TMT三镜面形评价过程

Prediction of TMT tertiary mirror surface figure

TMT望远镜是一台R-C式的30 m口径光学红外望远镜,其三镜为椭圆形平面镜,口径为3.594 m×2.568 m,质量达到1.8 t。三镜系统需要把来自于次镜的光折转到望远镜两侧奈氏平台的科学仪器上,具有跟踪和快速定向功能,因此三镜位置也是三轴正交运动的结果。三镜支撑系统必须能够在多工况、变载荷的条件下有效保证镜面的面形精度,所以依据TMT项目组要求面形结果需优于1/5波长(λ=632.5 nm)或者斜率均方根小于1μrad。以三镜支撑的一种方案为例详细阐述了TMT三镜面形评价的过程。这一方案中底支撑采用18点whiffletree结构形式,侧支撑采用8点A-frame结构形式。首先通过ANSYS建模仿真,然后使用MATLAB进行数据后期处理,依次去除结果中包含的刚体位移项、离焦和像散等,最后使用9个子孔径来拼接归一化的三镜,同时计算每一个子孔径的斜率均方根。这一数据处理方法有别于以往圆形主镜镜面评价的过程,但适合对TMT三镜椭圆镜面面形评价,且在光学系统中斜率均方根比均方根在镜面面形评价上更有意义。对于大口径反射镜镜面面型评价具有重要的指导意义。

TMT（Thirty Meter Telescope） is a thirty meter Ritchey-Chrétien optical-infrared telescope.The Third Mirror（M3） is an elliptical flat mirror,whose aperture is 3.594 m ×2.568 m,and the weight is1.8 ton.The M3 system reflects the light coming from the Secondary Mirror System to the science instruments located on the Nasmyth platforms,so the M3 system rotates and tilts as the telescope rotates about the elevation axis to track astronomical objects across the sky.Due to the M3 support system the mirror figure maintains a high precision.The specification requires that the RMS of the mirror surface figure will be less than λ/5（λ=632.5 nm） or the Slope RMS will be less than 1 μrad.This paper took one of the conceptual designs as an example to illustrate the method of evaluating the Slope RMS.The axial support system in this plan used an 18 points whiffletree structure while the lateral support system used an 8 points A-frame structure.The FE model was constructed in ANSYS.Then MATLAB was employed to carry out the data processing.In the data processing program,the rigid movement was removed at first.Then the elliptical mirror was normalized into a circle mirror and the defocus and the astigmatism were eliminated.In the end,the Slope RMS of nine sub-apertures was calculated.This method for evaluating the mirror surface figure is different from the classic one for the primary mirror,but it is effective to evaluate the elliptical mirror surface figure indeed.This method is meaningful in mirror surface figure evaluation.