Beam Measurements of Full Stokes Parameters for the FAST L-band 19-beam Receiver

TL;DR

This paper presents detailed measurements and analysis of the beam patterns and polarization characteristics of the FAST telescope's 19-beam L-band receiver, combining observational data with electromagnetic simulations to enhance understanding of beam structures and polarization effects.

Contribution

It provides the first comprehensive characterization of the full Stokes parameters for all beams of the FAST L-band receiver, including beam shapes, efficiencies, and polarization structures, validated by simulations.

Findings

Inner beams have higher efficiency and smaller size.

Outer beams show significant asymmetrical shapes.

Polarization beams exhibit characteristic squint and squash structures.

Abstract

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) has been fully operational since 11 January 2020. We present a comprehensive analysis of the beam structure for each of the 19 feed horns on FAST's L-band receiver across the Stokes I, Q, U, and V parameters. Using an on-the-fly mapping pattern, we conducted simultaneous sky mapping using all 19 beams directed towards polarization calibrators J1407+2827 and J0854+2006 from 2020 to 2022. Electromagnetic simulations were also performed to model the telescope's beam patterns in all Stokes parameters. Our findings reveal a symmetrical Gaussian pattern in the Stokes I parameter of the central beam without strong sidelobes, while the off-center beams exhibit significant asymmetrical shapes that can be fitted using a combination of log-normal and Gaussian distributions. The inner beams have higher relative beam efficiencies and smaller beam sizes compared to those of the outer beams. The sidelobes of the inner beams contribute approximately 2% of the total flux in the main lobe, increasing to 5% for outer beams, with a peak at 6.8%. In Stokes U, a distinct four-lobed cloverleaf beam squash structure is observed, with similar intensity levels in both inner and outer beams. In Stokes V, a two-lobed beam squint structure is observed in the central beam, along with a secondary eight-lobed structure. The highest squint peak in Stokes V is about 0.3% of the Stokes I in the outer beams. These results align closely with the simulations, providing valuable insights for the design of radio multi-beam observations.