Star-galaxy separation of detected objects. (Top) Observed sizemagnitude distribution of true galaxies and stars. The data points are the mean of r_e as a function of I_E . Also shown are the standard deviation of r_e and I_E in each bin. r_s/g = 0″.15 provides a good separation threshold working up to I_E < 24. (Bottom) Operating curve showing where our threshold (indicated with a cross) sits in terms of true positives (92.9%) and false positives (4.3%). The area under the operating curve is large, and the curve itself is reasonably flat for a wide range of false positive rate suggesting excellent discrimination and weak sensitivity on the threshold (the shear bias does not appreciably change). In real measurements this could be further optimised through access to external data or simulations.