Research On High Resolution Imaging Technology Of Segmented Diffractive Telescope

Because of using binary diffractive elements as primary mirror,the segmented diffractive optical telescope has the advantages of light weight,loose tolerance and foldable,et al,which has become the key development direction of lightweight,large aperture and high-resolution space optical telescope.However,from the current researches,it can be found that the imaging quality of the telescope is difficult to reach the limit resolution of its theoretical design.There are two main reasons for the degradation of telescope imaging quality: One is the common phase error caused by the misalignment of segmented Fresnel lens in assembly and daily operation;the other is the wavefront aberration introduced by the inevitable processing error of the diffractive element.These two problems are intertwined and influence each other,which leads to the dispersion of the point spread function(PSF)and the decrease of the modulation transfer function(MTF),and finally leads to the degradation of the imaging resolution of the telescope.In order to solve the above problems,the high-resolution imaging technology of the segmented diffractive telescope is studied in this dissertation.According to the research steps of theoretical analysis,simulation correction and experimental verification,this dissertation carries out relevant research from the following aspects.Firstly,the problem of image degradation caused by common phase error and wavefront aberration is analyzed.Based on the sparse aperture structure,the segmented Fresnel lens(SFL)model is established,and the mathematical relationship between all kinds of common phase errors and the MTF index of the imaging system is deduced.Through calculation and simulation,the influence degree of common phase errors on the PSF and MTF of the imaging system is clarified,and the evaluation index of misalignment degree under the point target and extended target imaging scene is established,which will provide a direct reference for the common phase error correction.In addition,the wavefront aberration of Fresnel lens caused by processing error is analyzed comprehensively,and the influence of various processing errors on the SR and MTF of Fresnel lens under the current processing ability is clarified,which lays a theoretical foundation for subsequent simulation correction.Secondly,the numerical simulation correction is carried out for the common phase error and wavefront aberration.According to the approximate analytical expression between common phase errors and MTF index,a method of common phase error correction based on far-field image is proposed.The simulation results show that when there is no wavefront aberration coupling,the common phase errors can be effectively corrected,and the corrected wavefront RMS value can be below ?/14,which makes the system reach the ideal imaging requirements.In addition,considering the coupling problem between wavefront aberration and common phase error,we propose a compound correction strategy based on blind optimization principle and adaptive optics technology.According to the wavefront aberration characteristics of the segmented Fresnel lens,we choose wavefront sensorless adaptive optical system(WFSless AO),which has simple structure and low control bandwidth,to realize the wavefront aberration correction of SFL.The reasonable parameter selection of deformable mirror in adaptive optics correction is studied.According to the selected parameter configuration of deformable mirror,the residual common phase error and wavefront aberration are corrected in depth.The simulation results show that when the residual commom phase errors after phase correction is not greater than the wavefront aberration,the adaptive optics technology can effectively compensate the wavefront aberration to less than ?/10 RMS,so that the SFL can meet the imaging requirements.Finally,the wavefront aberration correction experiment of diffractive telescope is carried out.Based on the experimental system of single aperture diffractive telescope,the measurement of wavefront aberration of the telescope is studied at first.The measurement results confirm that the thickness error is the main factor that causes the wavefront aberration of Fresnel lens,and further clarify the specific parameters of WFSless AO system.Then,the correction experiments were carried out in a variety of imaging scenes in the single aperture diffraction telescope.In the point target imaging,the peak intensity of far-field focal spot was increased from 692 ADU to 916 ADU,and the intermediate frequency component of MTF was significantly improved.In the extended target imaging,the resolution of the image is increased nearly 4 times after correction.Finally,in order to verify the correction ability of WFSless AO system for the wavefront correction of SFL,the correction and contrast experiment of the wavefront aberration in the SFL under manual adjustment common phase and noncommon phase conditions was carried out.The experimental results show that the WFSless AO system can correct the wavefront aberration of the SFL under the manual adjustment common phase conditions,which verify the validity and feasibility of the WFSless AO system for the wavefront correction of the SFL.This dissertation focuses on the problem of image resolution degradation caused by the coupling of the common phase errors and wavefront aberrations of the segmented diffractive optical telescope,and puts forward the correction method based on the image index directly and the compound correction strategy of common phase errors and wavefront aberrations.The research results are expected to provide a new idea and method for high resolution imaging of large aperture space diffractive optical telescope.