Over the last decade, the performance of space optical systems has been significantly improved through the efforts of optical researchers in China and abroad. Two critical specifications of an Earth Observing (EO) system are resolution and swath. By this measure, China's recently launched commercial EO systems have reached a world-class level. These inspiring achievements are the results of courageous innovations and substantial practices in reflective optical materials, optical design, aspheric fabricating and testing, and system assembly.

Advanced manufacturing technologies of optical systems include mirror blank preparation, optical design, fabrication, coating, testing, and system assembly, which is an enabling technology for high-performance optical systems. The imaging optical system is an information collecting system rather than a simple energy collecting system. An important indicator to assess its performance is the information collecting capacity. For example, the requirements for the EO payloads are higher resolution and wider imaging swath. However, constrained by launch costs and working environments, the volume and mass of space optical systems are strictly restricted. Therefore, some common requirements for its manufacturing technology are summarized as follows. First, the mirror materials should have high specific stiffness and thermal deformation resistance to reduce the cost of launch mass and thermal control. Second, the systems with large aperture, long focal length, and large field of view (FoV) simultaneously are needed to solve the contradiction of high resolution and wide swath, and this results in multiple-mirror on-axis or off-axis design. Aspheric surfaces or even freeform surfaces are employed to increase the design freedom and balance the large off-axis field aberrations. Nevertheless, the off-axis aspheric or freeform design causes asymmetric mirror shape and system layout. Consequently, unlike the conventional slow lapping process, the deterministic computer-controlled optical surfacing (CCOS) technique is necessary to achieve higher accuracy and efficiency. Meanwhile, aspheric surface digital testing and system computer-aided alignment techniques are utilized through the whole process of milling, grinding, polishing, and system assembly to guide deterministic processing and verify the payload's performance consistency in space and on earth.

In this paper, the latest progress of space optical systems over the last decade in China are introduced with the combination of several on-orbit examples. The technological advantages include silicon carbide (SiC) material, space optical system design and configuration evolution, digital measurement of complex optical surfaces, and computer-aided assembly and adjustment technology. In addition, the future trend of advanced optical system manufacturing technology is discussed.

The space qualified mirror materials should be easy to manufacture and of high dimensional stability to adapt to the working environments. Compared with optical glass and some metal materials, SiC ceramics exhibit excellent performance in specific stiffness and thermal stability (Table 1). For large-aperture space telescopes, reflective systems have been widely applied for their mirror light-weighting nature and chromatic aberration-free feature. Two-mirror systems are adopted traditionally in the early applications such as Ritchey-Chretien (RC) system (Fig. 5). Several designs based on three-mirror layouts have emerged in the 60s and 70s of 20th century to further correct astigmatism and increase the available FoV, and the most successful example is the three-mirror anastigmat (TMA) proposed by Korsch (Fig. 6). To further increase the field of view and correct the higher-order off-axis aberrations, this paper introduces freeform surfaces based on the conventional TMAs. The optical payload in GF-6 satellite is an off-axis four-mirror design, in which the second-and-fourth mirror employs the Zernike freeform surfaces (Fig. 11). The rapid development of advanced manufacturing technology has been greatly promoted with the evolution of complex optical system configurations. The CCOS, magnetorheological finishing (MRF) and ion beam figuring (IBF), and other advanced manufacturing technologies have been developed and applied to manufacturing aspherical optical systems (Fig. 16) with extremely high shape accuracy (Fig. 19). In addition, the aspheric surface testing methods have been developed and utilized in a combined way to measure the surface shape (Figs. 22 and 23). Finally, based on the co-reference alignment technology using computer-generated hologram (CGH), some testing results of the aligned system are shown (Figs. 25-26).

The advanced optical manufacturing technology based on multi-axis computer numerical control machining center has yielded remarkable results and has been extensively applied in numerous projects. However, for very large aperture monolithic or segmented mirror telescopes, optical manufacturing faces challenges in high quality and volume productions. On one hand, 8 m class aspheric or freeform mirrors need to be manufactured to the accuracy of sub-10 nm RMS. On the other hand, hundreds of 1 m class aspherical segmented mirrors need to be polished with high efficiency and consistency. In the future, an optical intelligent manufacturing system will be constructed with the combination of intelligent decision support, process sensing, collaborative manufacturing based on big data, cloud computing, and machine learning. The unmanned workshops together with intelligent green flexible manufacturing technology are highly expected in the following decade.