Q2237+0305 image A x and y pixel positions r_QSO(t) as a function of time (in the R band) determined by GALFIT on the combined images shown as upper (blue) and lower (orange) data points respectively. The lines are no fits to these positions, but simply lines with the proper motion µ_PSF of the PSF star from Gaia as slope and the median of all proper motion corrected positions as overall offset (see Eq. (6)). We did this for all six datasets and then used the lines as fixed quasar image A positions when applying GALFIT to the difference images to determine the light curves.