Study On Near-null Test Of Aspheres With Subaperture Stitching Technology

Large convex aspheres are finding increasingly more applications in optical systems including large telescopes, lithographic lens and inertial confinement fusion systems. They are important components with influence on the systems performance because the aperture and surface errors have direct impact on optical image quality. Modern deterministic optic machining technology is originally based on accurate measurement of surface errors. While traditional approaches typically including the null test now encounter a bottleneck due to their limited measuring aperture, insufficient aberration compensation or low accuracy, which significantly blocks the promotion of manufacturing quality. We first propose a novel method combining counter-rotating phase plates and subaperture stitching, which is promising to break through the bottleneck of limited aperture in null test and limited aspheric departure in subaperture test. The phase plates are able to generate variable aberrations, flexibly compensating most aberrations existing in subaperture of different convex aspheres with various shapes to meet the near-null test condition. Such near-null optics possess the advantage of compact design, single adjustment degree of freedom, easy calibration and alignment, and absence of circular pupil distortion. The main contents are as follows:1. Based on the working principle, residual aberration coefficients of every off-axis subaperture of convex aspheres are calculated to solve for the phase function of the plates. Meanwhile, the auxiliary alignment patterns and power carrier are introduced and the design of plates is finally obtained. It is then verified through simulations that the near-null optics can be applied to different aspheres. Finally, according to the analysis of plate misalignment induced aberrations, it is essential to design a multi-axis adjustment mechanism, and consider the process of alignment of the near-null optics, the interferometer and the tested mirror.2. A workstation is set up to carry out experiments for optical aspheres combining near-null compensation and subaperture stitching. Forward and inverse kinematic models are built on the basis of kinematic link. According to the theoretical calculation and simulation, it is verified that negligible distortion is induced to the tested mirror by gravitation using the designed mount. At the same time, the adjustment tolerance of the mirror is obtained by means of simulation with Zemax, which helps to design the device composed of two rotary tables used to adjust the mirror. Finally, the near-null subaperture stitching workstation is built and aligned, based on the current non-null subaperture stitching workstation.3. With a convex even asphere of clear aperture about 320 mm as the sample, the near-null subaperture stitching experiment is implemented. By means of ray tracing and convex hull algorithm, overlapping area and data are obtained between subapertures. Then the mathematic relation between subaperture maladjustment and aberration is constructed by using the first-order approximation and according to the principle that surface errors of different subaperture in the same overlapping area are identical, the mathematic model of near-null subaperture stitching is constructed. Next is to separate the aberration introduced by subaperture misalignment and residual aberration after compensation, and then final full-aperture surface error is obtained. Meanwhile, the sample is also tested with coordinate measuring machine and non-null subaperture stitching method. The results are compared and primarily verify the validity and advantage of the near-null subaperture stitching method.