Xuntian (CSST)

Xuntian mockup, showing its 2-meter diameter telescope
Mission type	Astronomy
Operator	CNSA
Mission duration	10+ years (planned)
Spacecraft properties
Dry mass	15,500 kg (34,200 lb)[1]
Start of mission
Launch date	Q4 2026 (planned)[2]
Rocket	Long March 5B
Launch site	Wenchang
Contractor	CASC
Orbital parameters
Reference system	Low Earth orbit
Main telescope
Diameter	2 m (6.6 ft)
Focal length	28 m (92 ft)
Wavelengths	255 ~ 1000 nm (Survey camera), 0.41~0.51 THz (590~730 μm) (Terahertz receiver)
Resolution	0.15arcsec
Instruments SC Survey camera HSTDM Terahertz receiver (high sensitivity terahertz detection module) MCI Multichannel imager IFS Integral field spectrograph CPI-C Cool planet imagingcoronagraph
Tiangong program ← Mengtian module

TheXuntian (Chinese:巡天;pinyin:Xúntiān;lit: Tour of Heaven)^[a] orChinese Space Station Telescope^[5] (CSST) (Chinese:巡天空间望远镜;pinyin:Xúntiān Kōngjiān Wàngyuǎnjìng) is currently under development.^[6]

Thisspace telescope will feature a 2-meter (6.6 foot) diameterprimary mirror and is expected to have a field of view 300–350 times larger than theHubble Space Telescope.^[7] This will allow the telescope to image up to 40 percent of the sky using its 2.5gigapixel camera.

As of 2024, Xuntian is scheduled for launch by late 2026^[2][8][9] on aLong March 5B rocket toco-orbit with theTiangong space station in slightly differentorbital phases, which will allow for periodic docking with the station.^[10]

This state-of-the-art telescope, characterized by itsoff-axis design without any obstruction, sidestepsdiffraction challenges associated with mirror support structures. As a result, itspoint spread function (PSF) remains unscathed, presenting a valuable asset forweak-lensing shear measurements.

The primary mission of the CSST revolves around high-resolution large-area multiband imaging and slitless spectroscopy surveys, spanning the wavelength range of 255–1,000 nm. Precise cosmology serves as the principal scientific driver behind this ambitious endeavor, with a focus on observing regions at median-to-high Galactic and ecliptic latitudes. Over a period of 10 years, the survey camera is slated to cover approximately 17,500square degrees of the sky in various bands, reaching point-source 5σlimiting magnitudes of about 26 (AB mag) in g and r bands.

The CSST'sspectral resolution (R=λ/Δλ) for the slitless spectrograph averages no less than 200, attaining wide-band-equivalent limiting magnitudes in GV (400–620 nm) and GI (620–1,000 nm) bands at about 23 mag. Beyond its wide-area survey, the CSST will target specific deep fields, aiming for observations that surpass the depth of the broader survey by at least one magnitude. The collective strengths of itsangular resolution, depth, wavelength range, and capacity for both imaging and spectroscopy, coupled with extensive sky coverage, render the CSST survey highly competitive.

The CSST's observations are poised to complement and enhance other contemporaneous large-scale projects, including theVera C. Rubin Observatory, theEuclid Space Telescope, and theNancy Grace Roman Space Telescope.

Xuntian is equipped with five first-generation instruments, including a survey camera, aterahertz receiver, a multichannel imager, anintegral field spectrograph, and a cool planet imagingcoronagraph.^[11]

The survey camera is also known as the multi-color photometry and slitless spectroscopy survey module. The module is located at the main focal plane and divided into the multi-color photometry submodule of 7 bands (NUV, u, g, r, i, z, y) and the slitless spectroscopy submodule of 3 bands (GU, GV, GI). The multi-color photometry submodule includes 18 filters, covering 60% of the area of this module. The slitless spectroscopy submodule includes 12 gratings, covering the other 40% of the area.

Theterahertz receiver, also known as thehigh sensitivity terahertz detection module (HSTDM), enablesterahertz (THz) astronomical observations from space. Conducting THz observations in space eliminates Earth's atmospheric absorption. HSTDM is a high-resolution spectrometer and the first spaceheterodyne receiver usingniobium nitride (NbN)-basedsuperconducting tunnel junction (Superconductor-Insulator-Superconductor (SIS)) mixer (the NbN SIS mixer).^[12]

TheMultichannel imager (MCI) has three channels covering the same wavelength range as the survey camera from the NUV to NIR bands, and these channels can work simultaneously. Three sets of filters, i.e., narrow-, medium-, and wide-band filters, will be installed on the MCI to perform extreme-deep field surveys with a field of view of 7.5′×7.5′. The magnitude limit can be stacked to a depth of 29–30 AB mag in three channels. It will study the formation and evolution of high-z galaxies, properties of dark matter and dark energy, and also can be used to calibrate the photo-z measurements with its nine medium-band filters for the main surveys.^[13]

The CSST-IFS (Integral Field Spectrograph) is one of the 5 instruments onboard the CSST. The key advantages of the CSST-IFS are the high spatial resolution of 0.2" and the full range optical wavelength coverage (0.35–1.0μm). Considering the limitation of the 2-meter aperture of the CSST, the CSST-IFS is optimal for targeting compact and bright sources, which therefore will be irreplaceable for studying galactic central regions (AGN feedback) andstar-forming regions.^[14]

The cool planet imaging coronagraph (CPI-C) aims to realize high-contrast (< 10⁻⁸) direct imaging ofexoplanets with an inner working angle (IWA) of 0.35′′ in the visible (0.6328μm). It plans to follow up exoplanets discovered byradial velocity observations, study planet formation and evolution, and probeprotoplanetary disks.^[15] CPI-C works at 0.53–1.6μm and is equipped with 7 broad passbands.

• 
  Estimated distribution of the observation pointing centers of the survey. The yellow circles in the bottom right figure are the selected deep fields.^[16]
• 
  Focal plane arrangement of the CSST survey camera
• 
  System throughput of CSST survey bands
• 
  Left: The transmission curves for the nine MCI medium-band filters from NUV to NIR bands. Right: The transmission curves for the seven survey camera filters.^[13]

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