Fringe detection laser metrology for differential astrometric stellar interferometers

Differential measurements with dual feed stellar interferometers using large baselines can deliver extremely accurate astrometric data. Separating the phase difference measured on the stars from the path length differences occurring within the interferometric instrument itself requires the use of laser interferometers. Usually heterodyne differential path techniques are used for nanometer precision measurements. With these methods it is usually difficult to track the full beam size and follow the light path up to the secondary mirror. We will report on the concept and first tests of a differential path metrology system, developed within the GRAVITY project, that allows one to measure the instrumental differential path over the full pupil size and up to the entrance pupil location. The differential phase is measured by detecting the laser fringe pattern created on the telescopes' secondaries. This novel method is almost free from systematic errors since the stellar and laser light are traveling along a common optical path.