Above value "13.4 seconds of arc" is shown in a report carried in a journal "Report of the national astronomical observatory of Japan ; Feb 1993". No book seems to touch on this value.
Two disks are rotating (in outer space ; in non-gravitational field ; horizontally). Rotating speed of one disk is 10 rotations per second and that of the other disk is 20. A body is detached from the edge of disk and is receding along a tangent line (and every tangent line does not rotate relative to space). It will be the same about a photon (emitted from a rotating source).