Null Test Of Off-axis Aspheres With Large-relative Aperture

Off-axis aspheric surface is popular for optical cameras with advantages of increasing the field of view and avoiding central obscuration.To meet the requirements of increasingly higher performance of space optical systems,the relative aperture and off-axis distance of off-axis aspheric surfaces are both increasing,and higher fabrication precision is required too.As a result,surface figure test with higher accuracy is required.To test an off-axis aspheric surface with a large relative aperture and large amount of off-axis distance,this thesis has designed a null comprising three pieces of lenses.The tolerance of the null test system is analyzed and monitored,and projection distortion is corrected for deterministic figuring.This thesis comprises three parts of contents:1.Firstly,the aspheric departure of aspheric surface with higher order terms is calculated.Then the primary parameters are determined on basis of third-order aberration theory and knowledge of geometric optics.After that,simulation is established by Zemax to obtain the optimal parameters with minimized wave aberration.The residual aberration meets the accuracy requirement.Finally,the performance of the null is analyzed such as ghost analysis and tolerance analysis of the fabricating parameters.Spherometer,wavefront interferometer,low coherence interferometer and laser alignment station are utilized for measurement of the radius of curvature,surface error,optical thickness and lens decenter,respectively.2.The sensitivity of misalignment to aberrations is calculated by ray tracing.In reverse the misalignment is estimated according to the measured aberrations and computer-aided alignment is realized.Computer-aided alignment technique is also analyzed under the condition of constrained off-axis distance.The misalignment under the off-axis distance constraint is obtaind by solving a constrained least-squares problem.During the practical test,the optical axis is aligned with the help of the paraxial interference fringes of the null optics,and the position of the test mirror is monitored by a meter bar.3.To deal with the projection distortion of the null test system,ray tracing is utilized to obtain the precise mapping between the CCD pixels and the test mirror coordinates.The projection distortion is corrected by curve fitting and subsequent coordinate transformation.The measurement result of the surface figure after distortion correction is used for deterministic figuring to assure convergence of the fabricating error to meet the design specification.