Illustration of the elliptical fit process on a Q_phi simulated disk image with multiple rings. Left: disk image with a radial cut at one azimuthal angle in white. Points used for ellipse fitting are shown in green and unwanted deleted points in smaller orange dots. Middle: radial profile extracted along the white line, where the purple curve corresponds to the smoothed profile. The maxima corresponding to the selected ring and the undesired maxima from other rings are shown in green and red, respectively. Right: disk image with the fit ellipse shown in white. A set of random points used to estimate uncertainties is plotted in blue.