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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. This procedure includes a correction of the images for the compound-beam shapes (we use the same beam models here as in 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. The mosaic image is shown in the left panel of Fig.~\ref{fig:mosaic}, and the noise map is shown in the right panel of this figure. The angular resolution of the mosaic image is 27x11''. The local noise varies over the image between 25 and 50 uJy/beam, with a median value of 40 uJy/beam.
- The Boötes field has been covered by Apertif with 187 compound beam images from 8 different survey observations performed between April 2019 and November 2021. The self-calibration and imaging procedure was performed with the newly developed pipeline described by Kutkin et al. 2023.
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