Photometric precision analysis. (a) Noise characterization of 20 s on-sky exposures obtained with the INS Mars640 camera. The noise is dominated by dark current. (b) Noise characterization of 20 s on-sky exposures obtained with the YNAOIR camera. The noise is dominated by the sky background. (c) Light curves of CSS J040017.9+382426 obtained with the SYSU 80 cm infrared telescope using the INS Mars640 camera on 2024 October 15 (red) and the YNAOIR camera on 2025 September 30 (green). ZTF r non-history data are shown by blue. Dots denote single 20 s exposures and triangles indicate 10 minute integrations. For this J = 13.9 mag source, the INS Mars640 camera delivers 40 mmag per frame and 7 mmag over 10 minutes, while YNAOIR achieves 25 mmag per frame and 4 mmag over 10 minutes.