大口径离轴非球面预应力研抛关键问题研究

In recent years, there are several 30m astronomic telescopes having taken development and research work in the world, including GMT and TMT in the United States, the E-ELT in European. and china has also put forward its own 30m-class telescopes as CFGT preliminary program, some pre-study have been taken on key issues. The research has made great breakthrough on the bulk of large diameter off-axis aspherical segments basing on Stressed Mirror Polishing technique. We have developed 2 aspherical segments with 1.1m in diameter and 8m, 12m in off-axis length respectively. The final residual surface error both have PV<2um. We find that there are some issues need further exploration during previous development process. these include following: (1) the interaction mechanism between pressure plate or workpiece and pitch polishing tool in continuous polishing program, the radius of curvature’s control method of polishing tool, and the impact characteristics of contact pressure for removing function.(2) the research on the optical measurement methods of surface medium-high frequency error on the edge in discrete points stressed loading program and mechanical low-pass filtering scheme.(3) The study on distribution of the contact pressure caused by the weight of loading mechanism; researching on distribution of the contact pressure result from asymmetric shape distribution of mirror during SMP.(4). In addition, whether Stressed Mirror technique can be used in fine grinding stage or not? We will take some try and explore on three points above mentioned. These problems will directly determine the overall development time of segments before IBF (Ion Beam Figuring). The research progress of related issues will improve the overall development efficiency of the bulk of off-axis aspheric mirror, shorten the development cycle.

近年来国内外天文界正在展开数个30m级望远镜研制工作，包括美国的GMT与TMT，欧洲的E-ELT。我国也提出了30m级望远镜CGFT初步方案，在关键问题上进行预研究。其中基于预应力抛光的大口径批量离轴子镜研制技术取得了较大突破，研制出口径1.1m、离轴量8m和12m的离轴子镜两块，面形残余误差PV<2um。同时研制过程中也发现若干亟需研究与解决的问题，包括：(1)环抛方案中压板、工件与抛光模之间的相互作用机制，环抛模R的控制方法，接触压强对磨削函数的影响特性；(2)离散点预应力加载方案产生的边缘中高频面形误差的光学测量方法和力学低通滤波方案研究；(3).加载机构重量引起的接触压力分布，预应力抛光过程中非对称镜面面形引起的压力分布问题；(4)另外预应力技术是否可用于研磨阶段？本课题将对上述几点问题进行尝试和探索。相关问题的研究进展将有助于提高批量子镜的研制效率、缩短研制周期。

本课题主要研究了以下方面的内容：(1)环抛方案中压板、工件与抛光模之间的相互作用机制，环抛模R的控制方法，接触压强对磨削函数的影响特性；(2)离散点预应力加载方案产生的边缘中高频面形误差的光学测量方法研究；(3).加载机构重量引起的接触压力分布，预应力抛光过程中非对称镜面面形引起的压力分布问题；(4)预应力技术用于研磨阶段的可行性和基于蓝牙传输的二维检测阵列的设计和检测精度研究。.研究成果中仿真得到的压板不同工位处与柏油盘接触压强分布、柏油模材料配比方式和材料物理性能测试结果将直接有助于5m环抛机的运行和预应力环抛加工1.5m级望远镜离轴非球面拼接主镜。研究得到了胶合和预应力加载产生的中高频面形残差分布特性，该结果有助于1.5m级拼接子镜的预应力环抛中高频面形误差分量的控制。研制出了基于蓝牙阵列的接触式二维检测装置、研究了基于气缸加载的预应力加载理论和加载方法、获得了预应力细磨收敛曲线和相关细磨工艺。细磨结束后经多次旋转拼接测量，在最大探测尺寸357mm内最终镜面误差PV=3.79μm，RMS=0.696μm。该研究成果可有助于中国LOT望远镜等拼接子镜的抛光前的快速非球面化，可缩短离轴非球面加工的周期，提高加工效率。