# Xuntian

Xuntian (Chinese: 巡天; pinyin: Xúntiān; lit: Tour of Heaven), also known as the Chinese Space Station Telescope (CSST) (Chinese: 巡天空间望远镜; pinyin: Xúntiān Kōngjiān Wàngyuǎnjìng) is a planned Chinese space telescope currently under development.
It will feature a 2-meter (6.6 foot) diameter primary mirror and is expected to have a field of view 300–350 times larger than the Hubble Space Telescope. This will allow the telescope to image up to 40 percent of the sky using its 2.5 gigapixel camera.
As of 2024, Xuntian is scheduled for launch no earlier than late 2026 on a Long March 5B rocket to co-orbit with the Tiangong space station in slightly different orbital phases, which will allow for periodic docking with the station.
This state-of-the-art telescope, characterized by its off-axis design without any obstruction, sidesteps diffraction challenges associated with mirror support structures. As a result, its point spread function (PSF) remains unscathed, presenting a valuable asset for weak-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 17,500 square degrees of the sky in various bands, reaching point-source 5σ limiting magnitudes of about 26 (AB mag) in g and r bands.
The CSST's spectral 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 its angular resolution, depth, wavelength range, and capacity for both imaging and spectroscopy, coupled with extensive sky coverage, render the CSST survey highly competitive.
Notably, the CSST's observations are poised to complement and enhance other contemporaneous large-scale projects, including the Vera C. Rubin Observatory, the Euclid Space Telescope, and the Nancy Grace Roman Space Telescope.

## Instruments

Xuntian is equipped with five first-generation instruments, including a survey camera, a terahertz receiver, a multichannel imager, an integral field spectrograph, and a cool planet imaging coronagraph.

### Survey camera

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.

### Terahertz receiver

The terahertz receiver, also known as the high sensitivity terahertz detection module (HSTDM), enables terahertz (THz) astronomical observations from space. Conducting THz observations in space eliminates Earth's atmospheric absorption. HSTDM is a high-resolution spectrometer and the first space heterodyne receiver using niobium nitride (NbN)-based superconducting tunnel junction (Superconductor-Insulator-Superconductor (SIS)) mixer (the NbN SIS mixer).

### Multichannel imager

The Multichannel 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.

### Integral field spectrograph

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 (0.35-1.0 μm). 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) and star-forming regions.

### Cool planet imaging coronagraph

The cool planet imaging coronagraph (CPI-C) aims to realize high-contrast (< 10$^{−8}$) direct imaging of exoplanets with an inner working angle (IWA) of 0.35′′ in the visible (0.6328 μm). It plans to follow up exoplanets discovered by radial velocity observations, study planet formation and evolution, and probe protoplanetary disks. CPI-C works at 0.53-1.6 μm 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.
* 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.

## Comparison with other optical space telescopes

| | Hubble (WFC3) | Xuntian | Nancy Grace Roman | Euclid |
| Launch date | 1990 (WFC3 installed 2009) | 2026 (planned) | 2026 (planned) | 2023 |
| Orbit | LEO | LEO (co-orbital with Tiangong) | Sun–Earth L$_2$ orbit | Sun–Earth L$_2$ orbit |
| Mirror diameter | 2.4m | 2m | 2.4m | 1.2 |
| Camera size (gigapixels) | 0.016 | 2.5 | 0.3 | 0.6 |
| Resolution (arcsec) | 0.05 | 0.15 | 0.11 | 0.1 |
| Field of view (deg$^2$) | 0.002 | 1.1 | 0.28 | 0.91 |
| Largest survey | Cosmic Evolution Survey | N/A | High-Latitude Wide-Area Survey | N/A |
| Survey duration | 2 years | 10 years | 5 years | 6 years |
| On-sky time | 640 orbits | 10 years | 1 years | 6 years |
| Survey area (deg$^2$) | 2 | 17,500 | 2,000 | 15,000 |
| Survey area (of sky) | 0.005% | 40% | 5% | 33% |