Synthetic images for different phase angles, showing O−C residuals in pixel (right) for centre-of figure positions measured by limb-fitting the images shown left. The maximum bias occurs for 90 degree phase and is zero for zero phase. All points in the right-hand displays are the same, with the particular O−C residual values corresponding to each of the four images represented by the small circles (phase angles are, from top to bottom, +90 deg, 0 deg, –90 deg, 180 deg).