First results from fringe tracking with the PRIMA fringe sensor unit

TL;DR

This paper reports initial results from commissioning the PRIMA fringe sensor unit at the VLTI, demonstrating spatial fringe sensing and increased limiting magnitude, while discussing calibration challenges and operational considerations.

Contribution

It presents the first commissioning results of the PRIMA fringe sensor unit, highlighting its capabilities and addressing calibration issues affecting fringe tracking performance.

Findings

Demonstrated spatially encoded fringe sensing

Increased VLTI limiting magnitude for fringe tracking

Identified calibration challenges due to polarization and photometric variations

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.