Aberration compensation and position scanning of a subaperture stitching algorithm

An aspheric testing system based on subaperture stitching interferometry has been developed. A procedure involving subaperture aberration compensation and radial position scanning was established to resolve discrepancies in the overlapped regions. During the aspheric measuring process, the Fizeau-interferometer axis, the optical axis of the asphere, and the mechanical rotation axis have to be aligned. Due to the tolerance of alignment mechanisms, subaperture interferograms would be contaminated by various amounts of aberrations associated with the rotation angle. These aberrations introduce large inconsistencies between adjacent subapertures in the stitching algorithm. Zernike coefficients of the subapertures in one annulus were examined and each coefficient term was found to be a sinusoidal function of the rotation angle. To eliminate the influence of misalignments, each subaperture was compensated with appropriate amounts of coma and astigmatism to make the resulting Zernike coefficients converge to the mean values of the sinusoidal functions. In addition, the determination of the overlapped regions relies on the precise estimate of the distance between the center of each subaperture and the center of the aspheric optics. This distance was first provided by the encoder and then estimated by position scanning along the radial direction pixel-by-pixel in numerical computations. The means of the standard deviation in the overlapped regions in the simulation and the experimental measurement of an aspheric lens were 0.00004 and 0.06 waves, respectively. This demonstrates the reliability of the subaperture aberration compensation and position scanning process.