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Context. The GRAVITY instrument was commissioned on the VLTI in 2016 and is now available to the astronomical community. It is the first optical interferometer capable of observing sources as faint as magnitude 19 in K band. This is possible through the fringe tracker, which compensates the differential piston based on measurements of a brighter off-axis astronomical reference source. Aims. The goal of this paper is to describe the main developments made in the context of the GRAVITY fringe tracker. This could serve as basis for future fringe-tracking systems. Methods. The paper therefore covers all aspects of the fringe tracker, from hardware to control software and on-sky observations. Special emphasis is placed on the interaction between the group-delay controller and the phase-delay controller. The group-delay control loop is a simple but robust integrator. The phase-delay controller is a state-space control loop based on an auto-regressive representation of the atmospheric and vibrational perturbations. A Kalman filter provides the best possible determination of the state of the system. Results. The fringe tracker shows good tracking performance on sources with coherent K magnitudes of 11 on the Unit Telescopes (UTs) and 9.5 on the Auxiliary Telescopes (ATs). It can track fringes with a signal-to-noise ratio of 1.5 per detector integration time, limited by photon and background noises. During good seeing conditions, the optical path delay residuals on the ATs can be as low as 75 nm root mean square. The performance is limited to around 250 nm on the UTs because of structural vibrations. [ABSTRACT FROM AUTHOR]

GRAVITY is a new instrument to coherently combine the light of the European Southern Observatory Very Large Telescope Interferometer to form a telescope with an equivalent 130m diameter angular resolution and a collecting area of 200m². The instrument comprises fiber fed integrated optics beam combination, high resolution spectroscopy, built-in beam analysis and control, near-infrared wavefront sensing, phasetracking, dual-beam operation, and laser metrology. GRAVITY opens up to optical/infrared interferometry the techniques of phase referenced imaging and narrow angle astrometry, in many aspects following the concepts of radio interferometry. This article gives an overview of GRAVITY and reports on the performance and the first astronomical observations during commissioning in 2015/16. We demonstrate phase-tracking on stars as faint as mK ≈ 10 mag, phase-referenced interferometry of objects fainter than mK ≈ 15 mag with a limiting magnitude of mK ≈ 17 mag, minute long coherent integrations, a visibility accuracy of better than 0:25%, and spectro-differential phase and closure phase accuracy better than 0:5°, corresponding to a differential astrometric precision of better than ten microarcseconds (µas). The dual-beam astrometry, measuring the phase difference of two objects with laser metrology, is still under commissioning. First observations show residuals as low as 50 µas when following objects over several months. We illustrate the instrument performance with the observations of archetypical objects for the different instrument modes. Examples include the Galactic center supermassive black hole and its fast orbiting star S2 for phase referenced dual-beam observations and infrared wavefront sensing, the high mass X-ray binary BP Cru and the active galactic nucleus of PDS 456 for a few µas spectro-differential astrometry, the T Tauri star S CrA for a spectro-differential visibility analysis, ζ Tel and 24 Cap for high accuracy visibility observations, and η Car for interferometric imaging with GRAVITY. [ABSTRACT FROM AUTHOR]

To enable optical long baseline interferometry toward faint objects, long integrations are necessary despite atmospheric turbulence. Fringe trackers are needed to stabilize the fringes and thus increase the fringe visibility and phase signal-to-noise ratio (S/N), with efficient controllers robust to instrumental vibrations and to subsequent path fluctuations and flux drop-outs. We report on simulations, analysis, and comparison of the performances of a classical integrator controller and of a Kalman controller, both optimized to track fringes under realistic observing conditions for different source magnitudes, disturbance conditions, and sampling frequencies. The key parameters of our simulations (instrument photometric performance, detection noise, turbulence, and vibrations statistics) are based on typical observing conditions at the Very Large Telescope observatory and on the design of the GRAVITY instrument, a 4-telescope single-mode long-baseline interferometer in the near-infrared, next in line to be installed at VLT Interferometer. We find that both controller performances follow a two-regime law with the star magnitude, a constant disturbance limited regime, and a diverging regime limited by both the detector and photon-noise. Moreover, we find that the Kalman controller is optimal in the high and medium S/N regime owing to its predictive commands based on an accurate disturbance model. In the low S/N regime, the model is not accurate enough to be more robust than an integrator controller. Identifying the disturbances from high S/N measurements improves the Kalman performances in the case of strong optical path difference disturbances. [ABSTRACT FROM AUTHOR]

PEGASE is a mission dedicated to the exploration of the environment (including habitable zone) of young and solar-type stars (particularly those in the DARWIN catalogue) and the observation of low mass companions around nearby stars. It is a space interferometer project composed of three free flying spacecraft, respectively featuring two 40 cm siderostats and a beam combiner working in the visible and near infrared. It has been proposed to ESA as an answer to the first “Cosmic Vision” call for proposals, as an M mission. The concept also enables full-scale demonstration of space nulling interferometry operation for DARWIN. [ABSTRACT FROM AUTHOR]