Difference between PSF and model magnitudes, used as morphology indicator, for random objects in Stripe 82. Point-like sources in the co-added images, overlaid in blue, have a small magnitude difference. Quasar selection requires magnitude differences of at most 0.05, relaxed to 0.1 for objects with a high significance of quasar-like variability (to recover low-z quasars where the host galaxy can make the source appear extended).