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摘要:
自适应光学系统在高分辨力成像望远镜中发挥了重要作用。自适应光学系统对于望远镜系统中的低阶像差几乎可以完全校正，但是会牺牲变形镜的校正行程量；对于中高阶像差，自适应光学系统不能完全校正，如何减小高阶残余误差是望远镜系统设计需要考虑的问题之一。本文首先分析了望远镜光学结构中的主镜结构、次镜遮拦、次镜支撑筋、主次镜装调和光学加工等静态和准静态像差情况，然后分析了这些因素如何影响自适应光学校正能力，最后给出了对望远镜光学结构的要求。
Abstract:Abstract: Adaptive optics has played an important role in high resolution telescope. The low order aberrations of the telescope can be completely compensated by adaptive optics, but it causes the loss of the compensation stroke of the deformable mirror. The middle and high order aberrations after compensating of the deformable mirror have some residual aberration, so we need control the residual aberration to ensure high resolution imaging quality, especially the high order residual aberration that can't be compensated, which should be strictly controlled in the beginning of the design of the telescope system. This paper analyzes the structure of primary mirror of the telescope optical system, secondary mirror block, secondary mirror support bars block, the primary mirror and secondary mirror alignment, and the static and quasi-static aberration of the optical machining. The influence of these factors on the adaptive optics compensation is analyzed, and the requirements of the aberration control are given.
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Key words:
- adaptive optics /
- optical structure /
- high aberration /
- secondary mirror support bars
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In the process of high resolution imaging of celestial objects, adaptive optics system plays an important rolein the compensation of atmospheric turbulence and the improvement of imaging quality. However, the adaptive opticssystem is in a certain condition between two extreme situations, which are fully uncompensated and fully compensated, and belongs to partially compensated optical system. Adaptive optics can achieve almost full compensation forlow order aberrations, but the compensation ability for high order aberration is limited. The low order aberrations ofthe telescope can be completely compensated by adaptive optics, but it causes the loss of the compensation stroke ofthe deformable mirror. The middle and high order aberrations after compensating of the deformable mirror, which areproduced mainly by telescope structures, alignment and processing, have some residual aberration. This residual aberrations result in severe degradation of imaging quality of the telescope. So we need control the residual aberration toensure high resolution imaging quality, especially the high order residual aberration that can’t be compensated, whichshould be strictly controlled in the beginning of the design of the telescope system.
We analyze the influence of the telescope optical structures on adaptive optics compensation, mainly for the 4 metertelescope. First of all, the simulation analysis of adaptive optics system layout of the 4 meter telescope is presented, inorder to analyze the residual aberrations with compensated by 4 meter adaptive optical system. The specific analysis ofthe optical structures on the layout correction capability of our adaptive optical system contains the following content:the structure of primary mirror of the telescope optical system, mainly the honeycomb structure, the primary mirrorsupport structure, the primary mirror temperature deformation, secondary mirror block, secondary mirror supportbars block, and the static and quasi-static aberration of the optical processing. The influence of these factors on theadaptive optics compensation is analyzed, so that the requirements of the aberrations control are given.
Low order aberrations such as defocus and astigmatism caused by primary and secondary mirror alignment, primary mirror support, and primary mirror thermal deformation, can be completely corrected by adjusting the secondary mirror or using a single deformable mirror which has large compensation stroke. High order aberrations out theability of adaptive optics compensation, such as the aberrations caused by honeycomb structure of primary mirror, canbe compensated by data processing. In the process of telescope design and processing, the factors that lead to a largenumber of high order aberrations should be strictly controlled, and high control requirements are put forward.
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图 1 变形镜驱动器和哈特曼子孔径布局.
Figure 1. Deformable mirror layout and Hartmann wavefront sensor layout.
图 4 变形镜驱动器间距变化对蜂窝镜面形校正效果.
Figure 4. Compensation effect in surface error of 1.8 m honeycomb mirror by deformable mirror actuator spacing changing.
图 5 主镜指向水平方向面形和变形镜校正残差面形.
Figure 5. Surface wavefront of primary mirror pointing horizontal direction and the residual surface wavefront by deformable mirror compensated.
图 6 温度变形引起的误差变化及变形镜拟合后的残差. (a)原始面形. (b)去除离焦之后的面形. (c)变形镜拟合残差面.
Figure 6. Surface wavefront of the temperature distortion and the residual surface wavefront by deformable mirror compensated. (a) Original surface. (b) The surface after removing defocus. (c) The residual surface by deformable mirror compensated.
图 7 望远镜次镜遮拦示意图. (a)正视图. (b)俯视图.
Figure 7. Secondary mirror block of telescope. (a) Front view. (b) Top view.
图 8 (a) 847单元变形镜驱动器分布. (b)哈特曼子孔径分布.
Figure 8. (a) Deformable mirror layout of the 847 actuators. (b) Hartmann wavefront sensor layout.
图 9 变形镜校正能力随次镜遮拦变化曲线.
Figure 9. Compensation capacity of deformable mirror by secondary mirror block changing.
图 10 次镜支撑筋分割波面的影响. (a)原始波面. (b)加入支撑筋. (c) DM拟合波面. (d)加筋校正残差波面.
Figure 10. Split wavefront of adding the secondary mirror support bars block. (a) Aberration source signal. (b) Aberration source signal. (c) DM's fitting surface. (d) Remainder error.
图 11 支撑筋尺寸大小(a)和旋转(b)对变形镜拟合效果的影响.
Figure 11. The effect of compensation by deformable mirror with the size (a) and rotation of the secondary mirror bars changing (b).
图 12 平面反射镜光学加工静态误差统计分布.
Figure 12. The statistical distribution of optical processing static error of planar mirror.
图 13 非球面反射镜光学加工静态误差统计分布.
Figure 13. The statistical distribution of optical processing static error of aspheric mirror.
表 1 1.8 m蜂窝镜面形像差量.
Table 1. Surface error of 1.8 m honeycomb mirror.
RMS/nm 原始检测面形 18 去除前36阶后的高阶残差 16 去除前65阶后的高阶残差 15 
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表 2 847单元次镜支撑筋分割波面对变形镜校正残差影响.
Table 2. The residual surface wavefront error of deformable mirror compensated for 847.
RMS 残差/RMS 信号 原始检测面形 0.04 加入50 mm支撑筋校正 0.07
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表 3 支撑筋大小对应所遮挡的子孔径比例.
Table 3. The size of the subaperture with the size of secondary mirror support bar changing.
支撑筋大小/mm 50 60 70 80 90 100 遮挡847单元子孔径比例 0.42 0.51 0.59 0.68 0.76 0.85
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表 4 光学加工静态误差校正分析.
Table 4. Static error compensation analysis of optical processing error.
原始RMS/nm 前35阶Zernike项RMS/nm 前阶Zernike项RMS/nm 65阶以后Zernike项RMS/nm 非球面镜 65 22 24.3 60 平面镜 32 18.5 21.5 23.7 系统静态误差 210 85 97 185 校正后残差 28 1.8 3.1 27.8
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表 5 光学结构对自适应光学校正能力的影响和控制要求.
Table 5. The influence of the optical structures on adaptive optics compensation.
影响因素 自适应光学控制要求 蜂窝主镜结构 主镜面形有很大的高阶残差 变形镜驱动器间距 < 7 mm 主镜支撑 主镜在指向水平时面形误差最为严重 校正后 < 2 nm 主镜温度变形 主要引人大的离焦 去除离焦后校正残差23.6 nm 次镜遮拦 变形镜布局像差校正能力 遮拦比0.2~0.25 次镜支撑筋 波面拟合像差 支撑筋遮挡子孔径比例 < 0.6个子孔径 光学加工静态像差 变形镜校正高阶残差 校正后30 nm 主次镜装调 变形镜校正行程 调整次镜位置
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