Design, fabrication, and testing of an all-metal ultra-compact telescope.

The design, fabrication, and testing of an all-metal four-mirror telescope for high-resolution remote sensing is presented in this paper. The system works in the visible (VIS) band and is designed with a focal length of 1406 mm, an aperture of 200 mm, and a full field of view (FOV) of 1.32°. The mechanical structure is designed to realize the snap-together alignment. The primary mirror (M1) and tertiary mirror (M3) are designed as a co-substrate element to simplify the fabrication and alignment. The telescope's weight is 3.5 kg, and the volume is just φ230×220mm3. Metallic mirrors are fabricated with single-point diamond turning, and post-polishing is used to correct the mirror's surface form deviation and remove turning tool marks effectively. After polishing, the RMS value of the mirror surface form deviation of the final mirror can reach 0.02λ at λ=632.8nm, and the surface roughness Ra value is about 1.83 nm. Benefiting from the all-metal mechanical design, the alignment process of the telescope is fast and accurate. The interferometric wavefront, modulation transfer function, and focal length of the telescope are measured, and the results demonstrated that it achieves the near-diffraction-limited imaging performance.