| With the deepening exploration of space by human,there is an urgent need for large-aperture or even super large aperture space optical telescopes with higher resolution and sensitivity.The 2.4m Hubble telescope,which have been put into use,and the 6.5m James Webb space telescope that is in research,in the future,optical system will have a aperture of 100 m or even larger.However,limited by such factors as mirror material,processing technology,spacecraft carrying capacity,launch volume and cost,the traditional design concept of space telescope with single aperture as the main body can hardly support the requirement of increasing aperture of space telescope.The optical system concept based on segmented-mirror primary mirror optic system provides a kind of structure that can surpass the limitation of spacecraft carrying capacity.The solution of the large space optical telescope based on segmented-mirrors with strong observing ability has become the research consensus of all countries in the world.In order to reduce the size of the segmented-mirror primary mirror optical system,it is necessary to fold the primary mirror before launched,and then expand the folded segmented-mirror into a complete primary mirror in orbit.Because the position error,all the deployed segmented-mirrors can't maintain the common phase,which affects the imaging quality of the optical system and can't be imaged in serious cases.The detection of position errors usually includes segmented-image point location search,alignment,stacking,coarse phase process and fine phase process.The fine phase detection process needs to adjust the piston error to nanometer level,which is the key to ensure the imaging equlity of the segmented-mirror primary mirror optical system.This paper used image point interferometry to detect piston error between segmentedmirrors based on interferometry technology.Compared with other technologies,image point interferometry in interferometric detection technology does not use optical elements such as quadric-pyramid and grating,and its optical path and algorithm are relatively simple.It can use a wide-band light source to detect the displacement errors and has high detection accuracy.In this paper,the detection and correction of the piston error of the segmentedmirror primary mirror after the expansion of the segmented-mirror is taken as the research object,and the main work and innovations are as follows:(1)Various detection methods for segmented-mirror piston errors at home and abroad are studied and analyzed.The principles of hartmann-shack detection,quadricpyramid detection,dispersion fringe detection and interferometric detection are studied.The detection range and accuracy of various detection methods are compared.The successful application of interferometric technology to detect displacement errors at home and abroad is introduced in detail.The optical path,detection range and accuracy of interferometric technology are compared with other detection technologies here.(2)A segmented-mirror primary mirror optical system with all aberrations is established.The influence of piston error on imaging quality of optical system is analyzed.A theoretical model of the influence of position error on the transfer function and diffusion function based on aberration theory is constructed.The piston error along the Z axis has the greatest impact,when the segmented-mirror moves 50 nm along Z axis,the wave-front aberration caused by piston error is 0.0542?(when ?=632.8nm).(3)A fine phase determination process based on image point interferometry is proposed to detect the displacement errors between segmented-mirror and complete the piston errors detection.The principle of measuring the piston error between segmentedmirror by image point interferometry is analyzed theoretically.The relationships between the position change and brightness change information of interference fringes and piston error are studied.When the piston errors of segmented-mirror are 50 and 300 nm,the positon change of bright fringes are 1 and 6 microns,respectively;when the piston errors are 10 and 30 nm,the magnitude of bright fringes are 1.48 and 3.195,respectively.The formulas for calculating the distance between fringes and the displacement errors of segmented-mirror are given.(4)Research the fitting method of curves between stripe position change and piston error.The piston error is solved by the fitting formula of the relationship curve between fringe position,brightness and piston error and the accuracy of the fitting formula for calculating the piston error of segmented mirror is compared.Based on the change of fringe position and brightness,the piston error between segmented-mirrors is used as piston error detection in this paper.The piston error can be detected from half wavelength to nanometer level,and the adjustment of the fine phase process after segmented-mirror deployment is completed.The influence of segmented-mirror processing error on interference detection results is studied.By setting the curvature radius of the segmented-mirror and the tolerance of the quadratic coefficients,the fringe information obtained from the analysis shows that the radius error between the two segmented-mirrors should be controlled within 1%,and the quadratic coefficients error should be controlled within 5‰,which will not have a significant impact on the test results.(5)Finally,the wave-front aberration caused by the piston error is 0.0204?.After repeated experiments,the average values of all PV and RMS are 0.2351? and 0.0346?,respectively,which meet the requirements of optical system with Strel ratio greater than 0.95 and wave aberration less than 0.02? caused by segmented-mirror piston error.It proves the feasibility of measuring piston error by image point interferometry. |
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