Optimization of sub-cells orientation for assembly of a high-quality transmission sphere

The transmission sphere (TS) provides a high-quality reference wavefront which is common path with a test wavefront to generate interference fringes in a Fizeau interferometer. The optical path difference (OPD) of the reference wavefront should be controlled within peak-to-valley (PV) 5 λ (λ=0.6328 um), because too large OPD makes interference fringes distorted. Therefore, the tolerance analysis of the reference wavefront is very critical. Because the surface irregularity of lens can change the phase of a wavefront, surface deformation after mounting highly impacts on the wavefront error. For large optics, such as φ 6-inch in this study, surface deformation dominates the quality of the reference wavefront. For minimizing surface deformation after mounting, semi-kinematic mounting technology is used in sub-cells design to avoid over-constraint forces and unpredictable deformation. Then, the deformation due to gravity force of each surface can be constrained around PV 0.3 λ with Zernike trefoil in vertical setup TS; however, the superposition of the distorted wavefront may deteriorate the optical performance. A method of optimizing orientation of each lens around optical axis is presented in this paper. Sub-cells are designed to be rotational around optical axis respectively. The wavefront error of the reference beam of the worst case is improved significantly after optimization. Consequently, the method can effectively reduce the difficulty of lenses fabrication and mounting, and then the specification of the surface irregularity can be lower for cost saving. Based on the optimization in our study, a good reference wavefront can be acquired without any tight tolerance or complicated assembly.