Mock Observations for the CSST Mission: CPI-C -- Instrument Simulation

TL;DR

This work presents CPISM, an end-to-end simulator for the CPI-C instrument on CSST, enabling development and validation of the CPI-C data-processing pipeline and scientific performance. It combines an optics module (apodized pupil filter, DM, focal plane mask) with a broadband high-contrast framework powered by Electric Field Conjugation to form a $10^{-8}$ dark hole, and a detailed EMCCD camera model that spans photon collection, transfer, multiplication, and readout while incorporating realistic noise sources. AGamma-based EM multiplication model and a calibrated EM gain–voltage relation improve simulation efficiency without sacrificing essential statistics, facilitating rapid generation of realistic CPI-C images. The tool outputs data products closely resembling real observations, supporting target selection, pipeline testing, and preparation for future CPI-C science cases.

Abstract

To support the development of the data processing pipeline and the scientific performance assessment for the Cool Planet Imaging Coronagraph (CPI-C) on the Chinese Space Station Survey Telescope (CSST), we have developed the end-to-end instrument simulation program, CPISM. This paper details the core modules of CPISM that simulate the CPI-C instrument, focusing on the simulation of the high-contrast imaging optical system and the visible-band science camera. We modeled key optical components, such as the transmission apodizing filter, the wavefront corrector, and the focal plane mask using the HCIPy package. A $10^{-8}$ contrast dark hole region, consistent with design specifications, was simulated using the Electric Field Conjugation (EFC) optimization method, and broadband observation effects were considered. For the science camera, which is an electron multiplying charge-coupled device (EMCCD), we established a detailed model encompassing photon collection, charge transfer, electron multiplication (EM), and readout processes, based on test data. This model simulates complex instrumental features including dark current, charge transfer efficiency, clock-induced charge, multiplication noise factor, and various readout effects like striping and drift. We also proposed and validated an improved statistical model for the EM process to enhance simulation efficiency. CPISM can generate simulated images containing rich instrumental details, closely similar to the expected real observational data, thus laying the foundation for the development and verification of CPI-C data processing algorithms and preparations for future scientific research.

Figures (9)

• Figure 1: The schematic of the CPI-C optical layout.
• Figure 2: Model and simulation results of the Apodizing Filter. a) Transmittance distribution of the Apodizing Filter, with white representing higher transmittance. b) The theoretical point spread function after pupil apodization at 662nm. Photon noise and Readout noise are not considered in the image c) The contrast curves after apodization (blue line) compared to that of an apodized circular aperture. The contrast curve is along the square diagonal direction.
• Figure 3: (a) The PSF with aberrations at the first focal plane of CPI-C at 662nm. Photon noise and readout noise are not considered. (b) $10^{-8}$ dark zone after high contrast optimization at 662nm. (c) The contrast curves. Orange line indicates the theoretical curve of the system. Blue line indicates the case with aberrations, and Red line the contrast curve after optimization. The contrast curve is along the square diagonal direction.
• Figure 4: Final focal plane image of the CPI-C system for a G0V star at the F662 band . Panel (a) displays the entire focal plane, while Panel (b) presents a detailed zoom-in view of the central region, as indicated by the red square in Panel (a). Note that the dark hole is based on simulations, and it will be calibrated to align with the future ground and on-orbit test results.
• Figure 5: The total efficiency curve of the four visible light bands of CPI-C.