In total, Apertif observed about 2300 square degree of the sky with some pointings covered several times. We process the Apertif data obtained for the field mentioned above.
The image of each compound beam of an observations is
calibrated independently. The new pipeline starts with the preflagged cross-calibrated continuum visibilities produced with the
initial steps of Apercal. For many of these data sets, additional
flagging is done of antenna-beam combinations for which the
direction-dependent errors are very large. Following this, a DI
calibration is performed. This step includes three cycles of self-calibration and imaging. First, a phase-only self-calibration with
a 10 minute solution interval is run, followed by a phase-only
calibration with 30 seconds solution interval. The final step in
the DI calibration is an amplitude and phase calibration with a
long solution interval of 1 hour. The second and third CLEAN
step in the DI self calibration are done using masks based on the
local signal-to-noise estimated from the residual images of the
previous step. The number of sub-steps, type of the calibration,
parameters for creating the local noise images, and the solution
intervals used were determined before running the pipeline on
all images by manually experimenting with these parameters and
analyzing the results through the validation procedure described
in Adams et al. (2022). This resulted in a final set of parameters
used for all images.
After the DI calibration, a clustering procedure is performed. The final CLEAN model obtained after the DI calibration is segmented using Voronoi tessellation with cluster centers located at the ten brightest sources. Using this segmented model, DD calibration is performed by calibrating each segment independently. This step is performed using DP3 ddecal with the parameter subtract=True, meaning that the visibilities are subtracted from the DI calibrated data with the DD calibration solutions applied. The residual visibilities, free from DDEs, are then imaged and the final DI model is restored on this image. This final compound beam image is produced with a size of 3072×3072 pixels and pixel scale of 3′′.
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) 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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