Trans-equatorial ghost responses from east-west aperture synthesis arrays

Abstract

A well-known property of radio synthesis telescopes is that spurious source responses decrease in amplitude with increasing angular separation between the interfering source and the field centre. This is due to both the delay beam (bandwidth smearing) and the primary beam envelope, as well as rejection by the synthesised beam itself. However, in certain special cases, ghost responses to sources far removed from the field centre can occur with significant amplitude. In particular, East-West linear arrays give rise to trans-equatorial ghosts, which are spurious responses to sources having declination equal in magnitude to that of the observed field, but opposite in sign. The trans-equatorial ghosts arise because the tracking required is independent of the sign of the declination. They can be present whenever the parent source is above the horizon and are then attenuated only by the primary beam – neither the delay beam nor the synthesised beam diminish the ghost response. Moreover, ghost sources may resemble real sources rather than easily recognised artifacts. Such ghosts have been seen in observations made with the Molonglo Observatory Synthesis Telescope, and in this case they are drawn out into small arcs by the minor departure of the telescope from an exact East-West array.