Astrometry with the VLTI: calibration of the Fringe Sensor Unit for the PRIMA astrometric camera

The future PRIMA facility at the Very Large Telescope Interferometer (VLTI) in astrometric mode offers the possibility to perform relative narrow-angle astrometry with 10 micro-arcsecond accuracy. This is achieved with a dual-beam interferometer concept, where a reference star and the scientific target, confined in a 60 arcsecond field, are observed simultaneously. The angular separation of the two stellar objects gives rise to an optical delay in the interferometer, which is measured by the Fringe Sensor Unit (FSU) and an internal laser metrology. PRIMA is using two FSU fringe detectors, each observing the interference of stellar beams coming from one of the two objects and measuring the corresponding phase and group delay. The astrometric observable, yielding the angular separation, is deduced from the group delay difference observed between the two objects. In addition, the FSU phase delay estimate is used as error signal for the fringe stabilisation loop of the VLTI. Both functions of the FSU require high precision fringe phase measurements with a goal of 1 nm rms (corresponding to λ/2000). These can only be achieved by applying a calibration procedure prior to the observing run. We discuss the FSU measurement principle and the applied algorithms. The calibration strategy and the methods used to derive the calibration parameters are presented. Special attention is given to the achieved measurement linearity and repeatability. The quality of the FSU calibration is crucial in order to achieve the ultimate accuracy and to fulfill the primary objective of PRIMA astrometry: the detection and characterisation of extrasolar planetary system

The PRIMA fringe sensor unit

The Fringe Sensor Unit (FSU) is the central element of the Phase Referenced Imaging and Micro-arcsecond Astrometry (PRIMA) dual-feed facility and provides fringe sensing for all observation modes, comprising off-axis fringe tracking, phase referenced imaging, and high-accuracy narrow-angle astrometry. It is installed at the Very Large Telescope Interferometer (VLTI) and successfully servoed the fringe tracking loop during the initial commissioning phase. Unique among interferometric beam combiners, the FSU uses spatial phase modulation in bulk optics to retrieve real-time estimates of fringe phase after spatial filtering. A R=20 spectrometer across the K-band makes the retrieval of the group delay signal possible. The FSU was integrated and aligned at the VLTI in summer 2008. It yields phase and group delay measurements at sampling rates up to 2 kHz, which are used to drive the fringe tracking control loop. During the first commissioning runs, the FSU was used to track the fringes of stars with K-band magnitudes as faint as m_K=9.0, using two VLTI Auxiliary Telescopes (AT) and baselines of up to 96 m. Fringe tracking using two Very Large Telescope (VLT) Unit Telescopes (UT) was demonstrated. During initial commissioning and combining stellar light with two ATs, the FSU showed its ability to improve the VLTI sensitivity in K-band by more than one magnitude towards fainter objects, which is of fundamental importance to achieve the scientific objectives of PRIMA.Comment: 19 pages, 23 figures. minor changes and language editing. this version equals the published articl

First sky validation of an optical polarimetric interferometer

International audienceAims.We present the first lab and sky validation of spectro-polarimetric equipment put at the combined focus of an optical long-baseline interferometer. We tested the polarimetric mode designed for the visible GI2T Interferometer to offer spectropolarimetric diagnosis at the milliarcsecond scale. Methods.We first checked the whole instrumental polarization in the lab with a fringe simulator, and then we observed α Cep and α Lyr as stellar calibrators of different declinations to tabulate the polarization effects throughout the GI2T declination range. Results.The difference between both linear polarizations is within the error bars and the visibilities recorded in natural light (i.e. without the polarimeter) for calibration purposes are the same order of magnitude as the polarized ones. We followed the α Cep visibility for 2 h after the transit and α Lyr for 1.5 h and detected no decrease with hour angle due to the fringe pattern smearing by instrumental polarization. Conclusions.Differential celestial rotation due to the dissymetric Coudé trains of the GI2T is well-compensated by the field rotators, so the instrumental polarization is controlled over a relatively wide hour angle range (±2 h around the transit at least). Such a polarimetric mode opens new opportunities especially for studies of circumstellar environments and significantly enhances both the potential of an optical array and its ability for accurate calibration

PRIMA for the VLTI : a status report

n/a, SPIE n/a, (2006)International audienc

The VLTI and its Subsystems

The Very Large Telescope (VLT) Observatory on Cerro Paranal (2635 m) in Northern Chile is approaching completion. After the four 8-m Unit Telescopes (UT) individually saw first light in the last years, two of them were combined for the first time on October 30, 2001 to form a stellar interferometer, the VLT Interferometer. The remaining two UTs will be integrated into the interferometric array later this year, so that any two UTs can be used for interferometry. In this article, we will describe the subsystems of the VLTI and the planning for the following years