A Star-based Method for Precise Wavelength Calibration of the Chinese Space Station Telescope (CSST) Slitless Spectroscopic Survey

TL;DR

This paper introduces a star-based wavelength calibration method for the CSST slitless spectroscopic survey, leveraging existing stellar radial velocity data to achieve high calibration precision without calibration lamps.

Contribution

It presents a novel star-based calibration technique that uses known stellar velocities to correct wavelength errors in slitless spectra, applicable to large-scale space surveys.

Findings

Achieves calibration precision of a few km/s in the GU band.

Attains 10-20 km/s precision in GV and GI bands.

Requires only a few hundred velocity standard stars.

Abstract

The Chinese Space Station Telescope (CSST) spectroscopic survey aims to deliver high-quality low-resolution ($R > 200$) slitless spectra for hundreds of millions of targets down to a limiting magnitude of about 21 mag, distributed within a large survey area (17500 deg$^2$) and covering a wide wavelength range (255-1000 nm by 3 bands GU, GV, and GI). As slitless spectroscopy precludes the usage of wavelength calibration lamps, wavelength calibration is one of the most challenging issues in the reduction of slitless spectra, yet it plays a key role in measuring precise radial velocities of stars and redshifts of galaxies. In this work, we propose a star-based method that can monitor and correct for possible errors in the CSST wavelength calibration using normal scientific observations, taking advantage of the facts that i) there are about ten million stars with reliable radial velocities now available thanks to spectroscopic surveys like LAMOST, ii) the large field of view of CSST enables efficient observations of such stars in a short period of time, and iii) radial velocities of such stars can be reliably measured using only a narrow segment of CSST spectra. We demonstrate that it is possible to achieve a wavelength calibration precision of a few $\mathrm{km}\,\mathrm{s}^{-1}$ for the GU band, and about 10 to 20 $\mathrm{km}\,\mathrm{s}^{-1}$ for the GV and GI bands, with only a few hundred velocity standard stars. Implementations of the method to other surveys are also discussed.