"Flying Giant Eye", made in China!

Original title: Planned to be launched this year, the "Flying Giant Eye" will fly independently in the same orbit as the Tiangong Space Station, made in China!

Many international mainstream media have made forecasts on possible world science and technology hotspots in 2023, and all of them have included the Chinese Space Station Project Survey Telescope (i.e. "China Survey Space Telescope", abbreviated as "CSST") among them. They believe that its flight into space will become a new annual milestone for human exploration or use of space, along with the United States' launch of a new rocket and the launch of an asteroid mining mission.

According to the latest version of China's space white paper, after the Tiangong space station is fully built, my country will start the launch and deployment of CSST in due course and carry out more extensive astronomical surveys. The China Manned Space Engineering Office has told the outside world that my country's first large-scale survey space telescope is planned to be launched in 2023 to carry out wide-area survey observations and conduct cutting-edge scientific research in the formation and evolution of cosmic structure, dark matter and dark energy, exoplanets and solar system bodies.

A rising star in the field of space optical observation

When it comes to astronomical telescopes, many people may first think of the "China Sky Eye", which is the Five-hundred-meter Aperture Spherical Telescope (FAST) located in Pingtang, Guizhou. As a major scientific and technological infrastructure built in my country in recent years, FAST has gained super popularity and is well-known at home and abroad for its large scale, excellent exploration capabilities and fruitful initial detection results. Compared with FAST, CSST is not as well-known at present, but for industry insiders, it is a rising star in the field of astronomical observations using optical telescopes.

In addition to their different levels of popularity, there are of course many differences between FAST and CSST. The most fundamental difference is that they belong to different types of astronomical telescopes.

FAST is a radio telescope that receives radio waves from celestial bodies, and the giant spherical mirror is its antenna for receiving radio waves. It collects weak cosmic radio signals and transmits them to the receiving system for amplification. The receiving system separates useful signals from noise and transmits them to the back-end computer for recording. The results recorded by the computer are displayed as many curves for astronomers to study and analyze, thereby obtaining various cosmic information. Because radio waves can penetrate the universe, radio telescopes are not easily affected by light and climate, and can work all day and night without interruption.

CSST is an optical telescope that captures wavelengths from near ultraviolet to visible light. It uses a large diameter, long focal length objective lens and a small diameter, short focal length eyepiece to achieve close imaging of distant objects. In this way, people can observe distant celestial bodies through an optical telescope. Due to the combined influence of factors such as the earth's thick atmosphere, ionosphere, ozone layer and geomagnetic field, the observation capabilities of ground-based optical telescopes are limited. With the advancement of aerospace technology, space optical telescopes that eliminate the influence of the above factors have emerged. These are the Hubble Space Telescope and the Webb Space Telescope, which were launched in April 1990 and December 2021 respectively. The China Space Telescope for Surveys will follow closely and become mankind's new "flying giant eye."

Divided into two parts, with 5 major equipment

CSST will be an extremely large space vehicle. Regarding its specific size, Liu Jifeng, director of the Joint Center for Scientific Work of the China Space Station Project Survey Telescope and deputy director of the National Astronomical Observatory, once described it as "the size of a large bus, three stories high when standing up, with a diameter of 2 meters." According to the information disclosed so far, the total length of the survey space telescope is about 14 meters, the maximum diameter is about 4.5 meters, and the launch mass is about 16 tons.

Zhan Hu, the responsible scientist of CSST's survey optical facility and researcher at the National Astronomical Observatory of the Chinese Academy of Sciences, introduced that CSST is mainly divided into two parts, namely the "platform section" and the "optical facility section". The former is actually the "resource cabin" of CSST, responsible for providing power for its space flight, and the latter is the main payload of CSST, including 5 observation equipment, namely the survey module, terahertz module, multi-channel imager, integral field of view spectrometer and exoplanet imaging coronagraph.

As its name suggests, the main mission of CSST is "sky survey observation", that is, to conduct a census of celestial bodies, to observe thousands of galaxies clearly and finely, and to bring a panoramic high-definition picture of the universe. Therefore, the sky survey module occupies the most observation time of the telescope. According to Zhan Hu, "sky survey observation" accounts for about 70% of the CSST's operating time.

According to Li Ran, a researcher at the National Astronomical Observatory of the Chinese Academy of Sciences, in order to ensure the imaging quality of "sky survey observation" and to be able to receive information from a wide field of view, the CSST sky survey module is equipped with 30 detectors with a total of 2.5 billion pixels. Among them, 18 detectors are equipped with different filters, which allows it to obtain images of celestial bodies in different bands, leaving a colorful appearance of the universe; the other 12 detectors are used for seamless spectral observation, and each exposure can obtain spectral information of at least 1,000 celestial bodies. During the entire sky survey cycle, the sky survey module will cover 40% of the entire sky area and accumulate high-quality data of nearly 2 billion galaxies.

While introducing the CSST's super-strong "sky survey observation" capability, Zhan Hu and Li Ran emphasized that its fine observation capability is also very strong. The telescope is equipped with a terahertz module, a multi-channel imager, an integral field spectrometer, and an exoplanet imaging coronagraph, all of which are precision measurement modules. They will rely on their own characteristics to carry out many special scientific observations, such as exoplanet detection, spatially resolvable spectral observations of galaxy core regions, research on neutral carbon in nearby galaxies, and super-deep field observations of the universe.

More suitable for sky surveys and easier to maintain and upgrade

As space optical telescopes, what are the characteristics of CSST compared with the Hubble Telescope?

In response, Zhan Hu said that CSST is more suitable for sky surveys. The lens diameter of its sky survey camera is 2 meters, which is slightly smaller than the Hubble telescope's 2.4 meters, but its field of view is about 300 times that of the Hubble telescope, and it can complete large-scale cosmic observations relatively quickly. Li Ran made a vivid analogy: it is like a group of sheep on a mountain. The Hubble telescope can see one of the sheep, while the CSST can take pictures of tens of thousands of sheep, and the clarity of each one is the same as that seen through the Hubble telescope.

CSST's expertise in sky surveys can be said to be "innate". As my country's manned space station flagship project, CSST is my country's largest space astronomical infrastructure to date. At the beginning of the project design, it aimed at large field of view, high image quality, wide band and other directions. In this regard, Li Ran explained that the Hubble Telescope is a pioneer of human space telescopes and has achieved fruitful scientific research results. Only by adopting innovative designs that are different from it can CSST further promote the development of space astronomy and further expand the boundaries of human cognition. There are two options for innovative designs: one is to build a larger-caliber space telescope to see deeper and obtain information on fainter celestial bodies; the other is to build a space telescope that can observe a wider area of ​​the sky, conduct sky surveys with higher efficiency, and study the universe more systematically. The relevant Chinese authorities chose the second idea for CSST based on relevant actual conditions.

Another notable feature of CSST is that it complements the space station. The close connection between the two can be seen from the full Chinese name of CSST, "China Space Station Engineering Survey Telescope". Specifically, CSST uses the Tiangong space station as its space home port. During normal observations, it stays away from the space station and flies independently in the same orbit with it. When it needs supplies or maintenance and upgrades, it actively rendezvouses and docks with the "Tiangong" and docks at the space home port. This can not only ensure its normal operation within its 10-year lifespan, effectively avoiding the situation similar to the Hubble Telescope that encountered a malfunction and could not be repaired for about 3 years, but also extend its life in orbit and achieve "service" beyond its term.

It took more than 10 years to continuously adjust and improve

CSST is the result of long-term cooperation among Chinese scientists, especially experts in optics and astronomy, and aerospace science and technology workers. Zhan Hu said that in December 2009, the headquarters of the Space Application System of China's manned space program organized a series of seminars to discuss the scientific goals and research directions of the space station in the fields of microgravity science, astronomy, life science, earth science, etc., which kicked off the CSST project. In November 2013, CSST was officially established.

Zhan Hu specifically pointed out that according to the plan at the time of project establishment, CSST is directly connected to the space station experimental cabin, but this brings some problems. For example, the attitude changes of the space station assembly, structural deformation, and disturbances caused by various vibration sources will seriously degrade the image quality of staring observations. For example, the polluted environment and particulate matter that may exist around the space station, the restrictions on the observation direction imposed by the structure of the space station's roughly earth-oriented attitude, and the stray light generated by the surfaces of the cabin and solar panels are not conducive to astronomical observations. In view of this, in 2015, the plan was adjusted to CSST and the space station to fly independently in the same orbit and was approved. After selection, five instruments including the sky survey module and terahertz module equipped with CSST were determined. In April 2022, the development of the CSST prototype entered a critical period. At the end of that year, the development of the prototype identification parts was completed. According to Xu Shuyan, chief designer of the survey optical facility and researcher at the Changchun Institute of Optics, Fine Mechanics and Physics of the Chinese Academy of Sciences, after completing the integration and testing of the subsystems, components and units of the telescope, they will move on to the development of prototypes and flight parts, and then conduct joint tests with the survey platform, carry out launch site tests, and finally launch at an appropriate time.

While the development of CSST is being carried out in full swing, the preparation work for observation data processing has been put into motion. According to Li Ran, CSST will produce 50PB of scientific data products in the full cycle. The relevant departments have established a team of astronomers, data experts and computer experts to develop the CSST scientific data processing system, produce astronomical images and star catalogs for use by astronomers across the country and even the world, and carry out further research.

It is expected to make important contributions to the development of world astronomy

CSST has not yet been launched, but its commitment to building an advanced space observatory open to the international community will provide new possibilities for humanity to understand the world and has attracted the attention of scientists around the world, especially astronomers and physicists.

On the website of the Joint Center for CSST Scientific Work, people can see the seven major scientific goals that CSST is aiming at, including cosmology, galaxies and active galactic nuclei, the Milky Way and nearby galaxies, stellar science, exoplanets and solar system objects, etc. Each of them points to the forefront of contemporary science. For example, using the rich observational data provided by the CSST Large Sky Survey and Ultra-Deep Field Observation, the accelerated expansion of the universe, dark energy, dark matter, etc., the morphological structure and evolution of galaxies, active galaxies and supermassive black holes, etc. are studied.

Li Ran said that CSST is expected to help humans explore and answer basic questions about the material composition, structure, and evolution of the universe. Zhan Hu pointed out that the improvement of astronomical detection capabilities has promoted human understanding of the universe. Every breakthrough in observation depth, breadth, band, and photometry accuracy will bring major discoveries and even lead to revolutionary developments in astronomy and physics. CSST has excellent overall performance, significantly surpassing previous projects in some indicators. It has the best image quality among the sky survey projects of the same period, and its observation capabilities in the near-ultraviolet band are unique. Li Ran expressed confidence that CSST is not only expected to make breakthroughs in the study of the mechanism of accelerated expansion of the universe, but will also open up a broader space for discovery and make important contributions to the development of world astronomy. (Reporter Zhang Baoshu)

Source: People's Daily Overseas Edition