The PRIMA fringe sensor unit

摘要

Context. The fringe sensor unit (FSU) is the central elementof the phase referenced imaging and micro-arcsecond astrometry (PRIMA)dual-feed facility and provides fringe sensing for all observationmodes, comprising off-axis fringe tracking, phase referenced imaging,and high-accuracy narrow-angle astrometry. It is installed at the VeryLarge Telescope Interferometer (VLTI) and successfully served thefringe-tracking loop during the initial commissioning phase. Aims. To maximise sensitivity, speed, and robustness, the FSU is designed to operate in the infrared K-bandand to include spatial filtering after beam combination and avery-low-resolution spectrometer without photometric channels. Itconsists of two identical fringe sensors for dual-star operation inPRIMA astrometric mode. Methods. Unique among interferometric beam combiners, the FSUuses spatial phase modulation in bulk optics to retrieve real-timeestimates of fringe phase after spatial filtering. The beam combinationdesign accommodates a laser metrology for pathlength monitoring. An R=20 spectrometer across the K-bandmakes the retrieval of the group delay signal possible. The calibrationprocedure uses the artificial light source of the VLTI laboratory andis based on Fourier transform spectroscopy to remove instrumentaleffects. Results. The FSU was integrated and aligned at the VLTI in Julyand August 2008. It yields phase and group delay measurements atsampling rates up to 2kHz, which are used to drive thefringe-tracking control loop. During the first commissioning runs, theFSU was used to track the fringes of stars with K-band magnitudes as faint as mK=9.0,using two VLTI auxiliary telescopes (AT) and baselines of up to96m. Fringe tracking using two Very Large Telescope (VLT) unittelescopes was demonstrated. Conclusions. The concept of spatial phase-modulation for fringesensing and tracking in stellar interferometry is demonstrated for thefirst time with the FSU. During initial commissioning and combiningstellar light with two ATs, the FSU showed its ability to improve theVLTI sensitivity in K-band by more than one magnitude towardsfainter objects, which is fundamental for achieving the scientificobjectives of PRIMA. Key words: instrumentation: interferometers - techniques: interferometric - astrometry