A new co-phasing detection technology of a segmented mirror based on broadband light
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摘要:
鉴于单块口径的光学望远镜不能无限增大，采用拼接镜技术才能造出10 m以上口径的光学望远镜，因此，拼接镜的共相检测技术成为了拼接过程和维持镜面质量的关键技术。针对目前最被接受的宽窄带夏克哈特曼法，本文提出使用宽波段（400~700 nm）光源的非相干性和相干性相结合方式实现250 nm粗共相，以及10 nm精共相，以此解决由于目标流量过低而引起测量时间过长的问题。即在粗共相时，以两个半圆孔的非相干衍射图样为模板，白光为光源，采用互相关算法计算互相关系数的值，通过设置合理的互相关系数阈值，以实现无限制的检测范围和0.25 μm 的检测精度；精共相时，以白光为光源、采用以一幅相干衍射图案（理想白光艾里斑）为模板的方式替代多幅不同平移误差下的相干衍射图案为模板方式，实现0.27 μm量程、0.01 μm以上精度的共相检测。对该共相方法进行了理论和仿真分析，结果表明：该新型共相检测方法的检测量程为无限量程，检测精度能达到 10 nm以上，该方法适用于拼接镜粗精共相的检测。
Abstract:Considering that the aperture of a monoblock telescope is limited in size, to build an aperture telescope that is greater than ten meters, the technology of segmented mirrors should be used. Therefore, the co-phasing detection technology of segmented mirrors has become the key technology in the segmented process and in maintaining the mirror quality. To solve the problem that the broadband method demands a long time consuming and the narrowband method has a small range in the most widely accepted broadband and narrowband shack Hartmann method, a new method is proposed combining the incoherent and coherent diffraction patterns of broadband light (400−700 nm) to realize coarse co-phasing of 250 nm precision and fine co-phasing of 10 nm precision. When a segmented mirror is coarse co-phasing, the incoherent diffraction pattern of two hale circular holes is used as a template and white light is used as the light source. The cross-correlation algorithm is used to calculate the value of cross-correlation coefficient, and then it can achieve the unlimited range and a detection precision of 0.25 μm by setting a reasonable threshold value for the cross-correlation coefficient. When segmented mirror is fine co-phasing, a disk pattern of white light instead of multiple coherent diffraction patterns with different piston errors is used as a template to achieve a range of 0.27 μm and a detection precision of 0.01 μm. The theoretical and simulation results show that the detection range is the range of actuator and the measurement accuracy is less than 10 nm. Both the theoretical analysis and simulation show that this method is suitable for the detection of a coarse and fine co-phasing of segmented mirror.
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Key words:
- astronomical optics /
- telescopes /
- sub-aperture /
- phase measurement
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图 1 放置在子镜间的圆孔示意图
Figure 1. Schematic diagram of circular hole placed between sub-mirrors
图 2 在400~700 nm带宽下，平移误差从1 μm到−1 μm变化时的理论圆孔衍射图
Figure 2. Theoretical circular diffraction patterns when piston error is varying from 1 μm to −1 μm at 400−700 nm bandwidth
图 3 在400~700 nm带宽下，平移误差从250 nm到−250 nm变化时的理论圆孔衍射图
Figure 3. Theoretical circular diffraction patterns when piston error is varying from 250 nm to −250 nm at 400−700 nm bandwidth
图 4 平移误差大于可见光相干长度时的（a）模板图案和（b）互相关系数Corr2随拼接镜平移误差变化关系图
Figure 4. (a) Template pattern and (b) Corr2 as a function of piston errors when piston error is greater than visible coherence length
图 5 平移误差在可见光相干长度以内时的（a）模板图案和（b）互相关系数Corr2随拼接镜平移误差变化关系图
Figure 5. (a) Template pattern and (b) Corr2 as a function of piston errors when piston error is within the coherence length of visible light
图 6 R1分别为0.2、0.3、0.4时，互相关系数Corr2值与平移误差关系图
Figure 6. Corr2 as a function of piston errors when R1 is 0.2, 0.3 and 0.4
图 7 R2分别为0.3、0.4、0.5时，互相关系数Corr2值与平移误差关系图
Figure 7. Corr2 as a function of piston errors when R2 is 0.3, 0.4 and 0.5
图 8 SNR分别为45、60、90时，互相关系数Corr2值与平移误差关系图
Figure 8. Corr2 as a function of piston errors when SNR is 45, 60 and 90
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