Error Estimation due to approximations in Shack-Hartmann sensor based measurement of high slope freeform wavefront

摘要

This paper addresses the challenges and limitations involved in the measurement of steep freeform wavefront by usingShack-Hartmann Sensor (SHS). To estimate the slope errors, Zemax simulation tool is used to design a SHS setup includingarray of lenslets and detector plane with predefined specifications. In first step, error due to approximation of tilted planewavefront over curved wavefront is simulated. Plane, tilted, curved and tilted-curved wavefronts are defined usingappropriate ray source objects. The centroids of the focal spots of lenslets are calculated based on the detector data obtainedby using ray tracing method, which is done by an in-plane scanning aperture for segmented local wavefrontssequentially.The scanning aperture is used to block rays from more than one lenslet array. Centroids from the focus spotsare calculated and the slopes are estimated with respect to collimated reference wavefront for each ray trace process.Further, matrix of slope errors is used as an input for MATLAB routines for surface reconstruction and error estimation.Based on the simulation data, it is found that the assumption used in the Shack-Hartmann wavefront measurementintroduce residual errors. For example a 50 wave peak to valley input and 1.19 mm thick lenslet array can give up to 9waves of residual form error. However, very thin lenslets can have very less residual error.The effect of shift of focal plane,tilted plane wavefront and curve wavefront during the reconstruction using SHS is reported.