MeerKAT Primary Beam Measurements in the L Band

TL;DR

This study measures and analyzes the frequency-dependent primary beam patterns of MeerKAT antennas in the L-band, highlighting effects of modeling simplifications, environmental factors, and pointing errors on beam accuracy.

Contribution

It provides detailed measurements of MeerKAT's primary beams, quantifies various effects influencing beam shape, and offers guidance for users on polarization and pointing accuracy impacts.

Findings

Pointing errors cause over 1% power measurement inaccuracies near beam half-power points.

Beam shape variability is around 0.3%, increasing above 1500 MHz due to waveguide mode effects.

Manufacturing differences in orthomode transducers significantly affect high-frequency beam properties.

Abstract

Full-polarization primary beam patterns of MeerKAT antennas have been measured in L-band (856 to 1711 MHz) by means of radio holography using celestial targets. This paper presents the observed frequency dependent properties of these beams, and guides users of this 64 antenna radio telescope that are concerned by its direction dependent polarization effects. In this work the effects on the primary beams due to modeling simplifications, bandwidth averaging, gravitational loading and ambient temperature are quantified within the half power region of the beam. A perspective is provided on the level of significance of typical use case effects. It is shown that antenna pointing is a leading cause of inaccuracy for telescope users in the presumed beam shape, introducing errors exceeding 1% in power near the half power point of beams, owing to a telescope pointing accuracy of $\sigma\approx 0.6$ arcminutes. Disregarding these pointing errors, variability in the Stokes I beam shape relative to the array average is most commonly around 0.3% in power; however, the impact above 1500 MHz is on average triple that of the lower half of the band. This happens because the proportion of higher order waveguide modes that are activated and propagate is sensitive to small manufacturing differences in the orthomode transducer for each receiver. Primary beam correction verification test results for an off-axis spectral index measurement experiment are included.