First results from fringe tracking with the PRIMA fringe sensor unit

Abstract

The fringe sensor unit (FSU) is the central element of the phase referenced imaging and micro-arcsecond astrometry (PRIMA) dual-feed facility for the Very Large Telescope interferometer (VLTI). It has been installed at the Paranal observatory in August 2008 and is undergoing commissioning and preparation for science operation. Commissioning observations began shortly after installation and first results include the demonstration of spatially encoded fringe sensing and the increase in VLTI limiting magnitude for fringe tracking. However, difficulties have been encountered because the FSU does not incorporate real-time photometric correction and its fringe encoding depends on polarisation. These factors affect the control signals, especially their linearity, and can disturb the tracking control loop. To account for this, additional calibration and characterisation efforts are required. We outline the instrument concept and give an overview of the commissioning results obtained so far. We describe the effects of photometric variations and beam-train polarisation on the instrument operation and propose possible solutions. Finally, we update on the current status in view of the start of astrometric science operation with PRIMA.Comment: 12 pages, 11 figures, SPIE 2010 conference proceeding

A dispersed heterodyne design for the planet formation imager

“…Based on simple SNR considerations and anticipated full system H-band efficiencies exceeding 1% (including effects of fiber coupling), fringe tracking on a H=7.5 star should in principle be relatively straightforward for telescope diameters greater than 1 m and modern, low noise detectors. 11 However, with the last generation of detectors, active fringe tracking has proved moderately difficult, with magnitude limits of H∼7.5 on the challenging end for the VLTI with the 1.8 m Auxiliary telescopes, 12 but certainly anticipated as routine with adaptive optics and new detectors. For this reason, a minimum telescope diameter for 2 m is needed for PFI.…”