Mock Observations for the CSST Mission: End-to-End Performance Modeling of Optical System

TL;DR

This work develops an end-to-end optical performance simulation for the CSST off-axis TMA, integrating five static and two dynamic error sub-models to propagate wavefront errors into PSF, ellipticity, REE$_{80}$, and field distortion across the full field of view. The framework combines design residuals, mirror figure errors, alignment, gravity-release, and thermal effects with dynamic vibrations and detector sampling, yielding a dynamic PSF via time-averaged PSFs. Quantitatively, REE$_{80}$ grows from $0.067''$ in the design state to $0.114''$ under full static and dynamic errors, while the center PSF Strehl drops from $0.964$ to $0.816$ under static errors; ellipticity shows a strong link to the wavefront distribution. The results highlight which error sources dominate image degradation (notably mirror surface errors) and demonstrate that end-to-end simulation can guide design optimization and in-orbit operation strategies for large-aperture, off-axis space telescopes, with plans to include additional factors such as attitude corrections and shutter effects in future work.

Abstract

This study presents a comprehensive end-to-end simulation analysis of the optical imaging performance of the China Survey Space Telescope (CSST) under in-orbit conditions. An integrated system model incorporating five static and two dynamic error sub-models was established. Wavefront errors were calculated for each sub-model and compared to the integrated system error to quantify the individual contributions to image degradation. At the detector level, wavefront error, point spread function (PSF), and ellipticity were evaluated across the full field of view (FOV). The average radius of 80\% encircled energy (REE80) of the PSF under full-error conditions was determined for 25 field points, yielding a value of 0.114 arcseconds. Furthermore, the calculations indicate a correlation between the wavefront distribution and the ellipticity distribution within the optical system. By optimizing the wavefront distribution, it is possible to adjust the ellipticity distribution of the PSF across the full FOV. The end-to-end simulation approach adopted in this paper provides a theoretical foundation for improving the image quality in large-aperture, off-axis space telescopes.

Figures (15)

• Figure 1: Schematic diagram of the optical system end-to-end simulation modeling.
• Figure 2: Schematic diagram of an off-axis three-mirror optical system. In the upper-left corner of the diagram, the three arrows denote the coordinate system of the optical system, while the single arrows indicate the labels of individual components within the optical system.
• Figure 3: Full-frequency surface shape simulation process. a) represents the low-frequency 36-term Zernike fitted surface figure, b) represents the mid-to-high frequency simulated surface figure, c) represents the sagittal height distribution curve of the mid-to-high frequency surface figure, d) represents the full-frequency simulated surface figure, obtained by superposition the surface figures described in a) and b), e) represents the power spectral density (PSD) curve of the full-frequency simulated surface figure as a function of spatial frequency.
• Figure 4: Analysis of frequency characteristics of mirror surface figure. The surface figure distributions are shown in Figures a) and d), the fitted curves (Figures b) and e)) both show a Gaussian distribution, the curve of PSD over the full frequency domain functions as a function of spatial frequency, as shown in Figures c) and f).
• Figure 5: Statistical results of wavefront values for 100 sets of assembly and adjustment errors.