Research On Imaging Characteristics And Image Restoration Method Of Large Aperture Spliced Diffraction Optical System

Large aperture spliced membrane diffraction optical system is one of the most promising approach to achieve a large caliber and ultra light optical system for space application,which can image at high-resolution by splicing sub-apertures made of ultrathin membrane.For large aperture spliced diffraction optical system,however,affected by the coupling effects of splice and diffraction imaging,the degradation of imaging can be more complex.Compared with the traditional fold/reflection type system,modulation transfer function(MTF)and signal-to-noise ratio(SNR)used to be lower,and has a complex characteristics vary with spatial and spectral changes,and imaging quality may meet the demands of remote sensing application.However,there are few reports on the imaging characteristics and image restoration methods of large aperture spliced diffractive optical system.Aiming at this problem,this paper analyzes the imaging characteristics of the large-aperture spliced diffraction optical system,proposes the image quality restoration method,and carries out the experimental verification of relevant models and methods.The main research work is as follows:(1)The imaging quality degradation mechanism of the spliced diffraction optical system is studied.The imaging mechanism and characteristics of the spliced diffractive optical system are analyzed from the perspectives of the splicing process of the primary lens and the diffraction imaging process of the primary lens.The effects of splicing parameters and splicing errors on the imaging quality of the system are analyzed.Based on the multistage phase grating,the dispersion effect and spatial shift characteristics of the diffraction primary lens are modeled and analyzed.The effect of the gate linear density on the diffraction efficiency of the large aperture spliced diffractive optical system is analyzed.The effect of diffraction efficiency and filling factor on MTF and SNR of the Mosaic diffractive optical system is studied,and the analysis model of MTF and SNR is established.(2)The imaging characteristic analysis method of large aperture splicing diffraction optical system based on strict vector analysis is established.Large aperture splicing type diffraction optics primary mirror with subwavelength surface microstructure,aiming at the problem of scalar analysis methods are no longer applicable,based on the three dimensional finite difference time domain method,set up suitable for non-rotational symmetrical structure of the primary mirror diffraction efficiency analysis method,this paper build up the large diameter of splicing type diffraction optical system MTF and SNR analysis model,realize the large caliber splicing type diffraction optical system imaging characteristics analysis.Experiments were carried out to analyze the effects of main mirror characteristic parameters such as working wavelength,field of view,filling factor and grid linear density on imaging characteristics.The analysis results are consistent with the theoretical analysis of imaging quality degradation of the large aperture spliced diffraction optical system,which verifies the correctness of the imaging characteristic analysis method and provides a basis for the establishment of the image restoration method.(3)A method for image restoration of large aperture spliced diffraction optical system is proposed.The large aperture spliced diffraction optical system has a large degree of imaging space variation and serious image quality degradation.In view of the disadvantages of the traditional blind restoration method,which cannot accurately estimate the point diffusion function of all positions in the full field of view,a non-blind restoration method is proposed.According to the degradation mechanism of spatially varying images,an efficient interpolation estimation method for the diffusion function of the whole field of view is designed when the diffusion function of part of the field of view has been obtained by prior or local measurements.And then,a direct spatial change recovery model with multi-parameter joint constraints is proposed.Finally,the solution method of the model is proposed based on the multivariate split Bregman iteration method.This method not only improves the image clarity and keeps the texture detail,but also reduces the degree of space variation,and restrains the noise and artifacts.(4)The image quality restoration experiment of large aperture spliced diffractive optical system is verified and analyzed.The image quality restoration experiment was carried out for typical spliced diffraction optical system simulation images.Combined with the subjective and objective evaluation results of image quality before and after processing,quantitative analysis was carried out on the number and accuracy of known point diffusion functions and the influence of image degradation void variation(CLE)on the image quality restoration effect.The experimental results show that: for the spliced diffraction optical system analyzed in the simulation experiment of this paper,when the number of known point spread function(PSF)used to estimate the PSF in whole field is more than 11?11,and mean square error is less than-510,and the CLE of system is less than 3.5,the recovery method can keep the SSIM of the restored image higher than 0.9,and make a 50% decline in DOV,which means the method proposed in this article can effectively improve the image quality of large aperture spliced diffraction optical system.