Boundary of the ELAIS-N1 field image made is defined by the outer blue square. The positions of direction-dependent calibrators are denoted by red small squares, while the pointing centre is marked by a blue solid dot. The left panel shows all self-calibration calibrator candidates, and the right panel displays the selected 28 calibrators. Each blue polygon in the right panel contains one calibrator, which is applied to correct the direction-dependent effects (DDEs) of the corresponding region.