Research On Ground Measurement Method Of PSF Ellipticity Of Space Sky Survey Telescope

Weak Gravitational Lensing(WL)is a good probe to study dark matter and dark energy in the universe so that it becomes one of the most powerful tools for cosmologists.WL is a slight deflection of the light from background galaxies through a gravitational field formed by dark matter and ordinary matte,it could be detected by the statistics of the galaxy ellipticities.However,galaxies ellipticities caused by WL are much smaller than the intrinsic ellipticity of galaxy itself,and even smaller than the Point Spread Function(PSF)ellipticity due to nonideal imaging of the telescope.PSF ellipticity change in the short term could easily cover the galaxy ellipticity which is used to detect dark matter and dark energy.In order to accurately measure the galaxy ellipticity caused by WL,the telescope should have superior optical properties.PSF ellipticity is one of optical properties of the telescope,it depends on many factors,such as optical design,manufacturing errors,optical alignment,and so on.Hence,there may be big differences between the actual performances of PSF ellipticity of telescopes and those in design status.To ensure the used telescopes to be competent in WL detections,it is very necessary to measure PSF ellipticities of telescopes on the ground.China Space Station Telescope(CSST)of China National Space Administration is an unobscured off-axis telescope with a 2 m aperture,28 m focal length,and 1.1×1°fields of view(FOVs).Over full FOVs,PSF ellipticities are not greater than 0.15,the average value is less than 0.05,and measurement errors of PSF ellipticity is less than0.01.When evaluating the imaging quality of the telescope,besides wavefront error measurement,it is also necessary to take the PSF ellipticity of the astronomical FOVs as the evaluation index.This topic takes the sky survey observation of space telescope as the background,according to the accuracy requirements of PSF ellipticity measurement of the telescope,develops the study on ground measurement method and key technologies of PSF ellipticity of an off-axis telescope used to detect WL.A way based on simulated star target imaging is proposed.Related errors are identified and modeled carefully.On this basis,performances of relevant device parameters in the actual measurement are analyzed,and the consistency between the simulation results of PSF ellipticity measurement and the experimental conclusions of that is verified.Specifically,this paper mainly includes the following contents:1.According to the characteristics of an unobstructed off-axis telescope,the accuracy of PSF ellipticity calculation based on sampling parameter setting is studied.It is obtained that the PSF image surface sampling space and pupil sampling space are important parameters affecting the accuracy of PSF ellipticity calculation.According to the accuracy requirements of PSF ellipticity calculation,through single and multivariable parameters control methods,the interpolation accuracy and the relative calculation accuracy of PSF ellipticity calculated by different PSF sampling parameter settings are obtained.The optimal parameter setting to meet the accuracy requirements of PSF ellipticity calculation of the telescope detecting WL is completed,and the highprecision PSF ellipticity calculation is realized.This setting is applied to the relevant characteristic analysis of PSF ellipticity,so that the values are comparable and reliable.2.According to the theoretical basis of PSF measurements,the PSF ellipticity measurement methods are studied.According to the measurement error requirements of PSF ellipticity of the telescope,the measurement accuracy of PSF ellipticity by different methods is analyzed through experiment and simulation.On this basis,according to the measurement principle of point source method,a way based on simulated star target imaging to measure PSF ellipticity is proposed,and the experimental simulation optical path is built.The differences between PSF measurements and PSF ellipticity measurements are analyzed and compared,and the quantitative results of different error sources affecting the accuracy of traditional PSF measurements and PSF ellipticity measurements are obtained.3.Analyze the error sources existing in the lab when PSF ellipticity of the telescope is measured,such as defocus error,detector noise,micro-vibration of optical platforms,bandwidth of filter,defocusing of the collimator,WFE of the collimator,central obscuration of the collimator etc.We analyze the influence of these errors on the accuracy of the PSF ellipticity measurement,and combine with the measurement scheme of PSF ellipticity and requirements of error distribution,according to the mathematical model of error,methods to suppress the influence of partial error and the conditions to restrict partial error are proposed.The influence of comprehensive error sources on PSF detection accuracy was analyzed by simulation.Based on the simulation results,the verification scheme of PSF ellipticity measurement method using collimator and telescope as the whole optical system to be measured on the basis of simulated star target imaging is adopted.4.The experimental platform of PSF ellipticity detection is designed and built to verify the feasibility and accuracy of PSF ellipticity measurement method based on star target imaging.The method of multi-degree of freedom misalignment of telescope secondary mirror is proposed to obtain the PSF ellipticity of different optical systems to be measured,and a large number of datas are used to ensure the authenticity and correctness of the PSF ellipticity measurement results.On this basis,a simulation analysis of the PSF ellipticity measurement experimental optical path is carried out,and then the feasibility of the measurement method,the accuracy of error calibration and related algorithms,and the correctness of the error suppression of error model are verified through experiments.