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Apertif Compound Beam Shapes

Submitter: Kelley M. Hess on behalf of Apertif Imaging Commissioning Team
Description: Apertif is a phased array feed upgrade to the Westerbork Synthesis Radio Telescope. The upgrade expands the field-of-view from a single beam 30 arcminutes in diameter to an area more than 2.8 deg x 2.3 deg by having 40 overlapping compound beams. The compound beam pattern is generated electronically, so understanding what the beams look like (their shape and overall sensitivity), how stable, and how reproducible they are are critical to Apertif commissioning, and to accurately measuring the flux of galaxies that Apertif detects during survey operations.

To this end, the Apertif Imaging Commissioning Team has recently measured the sensitivity and shape of the compound beams using drift scans of the bright radio source, Cyg A. Drift scans work by pointing the telescope at a fixed azimuth and elevation, and letting a bright source drift through the field-of-view at various distances from the center. The power measured depends on the response of the telescope in the direction of the source and the brightness of the source itself. We then use the power that is recorded to reconstruct an image of the beam pattern for each individual compound beam.

The gif cycles through each of the 40 compound beams as they have been reconstructed from the drifts scans. The top plot shows the XX polarization, and the bottom plot shows the YY polarization. The image values have been stretched in order to show the sidelobes in the four corners around the primary field-of-view of each compound beam. These sidelobes are at about the 3% level compared to the peak of the compound beams. Bright points which stay fixed between images in the gif are due to radio frequency interference during the observations.
Copyright: K. M. Hess
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