After nearly 20 years of hard work and a series of technical breakthroughs, scientists led by ZHANG Xuejun from Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences (CAS) have successfully created the world’s largest SiC aspheric mirror, which boasts an impressive diameter of 4 meters. The study was published in Light: Science & Applications.

Fig.1 World's largest aspheric SiC mirror (Image by ZHANG)

The key performance indicators of optical telescope systems are angular resolution (AR) and light collecting capacity (LCC), which are directly related to the optical aperture of the systems. The 4 m SiC aspheric mirror manufactured by CIOMP can greatly improve the AR and LCC of telescopes, which is of strategic significance to the development of space exploration and astronomical observation.

In the last two decades, CIOMP scientists scored a number of breakthroughs concerning core technologies such as the blank preparation, asphere fabrication, testing methods, as well as cladding and coating of large silicon carbide (SiC) mirrors. They also made innovations in advanced manufacturing technologies needed for the manufacturing of SiC mirrors with high structural stiffness and complex surfaces, established a full link integrated manufacturing system for 4 m large aperture SiC aspheric mirrors, which made the creation of the world's largest SiC aspheric mirror possible.

Materials used for the manufacturing of high-performance imaging systems have to meet strict requirements concerning aperture, surface type, surface error, surface roughness, etc., which is a challenge.

The complex physical phase changes and chemical reactions involved in the preparation of SiC mirror blanks once meant no mirror over two meters in diameter could be created. The team led by Prof. ZHANG solved technical challenges such as forming of ultra-lightweight blank with semi closed back, drying without macro cracks, and homogeneous microstructure reaction-formed joint technology, and was able to create a 4.03 m SiC lightweight mirror blank.

To solve the problems in trying to turn the mirror blank into a completed mirror with large diameter SiC aspheric surfaces, which included the poor precision and low efficiency of conventional manufacturing methods, the high temperature and large stress deformation of the chemical vapor deposition (CVD) cladding process, the scientists proposed the technology of efficient and high-precision compound processing, and developed SLAM, an optical manufacturing software platform. They were able to increase the convergence efficiency of full-spatial frequency (FSF) shape error by 30%, paving the way for the high-precision deterministic manufacturing of the 4 m SiC aspheric mirror.

To overcome problems of the low dynamic range of interferometric measurement and the air disturbance error in long optical path, the scientists proposed a cross scale/multi testing technology for large aperture aspheric surface shape error, which made high-precision testing of the 4 m mirror possible.

The 4 m SiC aspheric mirror, the largest SiC aspherical mirror in the world, has been used in astronomical observation and other fields, giving human beings a "big eye" to observe the universe.