Theory And Simulation Study On Imaging Characteristics Of Segmented Mirror

Large aperture optical telescopes play an irreplaceable role in astronomy because of their high angular resolution and strong light collecting ability.With the development of astronomy,the aperture of astronomical telescopes is getting bigger and bigger.Subject to the limitations of lens processing,transportation,support and other techniques,single primary mirror can not meet the needs of some astronomical detection projects.It is an effective method to improve the resolution of ground-based and space-based optical telescopes by combining several small aperture mirrors to form the primary mirror of large aperture telescopes.Compared with the traditional imaging system,the special structure of the segmented mirror determines its imaging characteristics and analysis methods are different.In the assembly process,co-focal and co-phase are the key factors to determine the imaging quality.However,whether the traditional imaging quality assessment method based on wave aberration is suitable for the segmented mirror system still needs further discussion.Therefore,it is necessary to study the imaging characteristics of the segmented mirror in order to provide theoretical guidance for the design and assembly process.In this field,the main work of this paper is as follows:1.The physical model of the segmented mirror is established.The imaging process of the segmented mirror is deduced theoretically based on the scalar diffraction theory,and the influence of the segment error on the imaging is analyzed.In this paper,the complex amplitude distribution on the observe surface is obtained by using Fourier transform on the generalized pupil function of the segmented mirror,and the effect of the segment error is reduced to the additional phase of the generalized pupil function.The corresponding relationship between different segment errors and additional phase is also given.As an extension of the existing imaging theory of segmented mirror,it is pointed out that the additional phase caused by the piston error should contain an extra defocusing term,and which will significantly affect the imaging quality when the piston error is large.2.Through the simulation calculation of the segmented mirror consists of two semicircle segment,the point spread function and the Strehl ratio with different Piston error,Tip-Tilt error and Decenter error are given,which show the basic imaging characteristics of the segmented mirror.It is found that the monotone descent characteristic of the envelope line of the Strehl ratio curve under the large piston error.In this paper,the possibility of coarse co-phase method of the segmented mirror by using this characteristic is proposed.3.The imaging quality evaluation method of segmented mirror is studied by using theoretical and simulate method.Taking hexagonal segmented mirror as the research object,the theoretical work of analyzing imaging quality based on wavefront phase error root-mean-square(RMS)is introduced,which gives the approximate expressions of Strehl ratio as a function of RMS when there are Piston errors or Tip-Tilt errors,respectively.Through numerical simulation of 10m-level segmented mirror,the point spread function and modulation transfer function under different segment errors are given to check the conclusion of theoretical analysis.The results show that there is a difference between the Strehl radio calculated by the traditional evaluation method based on wave aberration and Strehl radio obtained by simulation.