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摘要:
为解决地球遥感观测和星载激光雷达探测等领域对空间光学系统尺寸小型化、结构紧凑和高分辨率的要求，本文设计了一种基于Zernike自由曲面的紧凑式离轴三反系统，同时满足了长焦距、小畸变和宽工作波段的要求。该系统采用非对称、近圆形布局的离轴三反光学系统，并在第三反射镜采用了自由曲面的设计，通过在Zemax软件中设置适当的优化目标和方法，对光学系统进行设计优化。最终该系统有效焦距为800 mm，F数为4，视场角为12°×6°，畸变小于1%，工作波段覆盖可见光及近/中红外波段，在400 km轨道高度实现了1.5 m的地元分辨率(可见光)和2.5 m的地元分辨率(近红外)，地面幅宽为80 km×40 km。并进行了系统像差、点列图、MTF等性能指标评估的分析与验证，结果表明该设计方案带来了高分辨率并提升了信息获取能力。
Abstract:In order to meet the requirements of miniaturization, compact structure and high resolution of the space optical systems in the fields of Earth remote sensing observation and spaceborne Lidar detection, this paper designs a compact off-axis triple inverse system based on Zernike free-form surface, which simultaneously meets the requirements of long focal length, small distortion and wide working band. The system adopts an off-axis triple inverse optical system with the asymmetric and nearly circular layout, and the third mirror of the system adopts the free-form surface design. By setting appropriate optimization objectives and methods in Zemax software, the design of the optical system is optimized. Finally, the effective focal length of the system is 800 mm, the F-number is 4, the field of view is 12°×6°, and the distortion is less than 1%. The working band covers the visible and near/middle infrared bands, and the ground element resolutions of 1.5 m (visible light) and 2.5 m (near infrared) can be achieved at the orbit height of 400 km, and the ground width is 80 km×40 km. The analysis and verification of system aberration, dot plot, MTF and other performance indexes are carried out. The results show that the design scheme brings the high resolution and improves the information acquisition ability.
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Overview: With the continuous development of the space remote sensing technology, the resolution, operating band and compactness of the off-axis triplex optical systems have also generated the need for improvement to meet the design objectives of high resolution, broad spectrum and lightweight and compactness. Spatial optics, as the application frontier of basic science, the resolution and field of view of spatial optical systems are crucial for obtaining high-quality images in modern remote sensing technology. To address the special needs of the space remote sensing field, this research will be devoted to achieving an optimized design for lightweight and compactness, and realizing an off-axis triple-reversal structure with a near-circular layout, in order to reduce the mass and volume of the system and provide a more practical solution for applications such as satellite-mounted LIDAR detection. In order to further enhance the resolving power and information acquisition capability of the space optical system, a design scheme is proposed in this paper, which adopts a compact off-axis triple-reflector optical system with a long focal length, small aberration and a wide operating band, and the system structure adopts an asymmetric, near-circular layout of the off-axis triple-reflector optical system, which utilizes a special triple-reflector structure construction, which has been eccentrically tilted in the field of view, and designs an optical system based on even-ordered aspheric surface. On this basis, in order to improve the imaging quality and meet the design requirements, the design method of Zernike's free surface is studied, the third reflector is optimized, and finally the system is effective for a focal length of 800 mm, an F-number of 4, a field of view of 12°×6°, an aberration less than 1%, and an operating band covering the visible and the near/mid-infrared, and a geodetic resolution of 1.5 m has been achieved at the orbit altitude of 400 km. With 1.5 m ground element resolution (visible) and 2.5 m ground element resolution (near-infrared) at an orbital altitude of 400 km, and a ground width of 80 km×40 km, the system has been analyzed and verified in terms of aberration, dot-plot, MTF, and other performance indexes. And the results show that the design scheme brings high resolution capability and information acquisition capability. The analysis results show that the design scheme successfully realizes the design objectives of high resolution, wide spectrum and lightweight and small size, and meets the design requirements with the excellent resolution capability and the information acquisition capability.
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表 1 光学系统设计参数表
Table 1. Table of the optical system design parameters
指标项 参数值 焦距 800 mm 入瞳直径 200 mm F数 4 光谱范围 0.486~0.656 μm，0.78~2.5 μm，3~5 μm 视场 12°×6° 像元尺寸 可见光通道为3 μm，近红外通道为5 μm，中红外通道为17 μm 成像要求 可见光MTF(@160 lp/mm＞0.2)
近红外MTF(@100 lp/mm＞0.2)
中红外MTF(@30 lp/mm＞0.2)
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表 2 初始结构参数
Table 2. Initial structure parameter
曲率半径/mm 厚度 圆锥系数 主镜 −1641.935 −330 −4.118 次镜 −550.510 330 −0.919 三镜 −828.318 −730.4 0
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表 3 初始结构的倾斜和偏心
Table 3. Tilt and eccentricity of the initial structure
项目 倾斜X/倾斜Y 偏心X/偏心Y 主镜 ±18.4° ±800 次镜 ±45° ±565 三镜 ±45° ±1600
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表 4 Zernike standard自由曲面系数表
Table 4. Zernike standard free-form surface coefficient table
项目 Z1 Z2 Z3 Z4 Z5 Z6 Z7 −74.798 −1.701×10−5 10.674 0.164 −4.348×10-7 0.017 2.38×10-3 Z8 Z9 Z10 Z11 Z12 Z13 Z14 三镜 −2.385×10−8 −1.049×10−4 −5.268×10−8 −2.277×10−5 −1.079×10−5 7.318×10−9 3.574×10−7 Z15 Z16 Z17 Z18 Z19 Z20 3.034×10−9 5.154×10−11 7.969×10−7 7.156×10−10 8.6×10−9 8.421×10−10
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表 5 误差分配表
Table 5. Error distribution table
公差类别 公差项目 主镜 次镜 三镜 装调误差 X方向偏轴/mm 0.05 0.2 0 Y方向偏轴/mm 0.05 0.2 0 X方向倾斜/(°) 0.005 0.001 0 Y方向倾斜/(°) 0.005 0.001 0 面型误差 曲率半径/mm 0.2 0.2 0.2 波峰到波谷(PV值)/${\rm{\mu m}}$ 0.5 0.5 0.2
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表 6 蒙特卡罗累计概率统计表
Table 6. Monte Carlo cumulative probability statistic
累积概率/% 弥散斑RMS/mm MTF 90 0.00476342 0.11910409 80 0.00455730 0.15766662 50 0.00396765 0.18865949 20 0.00323214 0.23130557 10 0.00308628 0.24853763
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