Map of the HST sample. The heavy solid line delimits the HST WFPC2 FOV of our UV observations (Prop. 6607), the dashed line bounds the FOV of the optical HST WFPC2 observations by Piotto et al. (2002), and the dotted line marks the edge of the complementary ACS data set. The derived center of gravity C_grav is marked with a cross. BSSs (filled circles) and the concentric annuli used to study their radial distribution (see Table 2) are also shown. The inner and outer annuli correspond to ﹩r=r_{c}=27^{\prime \prime }﹩ and ﹩r=115^{\prime \prime }﹩, respectively.