Recently, the Research Laboratory of Astronomical and Space Mirror Technology at the Nanjing Institute of Astronomical Optics & Technology (NIAOT), Chinese Academy of Sciences (CAS), has completed the grinding of the 1.93-meter aspheric primary mirror of the Mootian Telescope. The Mootian Telescope was jointly initiated by CAS academicians Su Dingqiang, Cui Xiangqun, and Professor He Xiangtao from Beijing Normal University, and is co-constructed by Beijing Normal University, Xinjiang Astronomical Observatory of CAS, NIAOT, and Xinjiang University, with NIAOT responsible for its development. Both fine grinding and polishing of the mirror surface were accomplished using an active polishing disc, a technology pioneered and successfully developed in China by the team led by Cui Xiangqun. The active polishing disc, controlled by a computer, undergoes real-time deformation to match the required curved shape of the mirror surface. By optimizing various process parameters, it enables the processing of large mirrors with high speed, high smoothness, and high surface shape accuracy. In just two months, the root mean square (RMS) of the mirror surface shape accuracy reached 0.8 microns, demonstrating that NIAOT has achieved new improvements in the processing efficiency and accuracy of large mirror grinding through the development of active polishing disc technology in recent years. The project team combined a six-joint industrial robot with a swing-arm profiler to solve the problem of precision degradation caused by turntable runout errors. This provided sub-micron-level measurement data for the fine grinding and rough polishing stages of the mirror, achieving seamless integration with optical interference inspection. This method reduced the risk and time consumption associated with repeatedly moving large-aperture mirrors between processing and inspection positions, while also improving inspection efficiency. Fine polishing was completed using a parallel rotating planetary grinding tool driven by a robot. Stable grinding characteristics were achieved through precise control of revolution, rotation, and grinding pressure. Independently developed process analysis software integrates detection data processing, dwell time calculation, and processing path generation, enabling digital automatic grinding with good determinism and rapid error convergence. The RMS of the full-aperture surface shape error of the 1.93-meter mirror is better than 1/50 wavelength, exceeding the design index. The successful grinding of the 1.93-meter primary mirror of the Mootian Telescope marks that advanced processing and inspection methods represented by active polishing discs, robotic swing-arm profilers, and parallel rotating grinding tools at NIAOT have entered routine application. These technologies are expected to play a role in the development of large and extremely large astronomical optical telescopes in China. ### Related Information about Telescopes The Mozi Sky Survey Telescope began construction in July 2019. Unlike China’s Five-hundred-meter Aperture Spherical Radio Telescope (FAST), the Mozi Sky Survey Telescope is an optical telescope operating in the visible light band. The telescope is named in memory of Mozi, an ancient Chinese scientist who first discovered the principle that light travels in straight lines over 2,000 years ago and conducted the pinhole imaging experiment, earning him the title of "the world’s first optician." Boasting a large light-gathering area, low stray light, high system detection sensitivity, and strong sky survey capabilities, the Mozi Sky Survey Telescope can survey the entire northern sky every 3 nights, making it the most powerful optical time-domain survey equipment in the Northern Hemisphere. The Mozi Sky Survey Telescope consists of four major subsystems: the telescope itself, the prime focus camera, the telescope dome and site, and data storage and analysis. It is equipped with a 765-megapixel large-format prime focus camera. Its core scientific goals include searching for and monitoring astronomical transient events and conducting time-domain astronomical observations, making it currently the most powerful astronomical observation equipment for optical time-domain surveys in the Northern Hemisphere. Time-domain astronomy is a热门 research field in astronomy, which is crucial for answering important scientific questions such as the origin and evolution of stars, the nature of black holes, the state of neutron star matter, and the origin of ultra-heavy elements in the universe. It is reported that the Mozi Sky Survey Telescope targets time-domain astronomical events such as supernovae, gamma-ray bursts, electromagnetic counterparts of gravitational waves, solar system objects, and even new types of transients not predicted by theory. Meanwhile, the Mozi telescope will complement the Vera C. Rubin Observatory’s Large Synoptic Survey Telescope (VRO/LSST), which is expected to be operational in the Southern Hemisphere in 2025, in terms of sky coverage. This will enable full-sky time-domain monitoring and promote the development of time-domain astronomy. In recent years, news of near-Earth asteroids passing by Earth has often attracted public attention. The survey data from the Mozi Sky Survey Telescope can be used to monitor near-Earth objects, aiding in early warning and defense. Near-Earth objects refer to solar system objects whose orbits are close to Earth’s orbit, including asteroids and comets. It is reported that the superimposed survey data from the Mozi Sky Survey Telescope will provide the deepest, high-precision, large-area, multi-color photometry and position catalog of the northern sky. As a legacy survey dataset, it can be used for the identification and systematic study of various celestial objects in the universe for decades to come. Additionally, the Mozi Sky Survey Telescope will support the national strategy of becoming a space power by conducting research on the search and monitoring of near-Earth objects in the solar system, serving the strategic needs of space security and deep space exploration. Sources: Nanjing Institute of Astronomical Optics & Technology, Xinhua News Agency, Science and Technology Daily