Research On The Optical Testing And Co-phasing Technology For Large Aperture Segmented Mirror Systems Based On Computer-Generated Holograms

With the increasing demands for the detection capability,the sizes and weights of optical systems raise dramatically,rising up the risk and cost of mirror fabrication.Moreover,space telescopes face the constraints caused by the fairing size and carrying capacity of rockets.The segmented-mirror technology,as an effective method to solve the problem of manufacturing and launching large-aperture mirrors,has become a trend to realize large-aperture optical systems.For the large aperture segmented mirror systems,high-precision optical testing for mirror surfaces and high-precision & large-capture-range inspection for co-phase errors are the key problems:High-precision detection of surfaces errors.Large-aperture systems have strict requirements on the accuracy of mirror surfaces.Usually,the surface errors of the whole mirror and its sub regions should be less than ?/40 ~ ?/50 RMS.Thus high-precision optical testing technology is needed.Besides,optical testing for segmented mirror systems is involved by multiple off-axis aspheric surfaces of sub-mirrors,increasing the difficulty to achieve high-precision detection.High-precision inspection of co-phase errors.All the sub-mirrors of the segmented mirror system must be strictly co-phased.Relative misplacement among sub-mirrors breaks the continuity of the whole mirror surface,introduces co-phase errors,and hence seriously degrades the overall imaging quality of the system.Among the co-phase errors,the piston error suffers from the 2? phase ambiguity,hence the corresponding inspection technology or combined technologies should possess high precision and large capture range simultaneously.Besides,owing to the low tolerance,tip&tilt error must be detected highly accurately.In view of the development direction of 8-10 m large aperture optical telescopes for our country,it is of great theoretical and practical significance to study the key technologies,i.e.,surface errors detection and co-phase errors inspection for large aperture segmented mirror systems.Considering the inherent advantages of computer-generated holograms(CGHs)to inspect off-axis aspherics,this thesis is dedicated to study on the CGH-based optical testing and co-phasing technology for large-aperture segmented mirror systems.This thesis not only provides guidance for engineering practice,but also expands the application scope and improves the design efficiency of CGHs.The following researches in the text are carried out:1.By comparing the imaging properties of the three kinds of large-aperture optical systems,i.e.,the segmented mirror,sparse aperture,and membrane optics,it is confirmed that the optical testing of mirror surfaces and the wavefront sensing for co-phase errors are the key difficulties to realize large aperture optical systems.By focusing on the segmented mirror systems,the research on the application of CGH in optical testing and co-phasing is put forward.2.A deep investigation and comparison on the existing high-precision optical testing and co-phasing technologies are made.The specificities of optical testing methods are analyzed,clarifying the advantages of CGHs in the application of segmented mirror testing.The principles of detecting co-phase errors,the capture ranges,accuracy and the main limiting factors of co-phasing sensors are introduced,navigating the possible realization forms of CGH-based co-phasing technologies.3.The influences of surface errors and co-phase errors on imaging property are analyzed.A segmented mirror model is constructed,based on which the analytical formulas for the wave aberrations introduced by the 6-DOF(Degrees of Freedom,DOF)rigid body motion of sub-mirrors are obtained.The essence of and the relationship between the wave aberrations induced by each DOF motion are discussed.A 4 m coaxial three mirror Korsch telescope with segmented primary mirror is designed,and used as an example to show the effects of sub-mirror errors.The results match the segmented mirror model well.4.An optimal design method for CGHs based on the parametric model is proposed.Considering the ranges of the conic constant K and the f-number F/# of the aspheric mirror in common optical systems,a paraxial parameter model applicable to weak aspherics with-0.5 ? K<0 and F/# ? 2 is described firstly,and then an improved non-paraxial model is introduced and expands the application range to-4 ? K<0 and F/# ? 1.5.Analysis of the distribution of the CGH's parasitic diffraction orders is done,pointing out that the characteristic stray orders are(2,0)and(1,0).Finally,the detailed operating process of the optimal design method is provided,which can achieve optimization in four aspects: removing the stray rays,reducing the fringe density,limiting the mapping distortion and restricting the size of the CGH.5.An optimization design method for CGHs based on Matlab-Zemax interaction is put forward,and the corresponding program is written.It overcomes the inability of Zemax alone to design feasible CGHs for the mirrors with large asphericity or free-form surfaces,hence greatly improves the design efficiency.The program can realize global optimization for the inclination angle,carrier frequency and position of the desired CGH.6.CGHs used for surface testing of the 4 m Korsch segmented mirror system are designed.The primary and third mirror are tested with single CGHs,respectively,while the second mirror is inspected with a hybrid method composed of CGH and aberration-free point with subaperture stitching interferometry.A comparison between the CGH and Offner compensator is made,indicating the advantages of CGHs in optical testing for segmented mirror systems,i.e.,effectively utilizing interferometer's aperture,easy to align,and possessing small design error.7.Researches on the CGH-based co-phasing methods are done.Three ways are explored successively: the integration inspection to the surface errors and co-phase errors based on a dual-wavelength CGH,the coarse phasing technology with CGHbased dispersed fringe sensor,denoted as CGH-DFS for convenience,and the phasing method with CGH-based Shack Hartmann phasing sensor(CGH-SHAPS).CGH-SHAPS adopts a single CGH to achieve the dual function of the pupil mask and imaging lens array in the traditional SHAPS,and to realize simpler structure and easier alignment.Simulation test is done,and results show that CGH-SHAPS can detect piston and tip&tilt errors with high precision,and hence meets the requirements of co-phasing for large aperture segmented mirror systems.