Prediction of TMT tertiary mirror surface figure
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摘要: TMT 望远镜是一台R-C 式的30m口径光学红外望远镜,其三镜为椭圆形平面镜,口径为3.594m2.568m,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(=632.5 nm) 或者斜率均方根小于1 rad。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。
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关键词:
- 大口径反射镜支撑 /
- 斜率均方根 /
- TMT /
- Zernike 多项式拟合 /
- 平面拟合
Abstract: TMT (Thirty Meter Telescope) is a thirty meter Ritchey-Chr佴tien optical-infrared telescope. The Third Mirror (M3) is an elliptical flat mirror, whose aperture is 3.594 m2.568 m, and the weight is 1.8 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 /5(=632.5 nm) or the SlopeRMS will be less than 1rad. This paper took one of the conceptual designs as an example to illustrate the method of evaluating the SlopeRMS. 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 SlopeRMS 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.-
Key words:
- large flat mirror support /
- slope RMS /
- TMT /
- Zernike polynomial /
- plane fitting
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