Part of the field of view of WFI2033−4723 seen through the WFI instrument at the 2.2 m MPIA telescope. The field is a stack of 312 images with seeing ≤1″.4 and ellipticity ≤0.18, totalling ∼28 h of exposure. The insert represent a single, 320 s exposure of the lens in seeing. The stars labelled PSF 1 to PSF 6 in red were used to build the point spread function. The stars labelled N1 to N6 in green were used to compute the exposure-to-exposure normalisation coefficient.