| With the rapid development of muti-disciplines,the ground-based large aperture telescope as an astronomical observation equipment has made great progress.In the last few decades,the ground-based large aperture telescope is developing towards the trends of long focal,large aperture,thin mirror,and splicing.Under the influence of gravity and temperature,the lower order error of the optical system is obviously increased.Traditionally,the effect on the system is mainly eliminated by the support system,which is not efficient and convenient enough to correct the low spatial frequency error and it is difficult to design and process a set of low order error correction system for large ground-based telescope.In this paper,based on the tertiary mirror of the international cooperation project the Thirty Meter Telescope(TMT),a semi-active optical system is developed,which can be used to correct the low order surface error.The TMT tertiary mirror is an oval solid mirror with a size of 3.594 x 2.536 x 0.1m(long axis by short axis by thickness).Different from the traditional tertiary mirror,the TMT tertiary mirror system has the functions of two dimensional high precision pointing and tracking.Therefore,the mirror surface accuracy of TMT tertiary mirror must be strictly guaranteed in a variety of gravity conditions.The TMT tertiary mirror surface accuracy can be divided into high frequency surface error and low frequency surface error.In this paper,based on the principle of moment correction,a semi-active torque correction system called Warping Harness is designed.The following research work has been completed:(1)With a summary of the developing history in the domestic and foreign large telescope,the support methods for large aperture optical components were described in detail.A method of support between passive support and active optics which called semi-active optical support was proposed.(2)The relevant theory of the large aperture telescope primary mirror support system,including multi-point support theory for supporting point optimization,surface evaluation standard,the aberration theory and flexible hinge theory,these introductions laid the theoretical foundation for the design and optimization of semi-active optical support system.(3)According to the requirement of the low order mirror surface error of the TMT tertiary mirror,and by means of a 3D modeling software Solidworks,the preliminary structure design of the Warping Harness for the surface correction was completed.With the help of workbench,Hypermesh,ANSYS and other softwares,the simulation model of the Warping Harness calibration system was established,and some parameters of the structure were optimized.The two methods of surface fitting were compared: the fitting of free vibration and the fitting of Zernike polynomial.(4)With the optomechanical integrated analysis software Sigfit adaptive module,the best fitting correction force combination was solved by automation process,with RMS value of the surface error as optimal goal and the correction force as optimization variable.By using the fitting surface method with Zernike polynomial,the results before and after correction was compared,which showed that RMS value of the surface error was decreased from 64.9nm to 25.4nm.The correction rate of low order defocus and astigmatism reached 99.8%.The correction of the mirror surface error could meet the requirements of low order aberration of TMT tertiary mirror.Besides,the mirror surface error in six special conditions were corrected,and the correction rate is up to 68%.(5)Through the theoretical calculation of the actuator force of the Warping Harness,the design requirements of the corresponding displacement output stroke were obtained,and the mechanical and electrical testing system with high precision displacement-force-strain was built.Combined with the analysis and calculation results,the test of Warping Harness correction component of displacement,stress,strain output precision,linearity and repeatability were completed.At last,the transmission system error of Warping Harness was analyzed.This paper studied the theory of Warping Harness correction method based on semi-active optics.The analysis results showed that the mirror surface accuracy was improved significantly after the correction.The correction of low order aberrations like defocus and astigmatism could reach high accuracy.The research results play a role in promoting the construction of TMT tertiary mirror support structure and have important significance and reference value in the process of the large aperture telescope manufacture. |
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