Abstract
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.
Highlights
GRAVITY (Gravity Collaboration 2017) is an instrument used on the Very Large Telescope Interferometer (VLTI) situated at the Cerro Paranal Observatory
The phase delay Φi, j is derived from the complex coherent flux, but after a first step to correct for the phase curvature caused by the dispersion:
After the reference values are subtracted from the optical path differences (OPD), we can freely project the data in piston-state space as well as back to the OPD-state space
Summary
GRAVITY (Gravity Collaboration 2017) is an instrument used on the Very Large Telescope Interferometer (VLTI) situated at the Cerro Paranal Observatory. The specifications of the instrument were derived from the most demanding science case, which was to observe microarcsecond displacements of the light source causing the flares of the supermassive black hole Sgr A* (Genzel et al 2010, and references ) Such astrometric measurements are possible with 100 m baselines (Shao & Colavita 1992; Lacour et al 2014) and were recently demonstrated on-sky by Gravity Collaboration (2018a,b,c). The optical path differences (OPD) between each pair of telescopes are computed, and are used to control the displacement of mirrors on piezoelectric systems. This is the counterpart of the AO system, but at interferometric scale.
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