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To obtain a final mosaic image, the astrometric accuracy of the images was first improved by cross-matching sources from each individual image with those in the LOFAR HBA image and by correcting the astrometry of the Apertif images with the median position offset. The typical offset was found to be about 1'' with only a few exceptions. After this, the images were combined in the standard way using the mosaic package described in Kutkin et al. 2022 (K22). This procedure includes a correction of the images for the compound-beam shapes (we use the same beam models here as in K22 Kutkin et al. 2022) that reconvolves the images to a common angular resolution and reprojects them onto the common sky grid. Because individual Apertif images overlap, the mosaicking leads to an increase in sensitivity.
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