Knowing the primary beam shape of a radio telescope is critical for deriving accurate fluxes away from the beam center. In the case of PAFs, the primary beam response, also known as the compound beam shape, must be independently measured for each compound beam as they are not constrained to have the same shape. Generally, formed beams further from the pointing center of the PAF will have more elongated shapes.
Full characterization of the Apertif primary beam is ongoing work. In this documentation we describe the methods used to measure the compound beam shapes (drift scan measurements and Gaussian process regression); describe the first release of primary beam images and plans for near-term updates; and offer an initial characterization of these released primary beam images.
We wish to emphasize that the use of the classic WSRT primary beam correction is not appropriate for Apertif. In addition to the fact that the compound beams can have non circularly symmetric shapes (see Figure 1), the sizes of the primary beams are different from the classic WSRT. The Apertif front-ends fill the focal plane more efficiently than the old MFFE frontends, leading to a smaller primary beam shape. Figure 2 shows one set of measured compound beam shapes divided by the classic WSRT primary beam shape. In addition to the elongated shapes (and offsets) visible in outer beams, the Apertif primary beam value is generally smaller than the classic WSRT primary beam value, confirming the smaller primary beam shape for Apertif.
Figure 1. Beam maps for all 40 apertif beams reconstructed from drift scans. Contour levels are: 0.1, 0.2, 0.4, 0.5, 0.6, 0.8. Red contours highlight the 10% and the 50% sensitivity level. These drift scans were measured in September 2019 and channel 7 corresponds to a frequency of ~ 1.363 GHz.
Figure 2. Compound beam shapes derived from drift scans divided by the classic WSRT primary beam. Contours are: 0.2, 0.4, 0.6, 0.8, 1.0.