Xuntian

It is planned to operate in a co-orbital arrangement with the Tiangong space station, allowing periodic docking for servicing and maintenance.

• Name — Xuntian • Names List — Chinese space station survey telescope • Mission Type — Astronomy • Mission Duration — 10+ years (planned) • Launch Date — 2027 (planned) • Launch Rocket — Long March 5B (5B-Y5) • Launch Site — Wenchang, LC-101 • Launch Contractor — China Aerospace Science and Technology Corporation • Orbit Reference — Low Earth orbit • Apsis — gee • Telescope Diameter — ftin • Telescope Resolution — 0.15 arcsec

The telescope uses an unobstructed off-axis optical design, avoiding diffraction effects caused by mirror support structures. This design is intended to improve image quality for observations including weak gravitational lensing measurements. The primary mission consists of wide-field imaging and slitless spectroscopic surveys covering wavelengths from 255 to 1,000 nm. (Cosmological research is a major scientific objective) particularly observations at medium and high Galactic and ecliptic latitudes. During its planned ten-year mission, the survey camera is expected to cover approximately 17,500 square degrees of sky in multiple bands, reaching point-source 5σ limiting magnitudes of about 26 (AB magnitude) in the g and r bands. The slitless spectrograph is designed to achieve an average spectral resolution of at least 200. In addition to the main survey, the telescope will observe selected deep fields to greater depth than the wide-area survey. The combination of high angular resolution, broad wavelength coverage, imaging and spectroscopic capabilities, and large survey area is intended to support studies of cosmology, galaxy evolution, and related fields. Observations from Xuntian are expected to complement data collected by other optical space telescopes, including Hubble, Euclid, and Nancy Grace Roman. Comparison with other optical space telescopes • Xuntian — Hubble — Euclid — Roman • Launch — 2027 (planned) — 1990 — 2023 — 2026 • Mission duration — 10 years (planned) — 35+ years (ongoing) — 6 years (ongoing) — 5 years (ongoing) • Orbit — LEO: — LEO: — Sun–Earth L 2 — Sun–Earth L 2 • Mirror diameter • Camera size (gigapixels) — 2.5 — 0.016 — 0.6 — 0.3 • Resolution (arcsec) — 0.15 — 0.05 — 0.1 — 0.11 • Field of view (deg 2) — 1.1 — 0.002 — 0.91 — 0.28 • Survey area (deg 2) — 17,500 — 2 — 15,000 — 2,000 • Survey area (of sky) — 40% — 0.005% — 33% — 5%

• Illustration — Xuntian Space Telescope mockup, showing its docking port Xuntian will have six scientific instrument bays, with plans to carry five instruments at launch:

The survey camera (SC), also known as the multi-colour photometry and slitless spectroscopy survey module, occupies the main focal plane. It comprises a seven-band photometry subsystem (NUV, u, g, r, i, z and y) and a three-band slitless spectroscopy subsystem (GU, GV and GI). The photometry subsystem uses 18 filters and covers about 60 percent of the focal-plane area, while the spectroscopy subsystem uses 12 gratings and covers the remaining 40 percent.

The integral field spectrograph (IFS) provides spatial resolution of 0.2 arcseconds and covers wavelengths from 0.35 to 1.0 μm. It is primarily intended for observations of compact, bright targets, including galactic nuclei and star-forming regions. The IFS can observe in parallel with the MCI.

The multichannel imager (MCI) contains three channels covering wavelengths from the near-ultraviolet to the near-infrared. The channels operate simultaneously and use narrow-, medium-, and wide-band filters to conduct deep-field surveys across a field of view of 7.5′ × 7.5′. Combined observations are expected to reach depths of 29–30 AB magnitude. Planned scientific uses include studies of high-redshift galaxies, dark matter, dark energy, and calibration of photometric redshift measurements. The MCI can observe in parallel with the IFS.

The cool planet imaging coronagraph (CPI-C) is designed for high-contrast direct imaging of exoplanets, with an inner working angle of 0.35 arcseconds at visible wavelengths. It is intended to follow up planets identified through radial velocity observations and to study planetary formation, evolution, and protoplanetary disks. The instrument operates over wavelengths of 0.53–1.6 μm and includes seven broad-band filters.

The high sensitivity terahertz detection module (HSTDM) is designed for space-based observations of terahertz radiation, avoiding atmospheric absorption that limits ground-based observations. It is a high-resolution spectrometer and the first planned spaceborne heterodyne receiver to use a niobium nitride-based superconducting tunnel junction mixer. • Illustration — Estimated distribution of the observation pointing centers of the survey. The yellow circles in the bottom right figure are the selected deep fields. • Illustration — Focal plane arrangement of the CSST survey camera • Illustration — System throughput of CSST survey bands • Illustration — 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.