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为了满足轨道高度为500 km,幅宽为125 km实现对大范围气体监测[18]的星载CO2成像光谱仪探测器需求,系统的小F数有利于提高光谱仪器的光通量,采用像元合并的方案有利于进一步提高系统的信噪比。Offner光谱仪具体设计指标如表1所示。
Parameter Value Object space NA 0.2 Spectral range/nm 1594-1619 Spectral resolution/nm 0.1 Slit length/mm 30 Pixel dimension 15 μm×15 μm MTF ≥0.7 Keystone and smile/pixel <10% Table 1. Principal design indicators of the system
为了实现0.1 nm的光谱分辨率,采用刻线数为550 lp/mm的凸面光栅,基于成像光谱仪的设计指标以及Offner光谱仪光学系统初始结构参数计算公式,获取Offner成像光谱仪初始光学系统结构参数,如表2所示。
Surface Radius/mm Thickness/mm Material Lines/mm Object - 415.43 - - 1 −422 203.59 Mirror - Stop −214.58 203.59 Mirror 550 3 −415.5 203.59 Mirror - Image Infinity 219.03 - - Table 2. Structure parameter of initial optical
图5为Offner成像光谱仪光学系统初始结构示意图,该系统光阑在凸面光栅上,形成物方远心系统,便于与前置望远系统的光瞳衔接,提高系统能量的利用率。图6为初始光学系统在不同波长各视场处的点列图以及传递函数曲线。
Figure 6. Spot diagram of the initial structure with the wavelength of (a) 1.594 μm, (b) 1.6065 μm, (c) 1.619 μm; MTF of the initial structure with the wavelength of (d) 1.594 μm, (e) 1.6065 μm, (f) 1.619 μm
图6所示初始结构在1594、1606.5、1619 nm3个波长所对应的各视场点列图中,可以看到波长为1594 nm处弥散斑均方根(RMS)半径小于7 nm,1606.5 nm处弥散斑RMS半径小于5 μm,1619 nm处弥散斑RMS半径小于7 μm。边缘视场的弥散斑半径比中心视场处弥散斑半径大,约为3 μm,且不同视场处弥散斑RMS半径均约为像元尺寸的1.5倍。在图6中所示初始结构在1594、1606.5、 1619 nm 3个波长所对应的传递函数曲线中,3个波长不同视场的子午像面和弧矢像面在奈奎斯特频率33 lp/mm时的MTF相差较多。通过分析不同波长的各视场点列图和传递函数,此时,系统存在一定的像散、彗差及残余像差,难以满足高质量成像和高精度探测的需求。因此,需要进一步优化系统存在的像差,同时控制边缘光线与凸面光栅边缘之间的距离,避免发生渐晕。
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在初始光学系统光路中,放置两块弯月形厚透镜,分别位于狭缝和主镜之间光路,以及三镜和相面之间光路,可以更好的矫正系统的像散和彗差,提高系统的成像质量。在优化过程中,以RMS光斑半径作为优化函数,同时控制放大倍率和主、三镜的圆锥系数。最终,光学系统的结构参数如表3所示。
光路结构如图7(a)所示,设计的分光系统的机械结构图如图7(b)所示。
优化后各视场的点列图与MTF曲线如图8所示,在1594、1606.5、1619 nm 3个波长所对应的每个视场点列图中,优化后的光学系统相比于初始结构其像散和彗差均得到了明显的减小,并且弥散斑的均方根半径均小于5 μm,小于1/3像元尺寸。该系统在奈奎斯特频率33 lp/mm处,全视场的MTF值均达到0.7以上,满足高质量成像要求。
Surface Radius/mm Thickness/mm Diameter/mm Glass Lines/mm Object - 650.655 Φ200 - - 1 300.57 60 Φ375 H-QF3 - 2 373.35 999.812 - - - 3 −1703.59 −978.79 Φ500 Mirror - Stop −1004.4 −978.79 Φ120 Mirror 550 5 −1920.66 −999.812 Φ660 Mirror - 6 373.35 −60 Φ600 H-FK61 B - 7 300.57 −754 - - - Image Infinity - - - - Table 3. Structure parameter of optimization system
Optical system design of spaceborne CO2 imaging spectrometer based on Offner convex grating
doi: 10.3788/IRLA20220431
- Received Date: 2022-02-10
- Rev Recd Date: 2022-04-10
- Accepted Date: 2022-06-01
- Publish Date: 2022-08-05
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Key words:
- imaging spectrometer /
- optical design /
- short infrared /
- convex grating /
- pixel binning
Abstract: Spaceborne CO2 imaging spectrometer has become one of the important means to monitor global greenhouse gas changes due to its advantages of spectrum integration, high spatial resolution, high time resolution, non-contact and long-term monitoring. In order to solve the problems of large incident slit, low signal-to-noise ratio and poor imaging quality in large field of view, a new scheme was proposed for the initial structure design of the optical system of Offner imaging spectrometer. The scheme was based on the tangent of meridian ray and arc sagittal ray emitted at the 30 mm slit at the central wavelength to improve the utilization rate of incident light in the whole spectral range. Using optical design software, an imaging spectrometer with F number of 2.5 and spectral system resolution of 0.1 nm was designed in the range of 1594-1619 nm. The design results show that the root mean square (RMS) radius of the sequence diagram is less than 5 μm, and the modulation transfer function of the system at 33 lp/mm is better than 0.7. In addition, the system uses pixel merging (i.e. extended pixel) method to further improve the detection intensity of spectral signals. The design scheme satisfy remote sensing detection requirements of large field of view, high spectral resolution and high signal-to-noise ratio for spaceborne CO2 imaging spectrometer.