Alignment control in off-axis Gregorian system with reverse optimization method

Abstract

Off-axis refractive system with the noticeable advantages such as high resolution, large view field and central obscuration removed, has been one of the powerful systems for space astronomical telescopes in recent years. However, misalignment errors and surface error of mirrors are significant especially in the alignment progress of off-axis reflective telescope with large aperture. Computer aided assembling (CAA) jointly provide a robust misalignment correction method to ensure the accurate alignment of telescope. In this paper, system aberration of misalignment coaxial system with two mirrors is analyzed in detail, moreover, the off-axis system is studied further, especially in the off-axis Gregorian system. And the feasibility of correction values solution about off-axis refractive system is discussed. Both the simulation and experiment results demonstrate the feasibility of the proposed alignment method and high accuracy has been achieved. In the testing off-axis Gregorian system, the primary mirror is paraboloid with 1200 mm diameter, 210 mm off-axis distance, and the second mirror is ellipsoid with off-axis distance 129.0 mm, focal length 425 mm and 2125 mm, respectively. For the testing off-axis Gregorian system, the RMS value of primary mirror and second mirror are 0.021 λ and 0.027 λ (λ = 0.6328 nm), and the testing optimization result of system wavefront aberration with RMS value is better than 0.058 λ is achieved. The reverse optimization method testing can achieve high-accuracy measurement ability, which provides efficient and flexible way for the off axis refractive system from various types of elements with complex surfaces.