Research On Curvature Mismatch Detection Of Large Aperture Segmented Space Telescopes

The angular resolution and light-collecting ability of astronomical telescopes are positively correlated with the aperture of the primary mirror(PM),therefore,the most direct way to improve the performance of the telescope is to increase the PM’s aperture.The development of the traditional large-aperture monolithic PM telescope is close to the limit of today’s scientific and technological level,and the large-aperture segmented PM telescope has gradually become the mainstream of development.However,the segmented PM telescope needs to achieve high-precision alignment and phasing among the PM segments to achieve the diffraction limit imaging performance,and the radius of curvature(ROC)mismatch among the PM segments is one of the key problems.Each individually manufactured PM segment will have different ROC errors due to the influence of processing errors,the external environment,and other factors.When multiple PM segments are put together,the problem of ROC mismatch will occur.To make the segmented telescope achieve the diffraction limit imaging performance,it is usually necessary to control the ROC mismatch to micron order,but the current processing accuracy cannot meet this requirement,and also impossible to compensate for the influence of ROC mismatch by conventional means.Using active optical technology to control the ROC of each PM segment is a feasible solution to this problem.How to accurately detect the amount of adjustment is the key to active optical technology.At present,the ROC mismatch detection methods which have been applied in practical engineering abroad have the problems of few public data and unknown technical details.The other methods proposed by researchers from home and abroad have the problems of complex detection devices and processes which are not suitable for space telescopes’ on-orbit detection.The ROC mismatch detection itself also has the problem of being difficult to detect.Based on the project "Research on key technologies of super large aperture on-orbit assembly space telescope",this thesis focuses on "ROC mismatch detection of large aperture segment space telescope".The main contents are as follows:1.The ROC mismatch detection has the problem of there being few public data and unknown technical details abroad;the research started late and great limitations of the existing technology in our country;it is difficult to compensate by conventional methods and difficult to detect.In this thesis,first,the influence of the PM segment’s ROC error on wavefront aberrations is analyzed by the off-axis nodal aberration theory(NAT).Then a method suitable for a segmented space telescope,based on the phase retrieval algorithm,detected ROC mismatch according to sub-aperture defocus aberration is proposed.The simulation results show that the above method can effectively detect ROC mismatch;2.The aberrations induced by the secondary mirror(SM)lateral misalignments and ROC mismatch have coupling relationships.Therefore,the SM lateral misalignments will affect ROC mismatch detection.In this thesis,first,the influence of SM lateral misalignments on sub-aperture and full-aperture aberrations is analyzed by NAT.Then a joint calculation based on multiple sub-aperture aberrations is proposed to decouple the aberrations induced by SM lateral misalignments and ROC mismatch.The simulation results show that the above method can effectively reduce the influence of SM lateral misalignments on ROC mismatch detection.3.The wavefront detection error induced by image alignment errors will affect the ROC mismatch detection.First,this thesis analyzes the specific forms of wavefront detection errors induced by image alignment errors.Then a method that is suitable for segmented space telescopes and can compensate for the wavefront detection error induced by image alignment error in the iterative process of phase retrieval algorithm is proposed.The simulation results show that the above method can effectively reduce the influence of image alignment error on ROC mismatch detection.4.In this thesis,two sets of experimental schemes for detecting ROC mismatch are designed.First,the effects of the CCD or CMOS camera’s quantization and exposure state on wavefront sensing accuracy are simulated and analyzed.Then the ROC mismatch detection experiments are carried out on the segmented mirror simulated by single spherical mirror and the segmented mirror consisted by double spherical mirrors respectively.The ROC mismatch detection results of the segmented mirror simulated by single spherical mirror show that there is no difference in the ROC between the simulated sub-mirrors,and the ROC mismatch detection results of the segmented mirror consisted by double spherical mirrors can be verified with the simulation data.The above experiments prove the effectiveness and practical application value of the method for detecting the ROC mismatch among PM segments proposed in this thesis.The research of this thesis can lay a certain technical foundation for the development of active optical technology of super-large aperture space telescopes in China in the future.