Optical Imaging System For Three Sub-Mirror Sparse Aperture With Free-Form Surface

In order to improve the imaging quality of space telescope,it needs large aperture system under the condition of diffraction limit.However,larger aperture means higher weight,higher cost,larger volume and other problems.People have been looking for new ways to overcome these difficulties,one alternative is the sparse aperture optical imaging system.Sparse aperture optical system is composed of space distribution and mutual interference multiple small apertures.It has the characteristics of small volume,light weight,easy processing,low cost and its resolution which is similar to single-caliber system.The primary mirror of the traditional sparse aperture optical system is a spherical surface,and it has many aberrations to reduce the imaging quality.The imaging quality can be improved by adding some correcting lenses.However,the weight and the volume of the system will increase with the correcting lenses.This paper puts forward to use free-form surface instead of correcting lens to reduce aberration,weight and improve imaging quality with its special geometric structure and optical imaging.This paper is based on traditional telescope imaging theory,analyzing the sparse aperture imaging theory,in order to get the three sub-aperture sparse aperture's modulation transfer function and equivalent aperture.We discuss the effects of different structures on two anti-system aberrations,and add correction mirrors and free-form surface respectively to balance residual aberration after confirming the initial structure,then we compare them.Zernike polynomial will be theoretically used to characterize free-form surface.And we analyze it's contribution to aberrations and evaluate the changes of astigmatism after applying the free-form surface to the optical system.According to the imaging principle,we simulate the imagine respectively,using both the system whose primary mirror is spherical and the system whose primary mirror is free-from surface,and we also use relative ambiguity to analyze and evaluate the imagine.Finally,we analyze the tolerance based on the three sub-mirror sparse aperture with free-form surface to get reasonable tolerance,which is used to subsequently process.