为了提高大口径平面镜面形检测的精度和效率, 提出了一种新的五棱镜扫描法。该方法采用径向扫描的方式, 使用一个扫描的五棱镜和一台自准直仪来测量表面倾斜角的差值, 然后将被测平面镜的面形表示为Zernike多项式的线性组合, 再利用表面倾斜角的差值建立方程组, 最后采用最小二乘法计算得到被测平面镜的面形。在检测过程中, 该方法还可以对五棱镜在扫描过程中的倾斜变化量进行自动监视和调整, 减小了检测误差。误差分析表明, 该方法的面形检测精度为76 nm rms(均方根误差)。采用该方法对一块15 m口径的平面镜进行了面形检测, 并与Ritchey-Common法的检测结果进行了对比, 两种方法面形结果的差异为71 nm rms, 小于五棱镜扫描法的面形检测精度。证明了利用该五棱镜扫描法检测大口径平面镜面形的正确性。

To improve the accuracy and efficiency of the surface shape measurements of large flat mirrors, a new scanning pentaprism method is proposed. In this method, we use a scanning pentaprism and an autocollimator to radially scan and measure the differences between the tilt angles. The surface shape of the flat mirror under test is expressed by a linear combination of Zernike polynomials and an equation set is established on the basis of the angle differences. Finally, the surface shape of the flat mirror can be derived through a least squares calculation. In the measuring process, this method can automatically accommodate changes in the pentaprism tilts during scans, which can reduce measurement errors. The error analysis indicates that the surface shape measurement accuracy by this method is 76 nm rms(root-mean-square). This method is used to measure the surface shape of a 15 m flat mirror and the result is compared to that of the Ritchey-Common method. The difference between the two surface shape results is 71 nm rms, which is less than the surface shape measurement results of the scanning pentaprism method. This proves that it is feasible using the scanning pentaprism method to measure the topographies of large flat mirrors.