| While large-aperture off-axis reflecting space telescopes have many outstanding advantages in astronomical observation,the increase of the aperture of the primary mirror(PM)makes the image quality of off-axis reflecting space telescopes more vulnerable to the influence of perturbations: on the one hand,the increase in aperture size makes each mirror more vulnerable to the influence of vibration,gravity and temperature changes,and the image quality of the system is more sensitive to mirror misalignments;on the other hand,the large-aperture PM is susceptible to the change of the radius of curvature(ROC)due to the fluctuation of the external heat flow.Mirror misalignments(including various pose misalignments and ROC errors)will introduce wavefront aberration,resulting in image quality degradation.Therefore,it is necessary to correct the system in-orbit(using active optics technology)to restore and maintain the system image quality.To reduce the engineering cost of in-orbit application,aberration compensation is usually used to correct the image quality of large-aperture reflecting space telescopes in-orbit.Among them,it is necessary to select the appropriate adjustable degrees of freedom(DOFs)for aberration compensation and put forward a reasonable compensation amount solution method.Under the framework of nodal aberration theory,this paper studies the aberration coupling characteristics between different types of misalignments of off-axis reflecting telescopes systems,and further reveals its aberration compensation relationships,so as to provide a theoretical basis for the selection of compensation DOFs.Then,based on the research of aberration compensation relationships,a method suitable for in-orbit application is proposed.In this paper,the above theories and algorithms are applied to the analysis of aberration compensation characteristics and the calculation of compensation amount of off-axis reflecting space telescopess,and the corresponding research conclusions are effectively verified.Specifically,the main research work of this paper includes the following contents:Aberration coupling characteristics between different types of misalignments of off-axis reflecting telescopes are investigated first.The axial misalignments of off-axis reflecting telescopes can produce field-constant 0° astigmatism and 90° coma,while some lateral misalignments can also produce 0° astigmatism and 90° coma.There is a coupling relationship between them.The coupling intensity is mainly related to pupil eccentricity vector,the sensitivity of spherical aberration coefficient to axial misalignments and coma coefficient.Taking New Solar Telescope(NST)as an example,the astigmatism coupling ratio is about 1:2,the coma coupling ratio is about 1:1;when the two kinds of misalignments satisfy specific mathematical relationships,astigmatism nodes or coma nodes will appear in the center of the effective field of view.The mechanism of aberrations induced by ROC error is similar to that induced by axial misalignments.Both of them change the aberration field distribution by affecting the wave aberration coefficients of the optical system,resulting in field-constant 0°astigmatism and 90° coma.The coupling intensity of aberration caused by them is mainly related to the sensitivity of spherical aberration coefficient to the two kinds of errors.Taking NST as an example,the astigmatism coupling ratio between these two kinds of errors is about 1:2 and the coma coupling ratio between these two kinds of errors is about 1:2.The aberration compensation relationships between different misalignments of off-axis reflecting telescopes are then studied.The lateral misalignments of off-axis reflecting telescopes can compensate the field-constant 0° astigmatism or 90° coma caused by axial misalignments.Due to the difference between coupling ratios of astigmatism and coma,the adjustment amount for coma compensation is about twice of the adjustment amount required to compensate the astigmatism field.After compensation,there is some residual aberrations in the system,and certain amount of defocus aberration needs to be compensated by adjusting the focusing mirror.Taking600 mm-aperture off-axis three-mirror anastigmat(TMA)telescope as an example,when the axial misalignments is within ±0.1 mm,the residual wavefront error after compensation is better than 0.02 ?;axial misalignments can hardly introduce the fieldconstant 45° astigmatism and 0 ° coma,so it can not compensate the aberration caused by some transverse misalignments(decenter along the X direction or rotation around the Y axis).In the presence of ROC error for off-axis reflecting telescopes,in the ground installation and adjustment stage,the effect of ROC error can be compensated by adjusting the axial pose of PM,and the required adjustment amount is half of the ROC error.When the ROC error of PM is within ±1 mm,the residual wavefront error after compensation is better than 0.003 ?;In the in-orbit adjustment stage,the compensation is better to be carried out by adjusting the axial position and attitude of the secondary mirror(SM),and the adjustment amount of the SM is optimally determined according to the wave aberration of the full field of view.Among them,the compensation range is reduced due to the increase of the difference in aberration coupling ratio.When the ROC error of the PM is within ±0.2 mm,the residual wavefront error after compensation is better than 0.005 ??A calculation method for compensation amount based on star spot image feature matching at arbitrary multiple fields of view(FOVs)is further proposed.Based on nodal aberration theory and Fourier optics theory,the mapping relationship between the adjustment amounts required to compensate the wave aberration of the system and the gray image features of star spots at arbitrary FOV is established.Taking the minimum spatial distance between the actual image feature vector and the modeled image feature vector of arbitrary FOVs as the objective function,calculation of compensation amount is transformed into an optimization problem.Taking 600 mm-aperture off-axis TMA telescope as an example,considering noise(30 d B peak signal-to-noise)and 1%actuator error,the residual wavefront error of the system after compensation is better than 0.02 ?.This method has a comparatively small amount of calculation,has no restriction on the position of the field of view where the spot image is located,and therefore it is suitable for calculating the compensation amount in-orbit.This paper has certain guiding significance and reference value for aberration field characteristic analysis and aberration field correction of large-aperture off-axis reflecting space telescopes. |
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