Performance Calibration of the Wavefront Sensor's EMCCD Detector for the Cool Planets Imaging Coronagraph Aboard CSST

TL;DR

This work characterizes the EMCCD detector integrated in the CPI-C wavefront sensor on CSST, detailing chip screening, EM Gain calibration across cryogenic temperatures, noise modeling, and inter-channel non-uniformity mitigation. It shows that operating at -20 °C yields a practical balance between EM Gain (1x–150x) and channel uniformity (≈9.35%), while readout noise and CIC remain limiting factors at high gains. A robust dynamic per-channel non-uniformity correction algorithm is proposed and validated with thousands of frames, and hardware recommendations (independent clock drivers) are offered to further stabilize responses. The results establish a rigorous calibration framework for EMCCD-based WFS in space-based high-contrast imaging and guide future hardware and in-orbit validation toward sub-electron readout noise.

Abstract

The wavefront sensor (WFS), equipped with an electron-multiplying charge-coupled device (EMCCD) detector, is a critical component of the Cool Planets Imaging Coronagraph (CPI-C) on the Chinese Space Station Telescope (CSST). Precise calibration of the WFS's EMCCD detector is essential to meet the stringent requirements for high-contrast exoplanet imaging. This study comprehensively characterizes key performance parameters of the detector to ensure its suitability for astronomical observations. Through a multi-stage screening protocol, we identified an EMCCD chip exhibiting high resolution and low noise. The electron-multiplying gain (EM Gain) of the EMCCD was analyzed to determine its impact on signal amplification and noise characteristics, identifying the optimal operational range. Additionally, noise properties such as readout noise were investigated. Experimental results demonstrate that the optimized detector meets CPI-C's initial application requirements, achieving high resolution and low noise. This study provides theoretical and experimental foundations for the use of EMCCD-based WFS in adaptive optics and astronomical observations, ensuring their reliability for advanced space-based imaging applications

Figures (11)

• Figure 1: Physical configuration of the WFS's EMCCD detector: (a) imaging assembly incorporating the EMCCD chip; (b) electronic control and power supply unit; (c) integrated support structure with heat-pipe-based thermal management.
• Figure 2: EMCCD focal plane schematic: Storage section with 8 parallel readout channels, each channel: 60(H) $\times$ 120(V) pixels, and output amplifiers shared pairwise (channels 1-2, 3-4, 5-6, 7-8).
• Figure 3: Bias frame characteristics of candidate EMCCD chips (with 8 channels (Ch) per chip): (a) Chip 1, (b) Chip 2, (c) Chip 3, (d) Chip 4.
• Figure 4: Image noise characterization and inter-chip comparison.
• Figure 5: (a) Chip 1 with Laplacian variance of 1413, demonstrating moderate image sharpness. (b) Chip 2 with Laplacian variance of 1832, exhibiting enhanced edge contrast and superior clarity. (c) Chip 3 with Laplacian variance of 1319, presenting balanced noise suppression and resolution. (d) Chip 4 with Laplacian variance of 2165, achieving the highest level of detail retention and texture definition.The Laplace variance exhibits a positive correlation with resolution. Lower values suggest superior noise performance, whereas higher values indicate finer resolution capabilities.