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.
The mechanical position alignment of aspherical surfaces becomes a difficult challenge when the aspherical
departure of the surface is high. An optically obscured aspherical surface is even more difficult to be aligned due to the
missing obscured paraxial spherical vertex surface area for implementation of the auto-collimating technique. This
research paper aims to develop a method to align an obscured aspherical surface with respect to the mechanical axis of a
precision rotational stage by analyzing the multiple off-axis interferograms measured from a phase shifting Fizeau
interferometer. The alignment induced linearly varying coma is successfully shown in the simulation results. The method
can predict the vector direction of mis-alignment errors by least square fitting. An iterative process is possible to be
implemented to bring the misaligned aspherical surface back to aligned status.