The initial primary beams to accompany the first data release are those derived from the Gaussian process regression (GPR). The flux ratios between NVSS and Apertif were examined for both the compound beams derived from GPR and the drift scan methodology (see "Characterization of the primary beams"). Both sets of compound beams show similar behavior in the inner region, but the primary beam images from the drift scan methodology show systematics in the outskirts of the primary beam response, likely related to source confusion when performing the drift scans. This is under active investigation but currently only the GPR compound beam shapes are released as these appear to behave better in the outer regions.
These primary beams are appropriate for the center frequency of the continuum images, 1361.25 MHz, over the frequency range 1292.5-1430 MHz. The drift scan measurements demonstrate the size of the compound beam changes linearly with frequency, as expected.
The provided primary beam images are created at 100” resolution and should be regridded to match the images/cubes they are used to correct. In addition, they can be applied to other frequencies by scaling the cell size (cdelt1 and cdelt2 keywords) linearly with frequency (see subsection "Beam size change with frequency" in "Drift scan method" for the scaling) before regridding to match the data product of interest.
Two sets of primary beam images are released for each compound beam. The first set (“orig”) is not normalized to have a peak response 1. By construction, applying these primary beam images to the Apertif data should result in measured fluxes that are consistent with those in the NVSS catalog. Note, however, that the derivation of the primary beam response images did not account for the slightly different center frequency of the Apertif continuum images (1361.25 MHz) compared to NVSS and thus there may be small changes in the flux scale due to spectral indices of sources (2% for a typical spectral index of -0.7).
The second set (“norm”) are primary beam images normalized to have responses between 0-1, as is typical for primary beam images. Applying these primary beam images to the Apertif data will include systematic offsets between the measured Apertif fluxes and the NVSS catalog fluxes; this is discussed and quantified in "Characterization of the primary beams".