The metrology system of the VLTI instrument GRAVITY
  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. 
Magdalena Lippa, Stefan Gillessen, Nicolas Blind, Yipting Kok, Senol   Yazici, Johannes Weber, Oliver Pfuhl, Marcus Haug, St...
astro-ph.IM