Facet coverage of the Boötes field. The grey boxes show the positions of the calibrator directions, and the size of these boxes show the area used for self-calibration, i.e. larger boxes include more sources. The coloured polygons show the Voronoi tessellation of the image plane based on these calibrator positions. The maximum size of the facets is limited to 50 arcmin radius from the calibrator direction (2048 pixels at 1.5 arcsec pixel⁻¹) resulting in some incomplete coverage particularly outside the FWHM. The black circle has a radius of 2.44°, at the approximate 40 per cent power point of the average primary beam.