Top panel: the number density distribution of the RC star sample in the R−Z plane. The density map uses a bin size of 0.03 × 0.025 kpc. The Sun is located at (R, Z) = (8, 0.025) kpc. Bottom panel: sky maps of the selection function for our RC star sample, illustrating variations in detection probability across different Galactic coordinates. The scatter plot displays data points spanning Galactic latitudes from −60° to 80°. The accompanying color bar indicates the detection probability, scaled from 0.0 (dark purple) to 1.0 (bright green).