Machining position-attitude optimisation in ion beam figuring of high-precision off-axis aspheric surfaces

High-precision off-axis aspheric components are widely applied in various high-performance optical systems. However, the complicated manufacturing features of off-axis aspheric make it difficult to achieve high-precision surface for traditional optical manufacturing methods. Ion beam figuring (IBF) is a non-contact and highly deterministic method for the final precision optical figuring. In this paper, two different machining position-attitude models are discussed, which have great influence on the machining difficulty and figuring accuracy of IBF. The child mirror coordinate position-attitude model (CCPAM), which is based on the robustness of IBF removal function to small disturbance of target distance and incidence angle, can effectively decrease the sag height and incidence angle, thereby reduce the beam errors and machining difficulty. A linear three-axis IBF system is successfully applied for the figuring of a large-aperture parabolic off-axis aspheric mirror based on the CCPAM and its best-fit sphere. With two iterations of 113 min, surface error is down to 7.658 nm RMS (effective aperture 310 mm × 310 mm). The figuring results indicate IBF can realise the rapid fabrication of high-precision off-axis aspheric based on the machining position-attitude optimisation.