Xuntian Space Telescope

Xuntian will survey 17,500 square degrees of sky from orbit, from the near-ultraviolet to the near-infrared

Xuntian, the Chinese Survey Space Telescope or China Space Station Telescope (CSST), is a 2 m space telescope for multiband imaging and slitless spectroscopy from 0.255 to 2.5 micrometers, from the near-ultraviolet to the near-infrared, with a terahertz receiver besides. Its name means "surveying the sky." About seven years of its 10-year life go to the main survey: a wide field of 17,500 square degrees to magnitude 26 in the g band (5 sigma, AB, point source), a deep field of 400 square degrees, and an ultra-deep field of nine square degrees to magnitude 28 or fainter.

The survey is expected to yield more than one billion galaxy images, more than 100 million galaxy spectra and about four million spectra of active galactic nuclei (AGN). The field of view is about 300 times Hubble's by the count of the Chinese Academy of Sciences (CAS); the values in the Wikipedia table imply about 550 times.

Xuntian is a flagship project of China's crewed space program, funded through the China Manned Space Project. The China Manned Space Agency (CMSA) and the National Astronomical Observatories of China (NAOC) operate it, and the China Aerospace Science and Technology Corporation (CASC) is the launch contractor. Xu Shuyan of the Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) is chief designer of the optical facility, and Zhan Hu of NAOC is its chief scientist. China develops all five instruments itself, though three of them use CCDs from Teledyne e2v in the UK. The cost has not been published.

Launch has slipped several times. In July 2022 CIOMP reported the optical facility at the engineering prototype stage, with launch set for the end of 2023 and operations in 2024. In November 2023 Zhan Hu told Scientific American that launch had moved to mid-2025, and slides leaked from a 2024 CSST Science Conference targeted readiness by 21 December 2026. In January 2026 launch was given as early 2027 at the soonest. The CSST team's overview paper schedules it for 2027 on a Long March 5B from Wenchang LC-101; neither CMSA nor NAOC has published a month or day. See space telescopes working and planned and China's space telescopes.

Xuntian flies its own orbit beside Tiangong and docks with the station for servicing

Xuntian co-orbits with the China Space Station, Tiangong, at about 400 km in low Earth orbit. It flies independently on its own propellant and docks with the station for servicing. Astronauts can replace or upgrade the back-end instruments and parts of the main optics, and some parts can be brought into the station's lab module for repair. Designers proposed separating the telescope from the station around 2014 to keep it clear of station vibration and obstruction.

The spacecraft has a mass of 15.5 t by the overview paper (CGTN gave 10 t in 2022, at the prototype stage) and is about 16 m long when observing. With its fine guidance sensor it holds pointing to 0.85 arcsec over 1,200 s. Data come down at 20 Tb a day in Ka band, at 1.2 Gbps through the Tianlian relay satellites.

The 2 m primary mirror feeds an off-axis Cook-type three-mirror anastigmat with a focal length of 28 m. The full field is 1.72 square degrees, with a primary imaging area of 1.1 by 1 degrees, and the design puts 80% of a star's light inside a radius of 0.15 arcsec (R_EE80). CIOMP completed development of all optical facility components; the maker of record for the flight model is not separately published.

The builder of the spacecraft bus and platform is not publicly named. The China Academy of Space Technology (CAST) appears only as an author affiliation on the mission overview paper, alongside NAOC, CIOMP, Purple Mountain Observatory, the Technology and Engineering Center for Space Utilization, the Institute of High Energy Physics, NIAOT and Shanghai Astronomical Observatory.

The survey camera tiles 30 CCDs from CETC 44 and Teledyne e2v into a focal plane of 2.6 billion pixels

The focal plane of the survey camera holds 30 CCDs of 9k by 9k pixels, each pixel 10 micrometers across and 0.074 arcsec on the sky. The CSST team's overview paper gives 2.6 billion pixels: 30 x 9,216 x 9,232 is 2.55 billion, and the 2.5 billion CAS gave in 2022 is the same count rounded down. About 60% of the focal plane takes seven-band photometry (NUV, u, g, r, i, z and y) through 18 filters; the rest carries 12 gratings for slitless spectroscopy in three bands (GU, GV and GI) at a spectral resolution of at least 200.

Two makers supply the CCDs. The layout figure of the flight CCD paper shows 12 GM1201A sensors from China Electronics Technology Group Corporation No. 44 Research Institute (CETC 44), 10 with near-ultraviolet coatings and two with visible coatings, and 18 CCD290-99 sensors from Teledyne e2v, six standard visible and 12 deep-depletion. NAOC tested and characterized 31 flight-model CCDs, one more than the 30 positions; the paper does not say what the 31st is for.

The CCDs run at minus 121 F (minus 85 C). A full frame reads out in about 39 s at a pixel rate of 150 kHz.

Four more instruments range from a planet-imaging coronagraph to the first space receiver with SIS mixers

The Multi-Channel Imager (MCI), developed jointly by the Shanghai Institute of Technical Physics (SITP) and Shanghai Astronomical Observatory (SHAO) since 2018, images near-ultraviolet, blue and red channels at once, 7.5 by 7.5 arcmin each, through 30 filters in three wheels. Each channel has one Teledyne e2v CCD290-99 at 173 K; the maker of the cryocooler is not named. CIOMP ran the full-system point-source optical test, with pinhole image-quality tests at SITP.

The Integral Field Spectrograph (IFS) covers 0.35 to 1 micrometer with a 32-slice image slicer at a spectral resolution of at least 1,000, on Teledyne e2v CCD231-C6 and CCD230-84 frame-transfer CCDs. The text and the table of its instrument paper give different detector formats, and the builder is not stated; the lead author is at SHAO.

The Cool Planet Imaging Coronagraph (CPI-C) aims for a contrast of 1e-8 or better, one part in 100 million, at 600 to 900 nm. It carries an EMCCD visible camera and wavefront sensor, an InGaAs near-infrared camera and a deformable mirror with about 1,000 actuators; the makers of the cameras and the mirror are not named. The Nanjing Institute of Astronomical Optics and Technology (NIAOT) wrote the instrument paper. The inner working angle is 0.55 arcsec at 633 nm by the overview paper and 0.35 arcsec by Wikipedia.

The terahertz module (HSTDM) is the first heterodyne receiver in space with niobium nitride (NbN) superconductor-insulator-superconductor (SIS) mixers. Its two SIS mixers run below 8 K and cover 410 to 510 GHz, with 16,384 channels over a 1.2 GHz intermediate frequency (IF) band. The Technical Institute of Physics and Chemistry (TIPC) of CAS vibration-tested the demonstration model; the builder of the flight hardware is not stated.

Xuntian will send about 5 PB of raw data over 10 years to science centers across China

The ground segment has a Payload Operation Management Center, a Science and Application Data Center, a Scientific Data Processing System and an Astronomical Data Science Center. Science centers are planned at Peking University, at NAOC (the CSST Joint Science Center), in the Yangtze River Delta and in the Guangdong-Hong Kong-Macau Greater Bay Area, plus a CSST Data Science Center; the host institutions of the two regional centers are not publicly named.

Over 10 years the telescope is expected to return about 5 PB of raw data and at least 30 PB of processed data. The CSST science team gets the immediate products, and official data releases go to domestic and foreign teams every two years, after a proprietary period.

The pipeline is being rehearsed in simulation. Research in Astronomy and Astrophysics published a special issue on end-to-end simulation on 7 January 2026, and a survey camera simulator built by Purple Mountain Observatory (PMO) ran a 50 square degree drill of 2,442 pointings on 30 detectors, producing data in the Level-0 format. PMO also leads the data processing pipeline for the terahertz module.

Deep learning has been studied for CSST multicolor imaging, for classification and regression of stellar density.

Xuntian has not launched and has no visitor program

Xuntian is a spacecraft, so there is nothing to tour. No published source describes seeing it from the ground alongside passes of Tiangong.

Sources

1. arXiv: the CSST mission overview
2. Wikipedia: Xuntian
3. Tencent News: the CSST survey camera simulation drill
4. Huanqiu: the Xuntian optical facility
5. Next Spaceflight: Long March 5B with Xuntian
6. Yangtze Evening Post: CSST end-to-end simulation
7. SCMP: Xuntian faces further delay
8. arXiv: CSST survey camera flight CCDs (abstract)
9. arXiv: CSST survey camera flight CCDs
10. CAS: the CSST optical facility prototype
11. Space.com: China previews Xuntian
12. arXiv: the CSST Multi-Channel Imager
13. arXiv: the CSST Integral Field Spectrograph
14. arXiv: the CSST Cool Planet Imaging Coronagraph
15. Researching: the CSST terahertz module
16. arXiv: the CSST terahertz module data processing
17. Aerospace America: China's space telescope and international science
18. arXiv: deep learning for CSST stellar density
19. CGTN: China develops a prototype of its first space telescope