Imaging Performance Of Fractal Structure Sparse Aperture Array

The requirements for high-resolution images in space remote sensing are increasing.For subaperture(primary mirror)telescopes,higher spatial resolution require larger apertures.The angular resolution of the optical system is proportional to the ratio of the working wavelength to the entrance pupil diameter.The traditional method is to increase the spatial resolution by increasing the aperture of the optical system.However,the weight,volume and the difficulty of manufacturing and testing of the optical system will become unbearable with the rapid increase of the aperture.So the sparse aperture imaging technology emerged as the times require.The main research contents are as follows:1.Design a fractal sparse aperture array structureBased on the self-similarity of fractal theory,a sparse fractal aperture array configuration is proposed.The array is a kind of multi-layer fractal array configuration which takes Golay-3 as the fractal structure unit and expands in the way of self-similarity.Its structure is characterized by dimensionless reduction parameters,and the analytical expressions of pupil function and modulation transfer function are given.2.Study on imaging characteristics of fractal structure arrayNumerically calculate the modulation transfer function,actual cut-off frequency and the midfrequency frequency characteristics of the fractal structure under different fill factors and different outer rotation angles.Comparative analysis of the MTF and characteristic parameters of the array when the number of fractal layers is n = 1,n = 2,n = 3.The results show that when the filling factor is 9.52%(27)F ?22.46%,its change has little effect on the MTF curve.The rotation of outer layer is periodic,and the change of rotation angle has no great influence on the actual cut-off frequency.When the reduced aperture parameter and the fill factor is 22.46%,the mid-frequency characteristics of the n =18 array are more stable and the actual cut-off frequency is higher.Because of the reduced aperture parameters,the numerical results have scale invariance.When the number of sub-aperture is n =18,the MTF and characteristic parameters of fractal array structure and ring multiple-array configuration are compared.It can be concluded that the fractal array structure is better than the compound ring multiple-array configuration,which can effectively improve the performance of sparse aperture optical system.3.Direct imaging and image restorationThe image restoration method of sparse aperture system is discussed.This paper mainly introduces the restoration method of Wiener filter.Direct computer simulation imaging and image restoration of fractal structure in different fractal dimensions and layers.Calculating image quality evaluation parameters and objectively evaluate the overall image quality after array structure filtering.The experimental results verify the analysis results of MTF and characteristic index.It also shows that image restoration technology can effectively improve the imaging quality of sparse aperture system.Nevertheless,the information lost in zero value area can't be recovered.