Unobscured off-axis three-mirror freeform all-aluminum imaging telescope

Freeform optics has emerged as a significant design tool over the past decade. Non-rotationally symmetric optical surfaces enable high-performance imaging systems to achieve a large field of view (FOV), high resolution, low distortion, lightweight construction, and compact structure simultaneously. Advances in single-point diamond-turning fabrication and computer-generated hologram (CGH)-based metrology are gradually making optical imaging with freeform optics more practical. In this paper, we present the development and verification of an unobscured off-axis three-mirror freeform aluminum imaging telescope (FAIT). The design, fabrication, metrology, and assembly processes of the FAIT are discussed in detail. This system achieves an unobscured FOV of 4^∘×4^∘ with a maximum distortion of less than 1%. The optical modulation transfer function (MTF) across the full FOV exceeds 0.7 at the Nyquist spatial frequency of 90 lp/mm within the wavelength range of 400-900 nm. By integrating aluminum freeform reflectors with mechanical and electronic systems, the final FAIT successfully captures high-resolution photographs both on the ground and in near-Earth orbit. The all-freeform mirrors ensure that the FAIT maintains a compact envelope. Furthermore, the all-aluminum optomechanical structure facilitates effective athermalization while preserving cost-effectiveness and lightweight characteristics. Additionally, its weight of approximately 2.9 kg renders the entire FAIT system highly advantageous for space applications.