FAST observation and performance evaluation for our observations. Panels (a) and (c) delineate the relation between telescope gain G, system temperature T_sys, and the zenith angle for the central beam (M01) at 1100 MHz, 1250 MHz, and 1400 MHz, respectively. The middle panel (b) shows the evolution of the zenith angle during our 20220701 4 h and 20231227 1.5 h observations. The estimated mean G and T_sys are indicated with vertical lines in panels (a) and (c). The FAST parameters were taken from Jiang et al. (2020). The evolution of the gain in panel (a) arises because the aperture efficiency is mostly unchanged until a zenith angle of 26.4° and then starts to decrease. For zenith angles larger than 15°, local side lobe RFI increases T_sys (c).