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The PRIMA fringe sensor unit
Context: The fringe sensor unit (FSU) is the central element of thephase 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 isdesigned to operate in the infrared K-band and to include spatialfiltering after beam combination and a very-low-resolution spectrometerwithout photometric channels. It consists of two identical fringesensors for dual-star operation in PRIMA astrometric mode. Methods: Unique among interferometric beam combiners, the FSU usesspatial phase modulation in bulk optics to retrieve real-time estimatesof fringe phase after spatial filtering. The beam combination designaccommodates a laser metrology for pathlength monitoring. An R = 20spectrometer across the K-band makes the retrieval of the group delaysignal possible. The calibration procedure uses the artificial lightsource of the VLTI laboratory and is based on Fourier transformspectroscopy to remove instrumental effects. Results: TheFSU was integrated and aligned at the VLTI in July and August 2008. Ityields phase and group delay measurements at sampling rates up to 2 kHz,which are used to drive the fringe-tracking control loop. During thefirst commissioning runs, the FSU was used to track the fringes of starswith K-band magnitudes as faint as mK = 9.0, using two VLTIauxiliary telescopes (AT) and baselines of up to 96 m. Fringe trackingusing two Very Large Telescope (VLT) unit telescopes wasdemonstrated. Conclusions: The concept of spatialphase-modulation for fringe sensing and tracking in stellarinterferometry is demonstrated for the first time with the FSU. Duringinitial commissioning and combining stellar light with two ATs, the FSUshowed its ability to improve the VLTI sensitivity in K-band by morethan one magnitude towards fainter objects, which is fundamental forachieving the scientific objectives of PRIMA.Part of this work is based on technical observations collected at theEuropean Southern Observatory at Paranal, Chile. Public data can bedownloaded athttp://www.eso.org/sci/activities/vltcomm/prima/PACMAN_CommDataRelease_text.html.
| The VLTI PRIMA Facility
The Phase Referenced Imaging and Microarcsecond Astrometry (PRIMA)instrument was recently delivered to the summit of Cerro Paranal andinstalled as part of the Very Large Telescope Interferometer (VLTI)infrastructure. PRIMA is designed to (i) provide phase-referencedinterferometric imaging at milliarcsecond scales, (ii) enable faint starscience several magnitudes fainter than the current atmospheric limitsof the VLTI, and (iii) provide astrometric measurements at the tens ofmicro-arcsecond level. PRIMA has successfully seen first fringes and iscurrently (as of late 2008) undergoing initial commissioning tests.
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