High fidelity polarimetry across the full frequency range, and past the half-power point of the beam, will require advanced calibration techniques, including frequency dependent beams with a sampling interval of at least 10 MHz.
For structures extending past the beam half power point, it is advised to use a mosaic instead of a single pointing.
The latest results from holography measurements are presented in de Villiers (2023). Primary beam models accompany this publication and can be accessed here on the SARAO data repository. The main findings are outlined below.
Beamwidth and ellipticity
The MeerKAT primary beam, at all bands, is elongated in the vertical direction because the main reflector is wider in the horizontal direction, as projected onto the aperture plane. The beamwidth also oscillates as function of frequency towards the upper half of the band.
The beam pattern orientation relative to the sky
Antenna beamwidth as a function of frequency, for each polarisation channel. From de Villiers (2023).
Primary beam ellipticity as a function of frequency for each receiver band. From de Villiers (2023).
Frequency and polarization dependent pointing errors are observed in measured primary beam patterns. The range of variation amongst different receiver units are significant in the upper half of the band.
Small pointing offsets can have significant effect on instrumental polarisation in the upper half of the band. Antenna pointing errors, rather than variations in beam response from one feed to another, can be the dominant source of deviations from the average array beam.
Refinement of the global antenna pointing calibration procedures is currently under way. We are in the process of developing referenced pointing procedures which could be run prior to, and during, the observation.
In the videos below, we show the typical frequency dependence of the instrumental polarisation as Jones matrices and in Stokes parameters.
Variation of Stokes parameters as a function of frequency
Variation over frequency of the Stokes beam parameters for UHF band
Variation over frequency of the Stokes beam parameters for L band
Variation over frequency of the Stokes beam parameters for S band
Variation of Jones parameters as a function of frequency
Variation over frequency of the Jones matrices for UHF band
Variation over frequency of the Jones matrices for L band
Variation over frequency of the Jones matrices for S band
Dependence on receivers
Cross-polarisation beam shapes are dominated by the individual OMTs in the feeds, and are unique to the receiver rather than the antenna on which the receiver is mounted.
While we have beam models for individual antennas, it should be noted that the antenna to antenna variations are small, with deviations from the array average ranging from 0.3% on average up to 1.4% in the lower (900–1500 MHz) part of the L-band while in the upper half of the band (1500–1670 MHz) it is 0.9% on average and up to 5% in the worst case. Widening of the array-average beam is more likely to be caused by pointing errors, which at the moment have sigma = 0.6 arcmin.
Dependence on elevation
Gravitational loading has a minor effect on the copolarisation power, compared to antenna to antenna variations. The error is typically 0.05% over the 20° to 70° range compared to the nominal shape at 60°.
A note on sidelobes
MeerKAT is extremely sensitive, and strong sources in the sidelobes can limit imaging dynamic range if not corrected for.
At L-band, there is a far sidelobe at ~72 degrees from boresight at the 0.01% power level (Figure 2). This lobe can pick up significant signal from satellites, or the Sun.
An example of the beam sidelobes at more than 60 degrees from the antenna pointing centre. The panel on the right is a series of cuts across x-axis of the plot to the left.