大口径光学透镜的双级柔性支撑结构设计

Design and analysis of bi-flexible mounting structure for large optical lens

为了解决大视场巡天望远镜中大口径透镜支撑问题,提出了一种双级柔性支撑结构,既可以保证透镜位置精度和面形精度,又能够克服镜框结构弹性变形的影响。首先,详细阐述了大口径光学透镜支撑结构设计中的难点及所提出的双级柔性支撑结构方案,并详述了该支撑结构克服镜框弹性变形的机理和优势。然后,根据双级柔性支撑结构的组成部分以及结构特征,基于欧拉薄梁理论,推导了柔性支撑单元的结构力学模型。然后,假设透镜为刚体,根据在不同姿态下透镜的力平衡和柔性支撑单元的变形协调条件,推导了该柔性支撑结构的整体刚度模型,包括了轴向刚度、横向刚度以及转动刚度。最后,以640 mm口径实验透镜为例,采用数值仿真方法分别模拟了刚性支撑、单级柔性和双级柔性支撑情况下,镜框弹性变形对镜面面形精度的影响。在镜框弹性变形非常大的情况下,双级柔性支撑结构下的镜面面形精度由20 nm下降到50 nm仅下降了2.5倍。此外,利用Zygo干涉仪对实验透镜下表面干涉检测,最佳的面形精度为0.05λ,达到了加工状态,进一步验证了所提出的双级柔性支撑结构的优越性。

A new bi-flexible mounting structure is presented for the large lenses used in wide-field survey telescopes. Using this bi-flexible structure,the position and surface precision of the lens can be guaranteed,and the influence of barrel elastic deformation can be considerably reduced. First,the limitations of existing mounting structures for large optical lenses and the structure scheme are elaborated upon. Moreover,the mechanical principle and advantages of the proposed bi-flexible structure are discussed. Thereafter,according to the structural components and structural properties of the bi-flexible structure,a mechanical model of this structure is established based on Euler beam theory. Then,assuming the lens to be a rigid body,an integrated stiffness model of the mounting structure,which includes axial,transverse,and rotary stiffnesses,is derived based on the force equilibrium and considering compatible deformation conditions. Finally,a 640 mm test lens is used as an example for experimental verification. The influence of the barrel on the lens surface precision for three types of mounting structures are simulated. In the worst case,as the barrel undergoes the maximum elastic deformation,the lens surface precision is reduced to 50 nm;this indicates that the bi-flexible mounting structure performs better than the other two types of structures.In addition,the optical measurement results show that the minimum root mean square error is 0. 05λ,which further verifies the advantages of the proposed bi-flexible mounting structure.