R-band image centered on SDSS J1001+5027. The image is the combination of the 210 best exposures from the Mercator telescope, for a total exposure time of 21 h. We use the stars labeled 1, 2, and 3 to model the PSF and to cross-calibrate the photometry of each exposure. The position of the two lensing galaxies G1 and G2 are indicated in the zoomed image in the upper left. They are most clearly seen in the deconvolved images presented in Fig. 3.