The VLTI instrument GRAVITY combines the beams from four telescopes and provides phase-referenced imaging as well as precision-astrometry of order 10 microarcseconds by observing two celestial objects in dual-field mode. Their angular separation can be determined from their differential OPD (dOPD) when the internal dOPDs in the interferometer are known. Here, we present the general overview of the novel metrology system which performs these measurements. The metrology consists of a three-beam laser system and a homodyne detection scheme for three-beam interference using phase-shifting interferometry in combination with lock-in amplifiers. Via this approach the metrology system measures dOPDs on a nanometer-level.
@article{arxiv.1608.04888,
title = {The metrology system of the VLTI instrument GRAVITY},
author = {Magdalena Lippa and Stefan Gillessen and Nicolas Blind and Yipting Kok and Senol Yazici and Johannes Weber and Oliver Pfuhl and Marcus Haug and Stefan Kellner and Ekkehard Wieprecht and Frank Eisenhauer and Reinhard Genzel and Oliver Hans and Frank Haussmann and David Huber and Tobias Kratschmann and Thomas Ott and Markus Plattner and Christian Rau and Eckhard Sturm and Idel Waisberg and Erich Wiezorrek and Guy Perrin and Karine Perraut and Wolfgang Brandner and Christian Straubmeier and Antonio Amorim},
journal= {arXiv preprint arXiv:1608.04888},
year = {2016}
}