Experimental investigation of binodal astigmatism in nodal aberration theory (NAT) with a Cassegrain telescope system

We present simulations and an experimental investigation of binodal astigmatism in nodal aberration theory (NAT) for a customized, high-precision Cassegrain telescope system. The telescope system utilizes a five-axis, piezo-actuated flexural mechanism to introduce secondary mirror misalignments and generate aberrations intentionally. The induced aberrations are measured interferometrically and quantified for a grid of field points on the telescope system's image plane. For this purpose, a coma-free pivot point of the secondary mirror was simulated for isolating the binodal astigmatism field response. The separation of the nodes is proportional to the introduced misalignments. A simulation of Fringe Zernike coma and binodal astigmatism was generated using a real ray trace model of the optical system and analyzed to compare to the experimental results. A statistical analysis of the measurements was performed to show the experimental results' accuracy and stability. The experimental results were consistent with the simulations, hence experimentally validating NAT for binodal astigmatism for the first time.