Design of central support structure for large aperture mirror with wide working temperature
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摘要:目的
为提升低轨卫星与地面站间激光链路的通信质量，商业地面站内望远镜配备的大口径主镜需适应户外环境中恶劣的温度条件。
方法针对某通光口径φ500 mm的高精度主镜组件，提出了一种使用室温硫化硅橡胶的中心支撑方案。镜体采用微晶材料，衬套和支撑筒均为钛合金材质，1 mm厚的胶层在卸载镜体自身重力的同时，可有效减小组件内部热应力。胶层的厚度和高度通过仿真优化确定，特制的粘接工装可准确控制胶层形状和厚度，衬套上的通气孔促进了胶层的充分固化。
结果仿真分析表明，主镜在40 °C均匀温度变化工况下的面形精度RMS值为4.199 nm，光轴竖直重力工况下的RMS值为13.748 nm，光轴水平重力工况下的RMS值为4.187 nm、镜体最大倾角和位移分别为4.722″和3.597 μm，组件基频达到53.45 Hz。实测主镜的面形精度为RMS 0.017λ (λ=632.8 nm)，经大范围高低温循环试验及真空镀膜后，主镜均可保持高精度面形。
结论文中支撑结构可以显著提升高精度反射镜的温度适应能力，在地面大型光电设备中具有广阔的应用前景。
Abstract:ObjetiveIn order to improve the communication quality of LEO-OGS laser link, commercial ground station telescopes equipped with large aperture primary mirror need to adapt to harsh outdoor temperature conditions.
MethodA central support scheme based on room temperature vulcanized silicone rubber was proposed for a high-precision primary mirror with optical aperture of 500 mm. The structure consists of a Zerodur mirror blank, a pair of bushing and support made of titanium alloy, and a 1mm-thick adhesive layer which can effectively reduce the thermal stress inside assembly while temperature changes and unload the gravity of mirror blank. The thickness and height of the adhesive layer were determined by optimization. Specially designed fixture can accurately control the shape and thickness of the adhesive layer, meanwhile the ventilation holes on the bushing promote its full solidification.
ResultSimulation indicates that the surface shape accuracy of primary mirror is 4.199 nm in RMS under 40 °C temperature variation, with 13.748 nm under vertical gravity, and 4.187 nm under horizontal gravity, accompanied by the maximum mirror inclination and displacement of 4.722" and 3.597 μm, and the fundamental frequency of the assembly reaches 53.45 Hz. The measured surface shape accuracy of primary mirror is RMS 0.017λ (λ=632.8 nm), after extensive heat cycling test and vacuum coating, the surface can maintain high-precision.
ConclusionThe central support structure can significantly improve the temperature adaptability of precise mirrors, and has broad application in large-scale ground optoelectronic equipment.
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图 4 胶层高度对主镜面形精度的影响
Figure 4. Influence of adhesive height on surface accuracy of primary mirror
图 5 胶层厚度与主镜工作性能间的关系
Figure 5. Relationship between adhesive thickness and working performance of primary mirror
图 10 主镜在商业地面站中的应用
Figure 10. Application of primary mirror in co mmercial ground station
表 1 商业地面站中主镜组件主要设计指标
Table 1. Main design metrics for primary mirror assembly in co mmercial ground station
No. Item Requirement 1 Clear aperture Φ500 mm 2 Elevation during pointing From horizontal to vertical 3 Deformation under gravity Tilt: θX≤10″, θY≤10″
Displacement: δX≤20 μm, δY≤20 μm4 Working temperature range −20 °C~40 °C 5 Surface accuracy RMS≤1/30λ (λ=632.8 nm) 6 Mass ≤30 kg 7 Frequency ≥30 Hz 
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表 2 主镜组件材料物理属性
Table 2. Physical properties of materials used in primary mirror assembly
Property Mirror Adhesive Bushing Support Material & Type Zerodur RTV TC4 TC4 Density ρ (g·cm−3) 2.53 1.1 4.4 4.4 Elastic modulus E (Mpa) 91000 2.7 109000 109000 Poisson ratio μ 0.24 0.47 0.34 0.34 Thermal expansion
coefficient α (10−6·K−1)0.1 185 9.1 9.1
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表 3 主要工况下主镜仿真结果
Table 3. Simulation results of primary mirror under main loadcases (λ=632.8 nm)
Loadcase Deformation/nm Displacement/ μm Tilt/″ RMS δX δY δZ θX θY Temperature Variation（40ºC） 4.199 0 0 58.325 0.008 0.008 Horizontal gravity 4.187 3.597 0 0.001 0.235 4.722 Vertical gravity 13.748 0 0 13.415 0.004 0.004 Compound gravity（45°） 10.161 2.542 0 9.572 0.167 3.339 Design Criterion ≤1/30λ ≤20 ≤20 / ≤10 ≤10
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表 4 主镜组件模态分析结果
Table 4. Modal analysis results of primary mirror assembly
Order Frequency/Hz Vibration mode 1st 53.45 Rotation of mirror around Z axis 2nd 66.09 Rotation of mirror around X axis 3rd 66.10 Rotation of mirror around Y axis
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表 5 主镜面形精度检测结果
Table 5. Surface accuracy test results of primary mirror
Surface accuracy Polishing Heat recycle Coating RMS / λ (λ=632.8nm) 0.017 0.018 0.018
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