Mock Observations for the CSST Mission: Main Surveys-the Slitless Spectroscopy Simulation

TL;DR

This work builds a mock CSST slitless spectroscopy framework to accelerate pipeline development in the absence of real data. It combines grating physics (e.g., the grating equation $m \lambda = d (\sin \theta_i \pm \sin \theta_m)$), polynomial spectral representations, and least-squares trajectory fitting to produce multi-order slitless spectra across the CSST bands, enabling evaluation of detection performance and wavelength calibration through mappings between detector coordinates and wavelength via $L_{trace}$ and $\lambda$. The results show that 1st-order spectra are typically accompanied by 0th-order images, with GI providing the best detection efficiency, and quantify the typical wavelength-position mapping accuracy under idealized measurement conditions. The framework and insights lay groundwork for realistic data processing, while planned laboratory calibrations aim to refine the spectral-dispersion models for robust spectroscopic redshift measurements in upcoming CSST surveys.

Abstract

The China Space Station Telescope (CSST), slated to become China's largest space-based optical telescope in the coming decade, is designed to conduct wide-field sky surveys with high spatial resolution. Among its key observational modes, slitless spectral observation allows simultaneous imaging and spectral data acquisition over a wide field of view, offering significant advantages for astrophysical studies. Currently, the CSST is in the development phase and lacks real observational data. As a result, the development of its data processing pipeline and scientific pre-research must rely on the mock data generated through simulations. This work focuses on developing a simulation framework for the CSST slitless spectral imaging system, analyzing its spectral dispersing properties and structural design. Additionally, the detection performance of the slitless spectral system is assessed for various astrophysical targets. Simulation results demonstrate that nearly all 1st order spectra are accompanied by corresponding 0th order images, facilitating accurate source identification. Furthermore, the GI spectral band exhibits superior detection efficiency compared to the GV and GU bands, establishing it as the primary observational band for stellar and galactic studies. This work successfully develops a simulation framework for the CSST slitless spectroscopic equipment.

Figures (10)

• Figure 1: CSST focal plane layout. The L-shaped regions on both sides serve as the spectroscopic imaging areas, incorporating a total of 12 detectors, whereas the central region is allocated for integral imaging, comprising 18 detectors.
• Figure 2: CSST grating structure.The two gratings disperse towards the center avoid the lost of 1st order spectrum. (a) illustrates a schematic diagram of a slitless spectroscopic imaging region, which consists of a single detector and two opposing diffraction gratings designed for spectral dispersion. Solid lines indicate the propagation direction of light altered by the grating, while dashed lines indicate the propagation direction of light without the grating.(b) depicts the layout of the slitless spectroscopic imaging region on the focal plane. Each detector is divided into two segments due to the presence of the two opposing diffraction gratings positioned above it.
• Figure 3: The throughput of 0st order and 1st order spectrum of CSST three bands, including optical transmission, filter throughput, grating diffraction efficiency and the quantum efficiency (QE) of the CCD. The throughput of the 0st order spectrum is globally scaled according to the efficiency of the 1st order spectrum, adjusted to 10% of the 1st order spectral efficiency.
• Figure 4: the Schematic Diagram of CSST Slitless Spectrograph Optical Path. $\theta_i$ is the incident angle, defined as the angle between the incident light ray and the grating normal; $\theta_m$ is the diffraction angle, defined as the angle between the diffracted light ray and the grating normal; $\theta_f$ is the grating tilt angle, defined as the angle between the grating surface and the focal plane; $D$ represents the average distance between the grating and the focal plane, $D_m$ is the distance between the diffraction point and the origin where the grating normal intersects the focal plane.
• Figure 5: Schematic diagram of spectral dispersion image. This schematic diagram illustrates the pixel grid of the detector. The red star indicates the directly imaged position on the detector (virtual image), while the colored lines represent the dispersed first-order spectrum across the detector surface. Each pixel integrates photons within a wavelength interval $\Delta \lambda$.