2025AJ....170..116C

We present the polarization calibration of the 19-beam receiver at 1420 MHz within the full illumination of the Five-hundred-meter Aperture Spherical radio Telescope from 2018 October to 2023 March. We perform spider observations to characterize the on-axis Mueller matrix of the central beam. The calibrated polarization percentage and polarization angle of a source with strong linear polarization emission are about 0.2% and 0$\mathop{.}\limits^{\unicode{x000b0}}$5, respectively. Several parameters of the central-beam Mueller matrix show time variability from months to years, suggesting that relatively frequent polarization calibrations are needed. We obtain the Mueller matrix parameters of the 18 off-center beams with a combination of on-the-fly observations and spider observations. The polarization calibration provides consistent fractional Stokes parameters of the 19 beams, although the Mueller matrix parameters of the off-center beams are not as accurate as those of the central beam. The Mueller matrix parameters of the central beam do not show a strong dependence on the reflector surface. However, we notice different off-center Mueller matrix parameters between the eastern and western sides of the reflector surface. We provide average parameters of the 19-beam Mueller matrices that should be applicable to observations from 2020 to 2022, with several caveats. After applying the average parameters, on-axis fractional linear polarization measurements ≳10% and on-axis fractional circular polarization measurements ≳1.5% can be considered high-confidence detections. For sources with weak polarization, timely polarization calibrations using spider observations are required.