Outline of the relative positions of pointings in our adopted dither-pattern strategy. The 25 images are organized in a 5 × 5 array, where the center of the field falls in the central position 13. The other pointings are taken in a way that the gaps between the four detectors never cover the same point of sky more than once. The 25 images were designed for astrometric purposes allowing stars in frame 13 to be imaged in as many different locations of the detectors as possible. This enables us to self-calibrate the geometric distortion. The zoom-in in the blue panel shows an example of the adopted dither between two pointings. As described in Table 2, the shift step can change from field to field.