Introduction of the Expanding Aperture Segmented Telescope (EAST)
Peking University is leading a joint effort to build an Expanding Aperture Segmented Telescope (EAST) with the largest aperture, segmented mirror in Asia in 2024-2030, to quickly narrow the huge gap between China and foreign countries in optical astronomical telescopes, and to meet the urgent needs of Chinese astronomers for diversified optical astronomical observation. The construction will be carried out in two phases according to the expanding type (the aperture is 6 meters and 8 meters respectively), which will greatly improve the observation capability of China’s optical astronomy, provide necessary conditions for the follow-up optical observation of the upcoming China Space Station Telescope (CSST) and the completed Five-hundred-meter Aperture Spherical radio Telescope (FAST) and other waveband telescopes, and provide guarantee for the production of first-class scientific achievements. It will also accumulate valuable experience for China’s future construction of large aperture ground-based optical telescopes and the launch of large space segmented telescopes.
Astronomy plays an irreplaceable role in China’s strategic, scientific, and social development. It adopts the most advanced observation technology to obtain information from the universe, enabling humans to explore some of the most basic scientific problems in the universe. A large astronomical telescope is an important symbol of a country’s scientific and technological development level. At present, optical telescopes with an aperture of more than 6 meters are mainly owned by the United States, Europe, and Japan, and installed in the Americas, Europe, and Africa. The maximum aperture is 10 meters, including Keck and HET in the United States, GTC in Europe, and SALT in South Africa. Most of these telescopes are general-purpose telescopes, which are the main force of major astronomical discoveries. For example, the 2020 Nobel Prize in Physics was awarded to scientists who discovered the supermassive black hole in the center of the Milky Way using the Keck (two 10-meter telescopes) in the United States and the VLT (four 8-meter telescopes in Chile) owned by Europe.
Currently, the largest optical telescope in China is the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) with an average aperture of 4 meters, but this is a specificl telescope only used for spectral sky survey. The largest aperture of the domestic general-purpose optical telescope is the 2.4-meter telescope in Lijiang, Yunnan, and the 2.16-meter telescope in Xinglong, Hebei, which are much smaller than those in other countries. China has the world-class equipment in the radio and high-energy wavebands, such as the Five-hundred-meter Aperture Spherical radio Telescope (FAST), the High Altitude Cosmic Ray Observatory LHAASO, and the X-ray satellite HXMT. However, the lack of medium and large optical telescopes has seriously hindered the overall development of China’s astronomy. Observations at other wavebands often require follow-up observations by large optical telescopes to confirm their scientific value. The upcoming launch of the China Space Station Telescope (CSST) with a 2-meter-aperture and the Wide-Field Survey Telescope (WFST) with a 2.5-meter-aperture to be built in Lenghu, Qinghai, both require a larger aperture general-purpose optical telescope for the follow-up astronomical observations. In the future, the construction of ground-based optical/near-infrared telescopes with an aperture of 15-30 meters and the launch of space segmented telescopes with an aperture of more than 6 meters will also require the construction experience of ground-based segmented telescopes with an aperture of more than 6 meters. Therefore, it is very urgent and necessary to build a 6-8 meter aperture general-purpose optical telescope with the largest segmented mirror in Asia as soon as possible in 2024-2030.
The 6-8 meter general-purpose optical telescope adopts the segmented mirror technology, which can be constructed in two phases for a total of 7 years. In the first phase (the first five years, 2024-2028), the frame and dome will be built, and the main mirror will be segmented by 18 hexagonal mirrors (the sub-mirror size is 1.44 meters), with a diameter of about 6 meters (5.76 * 6.24 meters) and the effective aperture of 5.5 meters. In the second phase (the next two years, 2029-2030), a ring of 18 more hexagonal segments will be added around the mirror, expanding it to a diameter of about 8 meters (7.92 * 8.73 meters) and the effective aperture of 7.8 meters. The first phase of focal plane instruments includes imaging cameras and medium-low resolution imaging spectrometers, and the second phase includes high-resolution spectrometers, multi-object spectrometers, polarimeter, and adaptive optical systems. The total cost of the project is expected to be 500 to 600 million Chinese yuan.
EAST will be built in the Lenghu area in Haixi Prefecture, Qinghai Province, which is located at the northern edge of the Qaidam Basin, with a clear night sky, abundant sunshine, convenient traffic conditions, and a good regional security environment. The site selection team has carried out three-year comprehensive monitoring of the parameters of the Lenghu Saishiteng Mountain site at an altitude of 4200 meters, and obtained a large number of data including atmospheric visibility, skylight background, the ratio of the observable clear night, weather, dust, atmospheric turbulence, and settleable water vapor. The skylight background without a moon at the site of Saishiteng Mountain is darker than 22 magnitude per square arcsecond, the proportion of high-quality clear nighttime is about 70%, and the median value of seeing is 0.75 arcseconds. With superior basic parameters, it is a rare site of a world-class optical observatory in the eastern hemisphere.
The construction of a 6-8m EAST telescope in Lenghu will not only provide Chinese astronomers an advanced telescope for the scientific frontier research on the universe, but also provide opportunities and conditions for international cooperation in global astronomical observations. For example, the global relay observations of important astronomical phenomena (due to the alternation of day and night in the eastern and western hemispheres) and the timely observations of important transient astronomical events (they can be observed only in the eastern hemisphere if the western hemisphere is in daytime).
