Research On Optical Synthetic Aperture System With The Maximum Practical Cutoff Frequency

Optical Synthetic Aperture(OSA)system was one of the most promising techniques for high-resolution astronomical observation and high-resolution earth observation from GEO orbit.Muti-apertures in OSA were optimized to arrange in a certain style,and corresponding sub-apertures were co-phased,and the synthetic apertures were interfered to form a high-resolution image,which is approximative to the image quality of one singular optical aperture.To solve the current technical problems in optical synthetic aperture(OSA)system,the optimization of two-dimensional sub-aperture arrays in OSA system,the theoretical model of phase error and merit function of OSA system,imaging performance analysis of OSA complicated pupils and chromatic phase error,imaging co-phasing algorithm and relative test verification had been studied in this dissertation.Firstly,in the optimization of two-dimensional sub-aperture arrays in OSA system,an unequal improved multiple circular arrays(IMCAs)are put forward.When fill factor of the arrays was given,by using genetic algorithms with fitness function to maximize the practical cutoff frequency of MTF,the related parameters,such as the diameters of sub-apertures on different concentric circles,the radii and number of the concentric circles,and the rotation angles between two sub-aperture sets on different concentric circles of the two-dimensional OSA arrays are optimized.Compared to Golay arrays and multiple circular arrays,the uneaqual IMCAs had the maximum practical cutoff frequency of OSA arrays,smoother coverage in Fourier space,and better mid-frequency contrast than multiple circular arrays.The imaging quality of IMCAs can approximate a single annular aperture.The work can provide a feasible technology roadmap for layout optimization of high-resolution two-dimensional OSA arrays.Secondly,in phase error theory and merit functions for OSA system,an universal phase error analysis model was created,which was suitable for arbitrary OSA pupil structure types.Based on Fourier Optics,the mathematical analytic expressions of phase errors of subapertures between systematic MTF,PSF and SR were derived in the monochromatic incident light.The expressions were independent with the orders of phase error,the shape,size and lay-out of sub-apertures.The difference and relationship between the power series expressions and Zernike polynomials expressions of the phase error were investigated,which was prepared for subsequent phase errors sensitivity analysis.Thirdly,with different central obscuration ratios and different shapes of strut in sub-apertures of the different OSA systems,the SR was influenced by the lower order phase error such as piston,tilt and defocus,and the third order phase error.Different sensitivity between different phase error and SR was listed.In particular,piston is the most sensitive phase error,defocus was more sensitive than tilt error in IMCA-6-Cd,and coma was most variable phase error for OSA arrays with central obscuration.The lists can give the directions for the cophasing technology for OSA system.The chromatic incident wavelengths was coarsely sampled,the functions between chromatic bi-aperture SR and different order phase error were smoother than monochromatic situations.When the piston error is basically sampled in the range of coherent length of incient chromatic light,and chromatic MTF and PSF of bi-aperture system were simulated and compared to monochromatic situations.Again,based on piston error detection using MTF secondary peak height,the mathematic expression between them was improved by incident wide-band spectrum distribution,and the method which was based on MTF secondary peak height with different bandwidth and spectral distribution light source was advanced to detect piston in long range(in tens and hundreds wavelengths)and with high accuracy.In the bi-aperture system under ideal conditions,different band-width LED with real spectral distribution was selected in semi-physical simulation.Considerable sampled wavelengths(more than 100 sampled wavelengths)was used in the simulations by MATLAB,which showed that the bandwidth of light source(coherent length)and detection range of the piston error was highly correlated.Besides,the theory,methods and limitations of image-based piston error detection based on MTF secondary peak height for complicated pupil structure was investigated.Finally,experimental verification for the performance of IMCAs and imaging cophasing algorithm for OSA were investigated.Common secondary mirror experimental method and 4 different color light source was adopted,the two-dimensional PSFs of bi-aperture arrays and IMCAs with different fill factors were obtained,and the experimental results agreed well with simulation results from MATLAB and ZEMAX.Two identical liquid crystal retarder was chosen to produce the specified piston error as the reference signal,the principle verification experiment for the relationship between the pison multiwave errors and the MTF secondary peak height was carried out.The results show that they were in an one-to-one correspondence.And the mathematic relationship between them should be customized by the special system.