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　　What is large aperture optical synthetic aperture imaging?There are two types of optical synthetic aperture system imaging methods:direct imaging and interference imaging[4-5].Direct imaging works the same way as single aperture imaging.

　　According to the Van Cittert-Zernike theorem(Van Cittert-Zernike theorem),interferometric imaging uses the interference pattern to measure the complex coherence spectrum of the target source,and then uses the inverse Fourier transform to obtain the size and intensity distribution of the light source to achieve indirect imaging of the target.

　　The principles of single aperture,optical synthetic aperture direct imaging and indirect interference imaging are illustrated respectively.Only one-dimensional graphics are drawn in the figure,and the two-dimensional case can be deduced from the one-dimensional analogy.The left column in the figure indicates

　　The frequency domain graph,the right column represents the air domain graph.⌒means Fourier transform,means convolution,and×means multiplication.From the schematic diagram of its principle,it can be seen that when imaging with a single aperture,different spatial frequency information of the object can be obtained at the same time,but it is limited by the cutoff frequency,and the information higher than the cutoff frequency is lost.

　　When the optical synthetic aperture is used for direct imaging,the information of different spatial frequencies of the object is also obtained at the same time,but due to the expansion of the aperture,the obtained spatial frequency is more than that of a single aperture,that is,its cut-off frequency is generally greater than that of a single aperture.

　　Indirect imaging needs to obtain images of different spatial frequencies by changing the distance between the apertures.Therefore,it is necessary to apply the Van Sitter-Zernick theorem to synthesize different spatial frequency images into one image.Indirect imaging cannot achieve real-time imaging.