimally designed primary mirror assembly. Simulation results show that the root mean square (RMS) surface error is less than λ/50 (λ = 632.8 nm) under gravity, 4 °C temperature rise, and 0.01 mm assembly error. The mass of the primary mirror is 8.66 kg. The maximum displacement of the primary mirror assembly is less than 10 μm, and the maximum inclination angle is less than 5″. The fundamental frequency is 203.74 Hz. Finally, after the primary mirror assembly was precision manufactured and assembled, the surface shape accuracy of the primary mirror was tested by ZYGO interferometer, and the test value was 0.02 λ. The vibration test of the primary mirror assembly was conducted at a fundamental frequency of 208.25 Hz. This simulation and experimental results show that the optimized design of the primary mirror assembly meets the design requirements of the space camera.

The 1.2-m primary mirror supporting systems are composed of axial support system with whiffletree structure and lateral support system with 6 tangent links. With the simulation based on the finite element analysis (FEA), the 18 points positions of the axial support are determined and the bottom chamber structure of the primary mirror is improved. In order to reduce the mirror surface deformation, the assembly stress on the primary mirror is reduced by optimizing the lateral support structure. The analysis results demonstrated that the root-mean-square (RMS) of the surface deformation is 10.9nm when the primary mirror points vertically. Meanwhile the RMS of the surface deformation is 10.3nm when the primary mirror points horizontally.

The California Extremely Large Telescope (CELT) is a joint project of the University of California and the California Institute of Technology to build and operate a 30-meter diameter telescope for research in astronomy at visible and infrared wavelengths. The current optical design calls for a primary, secondary, and tertiary mirror with Ritchey-Chretien foci at two Nasmyth platforms. The primary mirror is a mosaic of 1080 actively stabilized hexagonal segments. This paper summarizes the recent progress on the conceptual design of this telescope.

Primary mirror is an important component of collimator. The surface figure error of primary mirror is a critical factor affecting the imaging quality of collimator. Besides, the support system of primary mirror of collimator must be steady, while collimator need be moved safely as an elementary optical measuring tool. The support system of the primary mirror is composed of axial support and lateral support. Due to the axis of the primary mirror is horizontal when collimator working, the lateral support of the primary mirror has a far greater impact on the figure error of the primary mirror. In this paper, static structure analysis with finite element method is carried out for a 5m collimator primary mirror with V-block support under gravity load. With the analysis, the relationship between the structure parameters in primary mirror V-block support and the deformation of the primary mirror is built. With this relationship, the optimization parameters are found out to reduce the gravity deformation of the primary mirror.

A so-called Semi-Active Support (SAS) system was proposed for the primary mirror of the YNAO 2-m Ring Solar Telescope (2-m RST of Yunnan Astronomical Observatory, China), whose primary mirror is distinctively figured in a ring with an outer diameter of 2.02 m and a ring width of 0.35 m. This paper reports the design and tests of the prototype of the pneumatic-based semi-active axial support system of the 2-m RST. The dummy mirror was a solid circular plate glass of 700 mm in diameter and 20 mm in thickness, which was support by nine Pneumatic Force Actuators (PFAs) organized in three groups, each group was regulated by one proportional regulating solenoid valve. Besides, three Displacement Actuators (DAs) were used to actively define the three Degrees Of Freedom (DOFs) of piston and tip/tilt of the dummy mirror. The pneumatic force actuators were to be actively pumped and regulated, meanwhile, the displacement actuators to actively define the primary's position, according to the variation of the elevation pointing of the telescope. The PFA was build with a metal bellow as its cylinder, and a pressing load cell of 200 N capacity with 5 mN resolution was integrated on its output tip. The DA was a step-motor based design with a travel range of 4 mm and a theoretical resolution of 50 nm. Basic technical requirements for the PFAs and DAs were reviewed first. General consideration and configuration of the prototype was elaborated, followed by detailed designs and comprehensive tests of the either type standalone actuator. The prototype was finally tested on a systematic level as well. This prototype study has paved a reasonable way for critical design of the axial support system of the 2-m RST primary mirror.

To enable the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), the McDonald Observatory (MDO) and the Center for Electro-mechanics (CEM) at the University of Texas at Austin are developing a new HET tracker in support of the Wide-Field Upgrade (WFU) and the Visible Integral-Field Replicable Unit Spectrograph (VIRUS). The precision tracker is required to maintain the position of a 3,100 kg payload within ten microns of its desired position relative to the telescope's primary mirror. The hardware system to accomplish this has ten precision controlled actuators. Prior to installation on the telescope, full performance verification is required of the completed tracker in CEM's lab, without a primary mirror or the telescope's final instrument package. This requires the development of a laboratory test stand capable of supporting the completed tracker over its full range of motion, as well as means of measurement and methodology that can verify the accuracy of the tracker motion over full travel (4m diameter circle, 400 mm deep, with 9 degrees of tip and tilt) at a cost and schedule in keeping with the HET WFU requirements. Several techniques have been evaluated to complete this series of tests including: photogrammetry, laser tracker, autocollimator, and a distance measuring interferometer, with the laser tracker ultimately being identified as the most viable method. The design of the proposed system and its implementation in the lab is presented along with the test processes, predicted accuracy, and the basis for using the chosen method^*.

Aspheric mirrors utilized in optical systems have, by tradition, relatively slow focal ratios (f/2.6 to f/5.0). The 5.0-meter diameter Hale telescope primary mirror, for example, has a focal ratio of 3.3. Future ground based telescopes are being considered with aperture diameters in the range of 10 meters to 25 meters. In order to keep the telescope length and building within realistic sizes, primary mirror focal ratios ranging from f/1.0 to f/1.5 will be required. In a similar manner, packaging limitations for future space telescopes will force even faster primary mirrors (f/0.8 to f/2.3). The 2.4-meter diameter, f/2.3, Space Telescope primary mirror has a faster focal ratio than any current ground based telescope larger than 2.0 meters in diameter. Both fabrication (processing) and metrology (testing) are issues that must be addressed in the manufacture of these fast aspheric mirrors.

With the aperture of telescope becoming larger, the mass of primary mirror and other relevant structures will become heavier as well. Therefore, lighting weight for large space-based telescope is necessary. This paper purposed a method based on Neural Network aims to build a math model for primary mirror of large space-based telescope, which can reduce weight of the telescope and smaller mirror deformation caused by gravity release effectively. In the meantime, it can also improve stiffness of structure and reduce thermal strain caused by on orbit temperature variation effectively. The model describes the relationship between the structure of primary mirror of large space-based telescope and corresponding deformation, and describes the optical performance of mirror by using Zernike Polynomial. To optimize the structure of primary mirror lightweight, we take the deformation of mirror and its optical performance into consideration. To apply the structures parameters and its corresponding deformations to Neural Network training, we use the combination samples of different mirror lightweight structure parameters and corresponding deformation which caused by gravity release and thermal condition. Finally, by taking advantage of the Neural Network model to optimize the primary mirror lightweight of 1-meter rectangle space-based telescope, which can make the RMS 0.024λ (λ=632.8nm)and areal density under 15kg/m². This method combines existing results and numerical simulation to establish numerical model based on Neural Network method. Research results can be applied to same processes of designing, analyzing, and processing of large space-based telescope directly.

The Thirty Meter Telescope (TMT) will be a ground-based, 30-m optical-IR telescope with a highly segmented primary mirror located in a remote location. From the start of operations, TMT will provide a rich and diverse mix of seeing-limited and diffraction-limited instrumentation. Initially, only classical observing will be supported, although remote observing will follow almost immediately. Queue (or service) observing may be supported at a later date. TMT users will expect high facility uptime and observing efficiency as well as effective user support for planning and execution of observations. Those expectations are captured in the high-level Operations Concept Definition (OCD) document. The services and staffing needed to implement those concepts are described in the TMT Operations Plan. In this paper, high-level TMT operational concepts are summarized followed by a description of the current operations plan, including staffing model.

The Ritchey-Chretien (RC) optical design of Thirty Meter Telescope (TMT) calls for a 3.1m diameter secondary mirror (M2M) and an elliptical tertiary mirror (M3M) of 3.5m along its major axis and 2.5m along its minor axis. The M3M is a thin, large, flat, solid elliptical mirror which directs the f/15 beam from the M2M to the multiple instruments on both Nasmyth platforms. The M3M will weigh approximately two metric tons and the mirror support system will maintain the mirror figure at different gravity orientations. A recent reduction of the field of view to 15 arc minutes allows a reduction in the size of the M3M, which in turn requires re-optimization of the mirror support system. The proposed M3M optimized support system consists of 60 tri-axial supports mounted at the mirror back surface. These tri-axial supports accommodate motions of M3M in three gravity directions. The print-though RMS surface errors of M3M are 10nm for axial gravity loadings and 1nm for lateral gravity loadings. The M3 system (M3S) has an active optics (aO) capability to accommodate potential mechanical or thermal errors; its ability to correct low-order aberrations has been analyzed. A structure function (SF) of the axial gravity support print-through was calculated.

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.

An overview of the Faint Object Camera and its performance to date is presented. In particular, the detector's efficiency, the spatial uniformity of response, distortion characteristics, detector and sky background, detector linearity, spectrography, and operation are discussed. The effect of the severe spherical aberration of the telescope's primary mirror on the camera's point spread function is reviewed, as well as the impact it has on the camera's general performance. The scientific implications of the performance and the spherical aberration are outlined, with emphasis on possible remedies for spherical aberration, hardware remedies, and stellar population studies.