To estimate the thermal distortion of a large aperture SiC lightweight primary mirror exactly, the calibration and calculation methods for the SiC lightweight mirror were researched in different thermal modes. Temperature sensors stuck on the lightweight mirror were divided into three levels a-long an optic axial direction, and the temperatures at each level were fitted by quasi-Zernike polynomials, respectively. By taking a 4 m SiC lightweight mirror for an example,the Finite Element Method (FEM) built by MSC. Patran was used to calibrate and calculate the thermal distortion in eighteen kind temperature fields of the first ninth quasi-Zernike polynomials and the Max deformation, surface error PV and RMS values due to the lightweight mirror thermal distortion were obtained for different temperature patterns. Calculation results show that the deformation of lightweight mirror is max under the first quasi-Zernike polynomial temperature pattern, and the surface error RMS value is 278. 3 nm. Using a least-square method, the aberration coefficients of surface errors are calculated by quasi-Zernike polynomial wavefront fitting program for each temperature field, and the main aberration caused by the thermal distortion is piston, tilt, defocus, coma and astigmatism.%为了准确地预算出大口径SiC轻量化主镜镜面的温度变形,研究了不同热模式下SiC轻量化主镜镜面面形的定标和计算方法.将SiC轻量化主镜上的温度传感器在轴向厚度方向上进行分层处理,对每层上的温度分布采用准Zernike多项式进行拟合.以4 m SiC轻量化主镜为例,采用有限元法分别对准Zernike前9项温度模式(18种温度场)下的镜面变形进行定标计算,得出各种单位载荷作用下轻量化主镜镜面的最大变形以及面形误差PV值和RMS值.计算结果表明:准Zernike第一项模式、单位载荷作用下镜面变形最大,其面形误差RMS为278.3 nm.采用最小二乘法对各种温差场下的镜面误差进行准Zernike多项式拟合,获得了准Zernike像差项的系数.采用最小二乘法拟合计算出镜面变形误差产生的准Zernike像差项的系数,结果表明,18种温度场下产生的像差形式主要有:平移、倾斜、离焦、彗差和像散.
展开▼
