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该成像系统用于可见光/近红外成像,探测器的响应波段覆盖可见光至750~1100 nm的近红外波段,像元尺寸为3.45 μm,像元数为2448×2 048,角分辨率优于0.5 mrad,视场大于10°。系统采用共轴折叠结构,在中心遮拦部分同轴嵌套一个可见光成像镜头(货架产品)。
共轴折叠成像系统参数如图1所示。系统入瞳为环形孔径,${d_{out}}$代表系统外径,${d_{in}}$代表系统内径,$d$代表系统厚度,光线经${d_{out}}$和${d_{in}}$之间的宽度为$w$的环形孔进入,经过多次反射后入射到像面上。遮拦比为$\alpha = {{{{d_{in}}} \mathord{\left/ {\vphantom {{{d_{in}}} d}} \right. } d}_{out}}$,有效口径为${d_{eff}} = \sqrt {d_{out}^2 - d_{in}^2} $。
光学系统的初始结构可以在常规同轴四反系统结构的基础上基于像差理论和附加边界约束条件计算得到,初始结构参数见表1。
表 1 初始结构参数
Table 1. Initial structure parameters
Surface Radius/mm Distance/mm Conic ${{M} }_{\text{1} }$ −43.576 −4.850 −3.272 ${{M} }_{\text{2} }$ 229.697 4.700 91.869 ${{M} }_{\text{3} }$ 107.812 −4.895 −336.062 ${{M} }_{\text{4} }$ −27.388 − −18.507 -
将各镜面的面形改为偶次非球面,非球面方程如公式(1)所示:
$$\begin{split} z=&\frac{c{r}^{2}}{1+\sqrt{1-(1+k){c}^{2}{r}^{2}}}+{A}_{2}{r}^{2}+{A}_{4}{r}^{4}+{A}_{6}{r}^{6}+{A}_{8}{r}^{8}+\\ &{A}_{10}{r}^{10}+{A}_{12}{r}^{12}+{A}_{14}{r}^{14}+{A}_{16}{r}^{16} \end{split} $$ (1) 式中:$c$代表顶点曲率;$r$代表径向坐标;$k$代表二次常数,并设置非球面的最高幂次为16。以点列图大小为评价函数对光学设计结果进行参数优化,得到如图2所示的成像系统,光线从环形入瞳入射到主镜S1,先后经过次镜S2、三镜S3和四镜S4反射后成像于探测器上。主镜和三镜、次镜和四镜分别共用镜坯,系统基本参数如表2所示,优化后的高次非球面系数如表3所示。
表 2 基本面型参数
Table 2. Basic surface parameters
Surface $ {{M}}_{\text{1}} $ $ {{M}}_{\text{2}} $ $ {{M}}_{\text{3}} $ $ {{M}}_{\text{4}} $ Radius/mm −67.998 −239.1149 −132.9414 −64.7218 Distance/mm −15.485 18.15515 −12.74705 24.65042 Diameter/mm 66.2129 52.86353 52.75673 32.0385 Conic coefficient 0.29199 67.76022 2.859459 −25.448 表 3 高次非球面系数
Table 3. High order aspheric coefficients
Surface $ {{M}}_{\text{1}} $ $ {{M}}_{\text{2}} $ $ {{M}}_{\text{3}} $ $ {{M}}_{\text{4}} $ A2 0.0015293882 −0.0075416624 0.0056460009 0.0015335974 A4 −1.0962896e-006 4.375154e-006 −2.6497218e-007 −1.296045e-005 A6 1.5847328e-010 3.5537337e-009 −1.6551466e-009 7.4891721e-009 A8 3.3404588e-013 3.7951706e-012 −3.5939478e-013 3.4871654e-011 A10 5.8294423e-016 −5.2626752e-015 1.6072703e-015 −4.9330029e-013 A12 −5.4089482e-019 4.9805961e-018 −3.6151876e-018 2.1115162e-015 A14 2.6248278e-022 −2.5360935e-021 2.9968098e-021 −4.6380052e-018 A16 5.322143e-026 −8.9904043e-025 −1.1493177e-024 4.0505877e-021 图3为系统的光学调制传递函数曲线,可以看出全视场内调制函数值接近衍射极限。图4为系统的场曲和畸变曲线,可以看出系统全视场畸变小于1%。
Interferometric test of coaxial folded mirrors for visible/near-infrared imaging systems (invited)
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摘要: 由共用镜坯、径向折叠的多个环带反射镜组成的成像系统具有紧凑化、免装调的特点。为确保各反射镜的面形精度和相互位姿精度,提出了计算全息(Computer Generated Hologram, CGH)补偿干涉测量方法。针对可见光/近红外成像需求,基于共轴折叠思路设计了环带四反射镜成像系统;应用金刚石车削工艺加工了多环带共体反射镜;重点针对其中共体的主镜、三镜和次镜、四镜分别设计了CGH补偿器,通过合理选择离焦载频和CGH轴向位置,有效分离了干扰衍射级次的鬼像,实现了多个反射镜面形与相互位姿误差的同步检测。干涉测量结果表明,多个反射镜同时达到接近零条纹状态,面形精度和相互位姿精度较高,且无鬼像干扰。系统对100 m远处目标探测实验表明,反射镜不需要额外装调即可实现良好成像,具有集成度高、研制周期短、成像质量高的优点。
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关键词:
- 可见光/近红外成像系统 /
- 折叠反射镜 /
- 干涉检测 /
- 计算全息 /
- 高次非球面
Abstract:Objective Photoelectric imaging system serves as the “eye” of all kinds of equipment, which plays an indispensable role in scene detection and target recognition. To acquire more abundant target information, one of the development directions is multi-band fusion detection. However, the existing multi-band imaging system mostly adopts the discrete structure, with large system volume architecture, high manufacturing cost, and lack of spatial consistency due to parallax between the discrete systems. The challenges pose difficulties in image fusion and other back-end processing. Multi-band common aperture, also a common configuration, is generally used to split the front optical path with optical components, and subsequently respond to the detection requirements of different bands through the discrete rear optical path. To address these issues, coaxial folded mirrors for visible/near-infrared imaging systems are designed in this paper. Methods To guarantee the surface accuracy and relative orientation accuracy for multiple mirrors, an interferometric null test with a computer-generated hologram (CGH) is proposed (Fig.5). Diamond turning technology is applied to machining the mirrors. In this approach, two CGHs are designed for the null test of the monolithic primary/tertiary mirrors and the monolithic secondary/fourth mirrors (Fig.6, Fig.9). Ghost image of disturbance orders of diffraction is effectively separated by properly choosing the power carrier and the axial position of the CGH. A single CGH is capable of simultaneously measuring both the surface error and the relative orientation error of multiple mirrors (Fig.8). The result of the interferometric null test shows multiple mirrors are measured with nearly null fringes, indicating high accuracy in terms of surface form and orientation. Moreover, no ghost disturbance is observed. Results and Discussions The optical components undergo the diamond turning process, and the mirror blank is shared among the primary mirror and the three additional mirrors, allowing for simultaneous processing (Fig.12). After processing, a CGH is used to conduct zero compensation measurements on both mirrors (Fig.13). The measured surface shape error is shown (Fig.14), and the primary mirror and the three mirrors demonstrate a combined surface shaper error of PV 0.87λ, RMS 0.12λ; Interference diagram reveals that the ghost image stripes only exist outside the main mirror and the three mirror stripes, and they do not form interference. The primary mirror and the three mirrors reach a near-zero fringe state at the same time, indicating a high level of surface shape accuracy and mutual pose accuracy (reaching the sub-wavelength level), which meets the imaging requirements of the system. Conclusions The study proposes an interferometric null test with a CGH for the coaxial folded mirrors in visible/near-infrared imaging systems. The method involves the creation of multiple holographic regions with different functions on the same CGH substrate, which allows for the generation of the aspheric wavefronts of different shapes after the diffraction of the incident test wavefront. Consequently, the zero position of different mirror shapes can be tested at the same time. Following ultra-precision machining based on CGH compensation measurement, the mirror shape accuracy and pose accuracy attain a sub-wavelength level, which realizes direct assembly without additional assembly and adjustment for optimal imaging performance. Similarly, by positioning reference processing, multiple similar systems are nested coaxially, which enables multi-band coaxial imaging from visible light to near-infrared. Such capability holds obvious advantages for unmanned platform target detection and fast image fusion processing. -
表 1 初始结构参数
Table 1. Initial structure parameters
Surface Radius/mm Distance/mm Conic ${{M} }_{\text{1} }$ −43.576 −4.850 −3.272 ${{M} }_{\text{2} }$ 229.697 4.700 91.869 ${{M} }_{\text{3} }$ 107.812 −4.895 −336.062 ${{M} }_{\text{4} }$ −27.388 − −18.507 表 2 基本面型参数
Table 2. Basic surface parameters
Surface $ {{M}}_{\text{1}} $ $ {{M}}_{\text{2}} $ $ {{M}}_{\text{3}} $ $ {{M}}_{\text{4}} $ Radius/mm −67.998 −239.1149 −132.9414 −64.7218 Distance/mm −15.485 18.15515 −12.74705 24.65042 Diameter/mm 66.2129 52.86353 52.75673 32.0385 Conic coefficient 0.29199 67.76022 2.859459 −25.448 表 3 高次非球面系数
Table 3. High order aspheric coefficients
Surface $ {{M}}_{\text{1}} $ $ {{M}}_{\text{2}} $ $ {{M}}_{\text{3}} $ $ {{M}}_{\text{4}} $ A2 0.0015293882 −0.0075416624 0.0056460009 0.0015335974 A4 −1.0962896e-006 4.375154e-006 −2.6497218e-007 −1.296045e-005 A6 1.5847328e-010 3.5537337e-009 −1.6551466e-009 7.4891721e-009 A8 3.3404588e-013 3.7951706e-012 −3.5939478e-013 3.4871654e-011 A10 5.8294423e-016 −5.2626752e-015 1.6072703e-015 −4.9330029e-013 A12 −5.4089482e-019 4.9805961e-018 −3.6151876e-018 2.1115162e-015 A14 2.6248278e-022 −2.5360935e-021 2.9968098e-021 −4.6380052e-018 A16 5.322143e-026 −8.9904043e-025 −1.1493177e-024 4.0505877e-021 -
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