A Robust Geometric Distortion Solution for the Main Survey Camera of CSST

The advancement in sensitivity and field of view of next-generation wide-field survey telescopes requires astrometric measurements with high precision, even in the presence of significant geometric distortions. To address this challenge, we develop a Weighted Polynomial Distortion Correction in 2-Phase (WPDC-2P) method. This approach enhances stellar cross matching, incorporates distance-based weighting into the traditional polynomial fitting, and employs a look-up table to absorb the remaining distortion residuals. Validated on simulated data from the Main Survey Camera of the Chinese Space Station Survey Telescope (CSST), incorporating geometric distortions up to approximately 200 pixels, the method achieves astrometric standard deviation ranging from 0.013 to 0.107 pixels (0.03 pixels for the g-1 detector) across all 18 detectors. Under extreme crowding conditions (e.g., globular cluster NGC 2298), the astrometric precision for the g-1 detector reaches 0.05 pixel level within the central region (rd < 4000), despite a centroiding precision of ∼0.04 pixels. When applied to the Beijing–Arizona Sky Survey data, for which the standard pipeline delivers an astrometric uncertainty of ∼20 mas, our method reduces the positional scatter to σΔα = 5.494 mas (0.01 pixels) and σΔδ = 9.981 mas (0.02 pixels) using only a weighted third-order polynomial correction. The method has been integrated into the CSST data processing pipeline and is prepared for further refinement using on-orbit calibration data.