The Research On Phase Retrieval In Space-borne Camera Based On Phase Diversity

It is important to maintain the high quality of space-borne cameras in orbit,because the environments of space are very risky and the image quality may bedegraded because of force and heat reasons. Traditional thermal and misfocus controltechnology are very complex with large source reqiremnts and limited effect.In order to improve the image quality of space camera in orbit, a phase retrievalmethod based on phase diversity (PD) is proposed to be applied to space opticalsystem in this dissertation. The wavefront aberration of optical system can be solvedby the images gained by space cameras, and the degraded function of space cameracan be reconstructed. At the same time, the misfocus can be tested and the imsfocus ofspace camera can be adjusted. There are mainly four contents in this dissertation:1. The improvement of merit function of traditional PD method, and thesimulation of solving wavefront aberration of optical system by PD method.On the basis of traditional theory of PD method, the Tikhonov regularizationfunction is increased, and the noise adaptability of PD method is improved. Thegenetic algorithm is selected, and solving wavefront aberration of optical system byPD method is simulated. The result shows that when the RMS of wavefront aberrationare0.1671λ,0.2931λ and0.3304λ, the presicion of calculation are all less than λ/100(RMS). 2. The adaptability analysis of traditional PD method, and the image restrorationbased on the calculation result by PD method.The relationships between the calculation precision and wavefront aberration ofsystem, wavefront defocus are analyzed. Also, the noise adaptability of PD method isanalyzed, and the effect of the defocus error and the contrast of target on thecalculation residual error of PD method are also analyzed. Furthermore, the imagesare restored according to the degraded function reconstructed by the calculation resultof PD method, and the structure similarity is introduced to evaluate the similarity ofrestored images and initial images.3. According to the wavelength sampling and monochromatic light assumption,expanding the traditional PD method to broadband illumination condition.In order to expand the traditional PD method to broadband illumination condition,the traditional theory of PD method is improved by wavelength sampling method. Thestrategy of wavelength sampling is analyzed. Also, the calculation precision and thesimilarity of restored image and initial image are analyzed when the wavelengthsamplings are different. Furthermore, when the width of spectrum is small, themonochromatic light assumption is proposed. The calculation precision and thesimilarity of restored image and initial image are simulated when the widths ofspectrum are different.4. The experimental validation of traditional PD method and PD broadbandillumination.Based on the former simulation and analysis, the following three experiments arecarried out: Firstly, the wavefront error of an off-axis three-mirror anastigmaticsystem is tested by traditional PD method. Compared to the interferometric testingresult, the relative deviation of PD result is only0.37%, and the quality of imagerestored by calculation result of PD method is obviously improved; Secondly, the PDwith broadband illumination is verified by digital camera. When different resolutionboards are selected, the results solved by PD method have good repeatability, and thewavefront aberration is about0.2λ (RMS); At last, the PD with broadbandillumination is verified by outfield imaging experiment of optical system. The wavefront aberration of optical system solved by PD method is about λ/13(RMS), theideal position of detector calculated by PD and the detector position confirmed inexperiment are basically the same.In this dissertation the phase retrieval in space-borne camera based on phasediversity is presented, including the improvement of merit function in traditional PDmethod, the adaptability analysis of traditional PD method, expanding the PD methodto broadband illumination, the simulation of PD method when the wavelengthsampling and the monochromatic light assumption is introduced respectively.Furthermore, the traditional PD method is verified by the wavefront testing of TMAoptical system, and the PD with broadband illumination are verified by the digitalcamera imaging experiment and the outfield imaging experiment of space camera. Itcan be concluded that it is feasible for PD method to solve the wavefront error ofoptical system when there is monochromatic light illumination or broadband spectrumillumination.