HST WFPC2 image of Mrk 509 through filter F547M. The directions North and East are marked with arrows of length 2.5′′. The black circle shows the placement of the 2.5′′ diameter COS aperture used for our observations. Lines indicating the position angles of various features associated with the structure of Mrk 509 are indicated. The small knot at the end of the linear structure visible in the F547M image directly above the nucleus lies at a position angle (PA, measured East of North) of −42° from the nucleus. The minor axis (PA = 135°) of the low-ionization gas disk is from Phillips et al. (1983). The PA of the polarized light in the core of the Hα emission line (60°), the broad wings of Hα (168°) and the optical continuum (144°) are from Young et al. (1999). The radio structures (PA = −65° and 165°) are from Singh & Westergaard (1992). Note that extended radio emission to the NW rotates from PA = −40° to PA = −65° with increasing distance from the nucleus.