Extragalactic reference targets for PRIMA

The VLTI's PRIMA (Phase-Referenced Imaging and Microarcsecond Astrometry) instrument is designed to provide tens of microarcsecond astrometry and faint-object pairs that are separated on the sky by one isoplanatic patch (roughly 60 seconds of arc at Cerro Paranal in the K band), with atleast one of the two objects being bright (K<10), and a second fainter object (Delta K < 7) that is nominally a 'stable' astrometric reference. The original expectation was for star-star pairs to be abserved by PRIMA in its astrometric mode; however, we are exploring the possibility of also utilizing background galaxies in this role. Advantages of such source selection is climinating the need to solve for dim object parallax and proper motion before obtaining similar values for the bright foreground star. Additionally, data from the galaxy may be of scientific interest as well, potentially leading to characterizations of object morphology at milliarcsecond scales. Towards that end, we have begun observations with ESO's NTT to explore the sustainability of qualifying star-galaxy asterisms as potential PRIMA targets. Commissioning observations for PRIMA are slated to begin in Fall of 2008.