制冷型大面阵自由曲面离轴三反光学系统设计(特邀)

Design of cooled freeform off-axis three-mirror system with large rectangular field(invited)

针对超大面阵红外遥感探测的需求,设计了一个基于自由曲面的超大矩形视场制冷型离三反光学系统。系统采用一个偶次非球面反射镜和两个自由曲面反射镜组成二次成像的结构,具有实出瞳并与冷光阑匹配,能够实现100%的冷光阑效率。与其他离轴三反系统相比,该系统最大特点在于其适配了4 k分辨率的大面阵红外探测器,具有视场大、无遮拦、成像质量好等技术特点。系统焦距为150 mm,工作波段为1.5~5μm,工作F数为5,视场为30°×25°。结构上,主镜采用偶次非球面,次镜和三镜采用XY多项式自由曲面,以校正大视场下的各种像差,系统在各个视场下调制传递函数在25 lp/mm处均大于0.4,满足大面阵红外探测器的成像质量要求。

Objective A cooled off-axis three-mirror system with a large rectangular field of view based on freeform surface is designed to satisfy the requirement of infrared remote sensing using a large plane array detector.The off-axis three-mirror system is composed of one even aspherical surface and two freeform surfaces,achieving secondary imaging with a real exit pupil that matches the cold shield,resulting in 100%cold shield efficiency.The system has larger rectangular field and decent imaging quality compared to other off-axis three-mirror systems,which ensures the adaption to large-format infrared detectors with a 4 k resolution.The system has a focal length of 150 mm,working waveband of 1.5-5μm,F-number of 5,and field of view of 30°×25°.The primary mirror is even-order aspherical surface,and the secondary and third mirror are XY polynomial surfaces.High-order aberrations are properly corrected with the adoption of freeform surfaces,so the modulation transfer function of the system at 25 lp/mm exceeds 0.4 across all fields of view,meeting the imaging quality requirements of large-format infrared detectors.Methods An off-axis three-mirror systems with large rectangular field of view is presented in this paper.The initial structure is a coaxial three-mirror system with its optical power distribution being convex-concave-concave(Fig.1).The curvatures of three mirrors are calculated by eliminating primary aberrations based on Seidel aberration theory.The off-axis three-mirror system is derived from the coaxial structure by shifting the field center.According to Nodal aberration theory,even aspherical surfaces are adopted to shift aberration contributions of surfaces to new field centers so they can compensate for each other(Fig.3).The off-axis threemirror system with large field of view in tangential direction is further optimized with pupil shifting(Tab.1).The secondary and third mirror are then converted to XY polynomial surface to expand field of view in horizontal direction while the image quality is not degenerating.Results and Discussions The optimized off-axis three-mirror system is presented(Fig.5)with primary mirror being even aspherical surface,the secondary and third mirror being XY polynomial free-form surfaces.The system meets the requirements of the detector and the design specifications(Tab.2)and the efficiency of cold diaphragmis 100%.The modulation transfer function of the system at 25 lp/mm exceeds 0.4 across all fields of view(Fig.6).RMS radius of spot diagram for all fields of view are less than Airy disk radius(Fig.7),indicating a good imagingquality.The maximum distortion of the system is−4.88%,which is acceptable and can be corrected by specificimage processing algorithm.A tolerance analysis is conducted on the system,proving a good instrumentationfeasibility(Fig.9).Conclusions A cooled off-axis three-mirror system with a large rectangular field of view is presented in thispaper.The field of view of the system is 30°×25°,and F-number is 5,ensuring the adaption to 4000×3400@20μm infrared detector.Of three mirrors of the system,the primary mirror is even aspherical surface,and thesecondary and third mirror are XY polynomial free-form surfaces.The system is a re-imaging structure with noobscuration and a real exit pupil matching cold shield of the detector,achieving 100%cold shield efficiency.Theimage quality is good when the system works in 1.5-5μm waveband,thus the system has broad application inoptical remote imaging and sensing field.