| In recent years,the airborne photoelectric detection system for military helicopters has developed rapidly.On the premise of satisfying good imaging performance,the optical system with small volume and multi-spectral application become a research hotspot.In order to improve the combat capability and reduce the air resistance caused by the hanging pod type optoelectronic detection system,the conformal requirements for the optoelectronic detection system and its optical window are put forward.Conformal photoelectric collision avoidance system has all-weather imaging performance,and adds laser three-dimensional radar to provide obstacle warning,avoid collision as well as improve survivability.At the same time,the shape of the optical system has the advantage of small air resistance.The application value of this paper lies in the design of an optical window matching with the conformal photoelectric collision avoidance system,whose surface is integrated with the fuselage,which not only meets the requirements of good aerodynamic performance,but also has a compact structure and good imaging effect.Under the above background,this paper carries out the following research work:Firstly,the technical indexes of the conformal photoelectric collision avoidance system were determined,and the design scheme of the common-aperture sub-band was adopted.Based on the imaging principle of visible light,laser as well as mid-wave infrared system and the selected detector parameters,the focal length,field of view and entrance pupil diameter of the imaging system of each band were reasonably determined.The detection capability of each band imaging system was verified to provide design parameters for the construction of an optical window matching the conformal photoelectric collision avoidance system.Secondly,the applicable environment of different surface shapes was analyzed,and an ellipsoid with aspect ratio of 1,diameter of 100mm,thickness of 5mm and material of MgF2 was established as the surface shape of the optical window.Based on the finite element theory,the division model of optical window and its outflow field was established by using ICEM-CFD.The S-A turbulence equation model was selected,and the pressure as well as temperature distribution of the optical window and its outflow field were calculated by Fluent.Then,ZEMAX was used to construct the conformal window scanning optical path model.The relation between Zernike polynomial and aberration was used to determine that the universal scanning node is 60mm away from the optical window vertex.MTF was used to preliminarily evaluate the imaging quality of each band system in the scanning process.The aberration characteristics of the window were analyzed by the P-V value of Zernike aberration coefficient The results show that aberrations,which are mainly astigmatism and coma and vary with the scanning field of view,are introduced into the optical window.Finally,the generalized coddington aberration correction mathematical model and the binary plane monochromatic aberration theory were derived.A fixed aberration correction system composed of Zernike Sag surface and binary diffraction surface were used to correct the aberrations introduced by scanning of optical window,ZEMAX software was used to optimize the structure design of the aberration correction system.After comparison,the MTF curve of the imaging system in all bands was significantly improved before and after the correction,the P-V value of Zernike aberration coefficient was significantly decreased.The results show that the designed fixed aberration correction system makes the aberration P-V values of the imaging system in all bands less than 0.6λ,and the correction effect is good.To sum up,the optical window and fixed aberration correction system designed in this paper meet the multi-band application requirements of conformal photoelectric collision avoidance system,which lay a foundation for subsequent analysis of aerodynamic optical effects and processing of optical components. |
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