Residue distribution of the track positions obtained with a MIMOSA-1 sensor equipped with 1 diode per pixel. The grey distribution labelled "CoG" shows the residues observed with 3x3 pixel clusters, using the charge density for reconstructing the cluster barycenter. A fit to the distribution leads to 1.8 µm standard deviation. Once the accuracy on the telescope extrapolation has been taken into account (i.e. about 1µm), the standard deviation drops to about 1.4 µ m. The distribution labelled "Binary" shows the residue between the seed pixel of each cluster and the hit extrapoled from the reference telescope.