Described is a device comprising a retro-reflective marker sphere comprising: a core ball comprising: a generally spherical body portion, a mounting base extending from one end of the body portion of the core ball, and a mounting recess having a recess opening in the mounting base and extending into the body portion of the core ball, and a retro-reflective covering on the core ball, the retro-reflective covering comprising an opening through which the mounting base of the core ball extends, wherein a flat lower surface of the mounting base is spaced proximally from a lower edge of the opening of the retro-reflective covering, and wherein the mounting recess includes an interior screw thread for engaging an exterior screw thread of a mounting post when the retro-reflective marker sphere is mounted on the mounting post.