Researchers check the surface of a four-meter-aperture optical mirror developed by Changchun Institute of Optics, Fine Mechanics, and Physics of the Chinese Academy of Sciences in Changchun, northeast China's Jilin Province, Aug. 21, 2018. China has developed a high accuracy four-meter-aperture optical mirror, an important tool for deep space and astronomical observation. The silicon carbide aspheric optical mirror measures 1.6 tonnes. The silicon carbide used in production provides more stability to the surface of the mirror, allowing for greater accuracy at 20 nanometers. (Xinhua/Xu Chang)