Optimization design for the supporting system of 5m collimator primary mirror

The Large Binocular Telescope (LBT) is built around two lightweight borosilicate honeycomb mirrors which, at 8.4 meters in diameter, are the largest operational examples of this technology. Since the mirrors are relatively stiff, the LBT mirror support system relies on passive position control and active force control. Passive position control is performed by six extendable hardpoints organized as a truncated hexapod, which may be positioned as required by the active optics control loop. The hardpoints rely on their axial stiffness to maintain the mirror position against residual external disturbances. The active force control system minimizes the force exerted by the hardpoints on the glass. Additionally, the axial component of the nominally uniform active support forces can be perturbed to distort the mirror as required by the active optics control loop. Because of the relatively large CTE of borosilicate glass, the differential temperature of the mirror is critical. Thus, the force control system must support a 16 metric ton mirror using less than 100 Watts of electrical power. The authors present a description of the primary mirror support system as implemented at the LBT. Initial stability problems made the mirrors nearly unusable in freezing temperatures. The authors explain the reason for this instability and describe the solutions implemented. Data demonstrating the current performance of the primary mirror support system are also presented.

This paper describes the optimization solution improving the total quality of the primary mirror supporting type. With the methods of Finite element analysis(FEA), Orthogonal experiment and BP Neural Network, the relationship between the structure parameters in primary mirror supporting type and the deformation of the primary mirror is built. With this relationship and Genetic Algorithm(GA) optimization design, a group of reasonable technology parameters is found that can improve the static stiffness of the primary mirror supporting type so as to reduce the gravity deformation of the primary mirror. The modal analysis and random vibration analysis are also discussed in detail, and the results indicate that the dynamic stiffness of the primary mirror supporting type is also improved.

The Mexican Infrared Telescope is one of the most important projects in the Institute for Astronomy of the National University of Mexico. As part of the design we pretend to simulate different components of the telescope by the Finite Element Method (FEM). One of the most important parts of the structure is the primary mirror support. This structure is under stress, causing deformations in the primary mirror; these deformations shouldn't be over 40 nanometers, which is the maximum permissible tolerance. One of the most interesting subjects to develop in this project is to make the segmented primary mirror to work like if it were a monolithic one. Each segment has six degrees of freedom, whose control needs actuators and sensors with stiff mechanical structures. Our purpose is to achieve these levels of design using FEM aided by computer and we pretend to study several models of the structure array using the Conceptual Design Method, in an effort to optimize the design.

With the rapid progress of technology and manufacture, the number of telescopes in space is mounting dramatically. Considering launch cost, more and more engineers nowadays pursue lightweight and compact design of telescope in space. Traditional design scheme usually has a support plate for primary reflective mirror. It will take some space for other parts. The paper, taking a primary reflective mirror with 600mm in diameter as an example, put forward a novel design of circle support component instead of traditional support plate. It utilized a special structural layout to save more space. The paper specified this design scheme and made comparison between these two design schemes. The paper also gave the assembling method of the primary reflective mirror and circle support component, which would verify its feasibility in engineering practice in a virtual way. The novel structure design in this paper will give some guidance to engineers and designers, who are devoting themselves to designing space telescopes.

Segmented deployable space imaging systems represent a promising solution for future ultra-large aperture space optical systems. With the proposal of larger aperture segmented telescope designs, ultra-lightweight primary mirror (PM) segments with lower areal density have become a research hotspot. However, the lightweight structure poses more stringent challenges in maintaining a high-precision surface figure of PM segments. Active deformable PM segments with surface shape adjustment capability serve as an effective approach to address these challenges. To achieve surface figure control at the PM segments scale, accurate determination of the PM segments' surface figure error magnitude is fundamentally required. After in-orbit deployment of segmented mirror systems, deviations between the actual position and ideal position of PM segments will induce optical pupil localization errors in the system's overall pupil plane. The optical pupil localization error (OPLE) of PM segments leads to an inaccurate decoupling of wavefront errors into individual PM segments, thereby compromising the accuracy of surface figure correction. This paper proposes an active segmented mirror optical pupil localization method based on local wavefront sensitivity analysis. By utilizing the deformation capability of active optics, we establish sensitivity equations through wavefront variations within the PM segments' pupil region before and after surface shape adjustments, enabling the inverse solution for the OPLE of PM segments. The proposed method achieves micrometer-level positioning accuracy of PM segments' positional errors solely relying on focal plane information, while avoiding the introduction of additional mechanisms. Experimental validations were designed to verify the feasibility of the method.

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.

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.

To meet the stringent requirements of the ESST primary mirror cell truss in terms of weight, stiffness, and thermal stability, this study proposes a three-layer spatial truss structure for the primary mirror support system. A hybrid material strategy is developed to achieve an optimal balance between structural rigidity and cost efficiency. The optimization objectives are defined as maintaining the total structural mass below 16 tonnes while minimizing the displacement at the segment support points. The results show that an all-steel design limits the maximum support deformation to 0.518 mm, with a material cost of 1.92 million CNY. By incorporating 0.89% carbon fiber reinforced polymer (CFRP), the maximum deformation is reduced to 0.49 mm, with a material cost of 2.016 million CNY, thereby satisfying the ESST performance criteria. This study provides a cost-effective structural design solution for the next-generation large spectroscopic survey telescope in China.

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.

Support structure and optical alignment technology of large-aperture secondary mirror measured by back transmission method

The primary mirror component is an important part of the Cassegrain system. As the first-stage imaging component, the RMS surface error directly affects the image quality of the whole optical system. In this article, taking the primary mirror component of a certain type of Cassegrain aerial camera as the research object, the factors affecting the RMS precision of the primary mirror surface are analyzed in detail from aspects of back supporting structure design, platen elastic crimping design, simulation analysis, test verification and so on. Using the finite element method to simulate the primary mirror supporting structure, analyzes the influence on the primary surface error by the three-point supporting structure in different positions. Furthermore, analyzes the variations of the primary mirror surface error under the influence of three-point supporting structure and pressure plate. The last but not the least, analyzes the primary mirror surface error under the different pressure conditions, concludes the optimal supporting point position and the excellent elastic compression. After the primary mirror assembling, through test verification, the RMS is 0.0270λ, which is better than the original design requirement of λ/35(0.0286λ). And the RMS variation between before and after assembling is less than 0.005λ. Performing the high and low temperature test on the primary component, after test, the RMS values is 0.0269λ, it proves that the primary frame structure and its axial supporting structure have little effects on the RMS precision of the primary mirror. It can also meet the requirement of the large-aperture primary mirror surface in the co-optical system under complex conditions. The feasibility of the structure design has been verified.

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 surface figure precision, weight, and dynamic performance of spaceborne primary mirrors depend on mirror structure parameters, which are usually optimized to improve the overall performance. To realize rapid multi-objective design optimization of a primary mirror with multiple apertures, a fully parameterized primary mirror structure is established. A surrogate model based on a hybrid of improved particle swarm optimization (IPSO), adaptive genetic algorithm (IAGA), and optimized back propagation neural network (IPSO-IAGA-BPNN) is developed to replace optomechanical simulation with its high computational cost. In this model, a self-adaptive inertia weight and a modified genetic operator are introduced into the particle swarm optimization (PSO) and adaptive genetic algorithm (AGA), respectively. The connection parameters of BPNN are optimized by the IPSO-IAGA algorithm for global searching capability. Further, the proposed IPSO-IAGA-BPNN, based on a rapid multi-objective optimization framework for a fully parameterized primary mirror structure, is established. Moreover, in addition to the proposed IPSO-IAGA-BPNN model, the Kriging, RSM, BPNN, GA-BPNN, PSO-BPNN, and PSO-GA-BPNN models are also analyzed as contrast models. The comparison results indicate that the predicted value obtained by IPSO-IAGA-BPNN is superior to the six other surrogate models since its mean absolute percentage error is less than 3% and its R2 is more than 0.99. Finally, we present a Pareto-optimal primary mirror design and implement it through three optimization methods. The verification results show that the proposed method predicts mirror structural performance more accurately than existing surrogate-based methods, and promotes significantly superior computational efficiency compared to the conventional integration-based method.

Lateral support has a significant effect on the deformation of thin primary mirror. This paper deals with the analysis and optimization procedure on the “push-pull-shear” lateral support of thin meniscus mirror. Three steps were carried out in the lateral support optimization. Firstly a finite element model (FEM) was build for further analysis, then according to its structural characteristic, the ratio between the tangential force and the radial force was analyzed during the first optimization; In order to make the lateral forces with the same value, equal angle distance was changed to different angle distances during the second optimization, also lateral support position and magnitude of the lateral forces were given; during the last analysis, a portion of axial force was exerted at the inner ring, the response curve between the inner ring axial force and the RMS in Z direction of the mirror was plotted. The final result showed that the RMS value was decreased from 12.168nm to 3.253nm in Z direction. The optimization procedure provided a feasible way for lateral support of thin meniscus mirror.

An off-axis three-mirror detection system with a large field of view is designed in order to improve the space target detection capability. The optical system is a Cook-TMA with the focal length of 127mm, the F number of 2, the field angle of 25° × 25° and the spectral range of 400-700nm. The primary mirror and the tertiary mirror of the off-axis three mirror system are all designed by free form surfaces: the primary mirror is characterized by Zernike polynomial and the tertiary mirror is described by XY polynomial. At the same time, we analyze the related characteristics of Zernike polynomial and XY polynomial. The results show that the free form surfaces have great advantages in improving the field of view and the imaging quality of the off-axis optical system. The system has high energy concentration and good imaging quality, which can capture and track the target in a wide range is suitable for wide area target monitoring.