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Interferometric measurements of optical path length differences of stars overlarge baselines can deliver extremely accurate astrometric data. Theinterferometer GRAVITY will simultaneously measure two objects in the field ofview of the Very Large Telescope Interferometer (VLTI) of the European SouthernObservatory (ESO) and determine their angular separation to a precision of 10micro arcseconds in only 5 minutes. To perform the astrometric measurement withsuch a high accuracy, the differential path length through the VLTI and theinstrument has to be measured (and tracked since Earth's rotation willpermanently change it) by a laser metrology to an even higher level of accuracy(corresponding to 1 nm in 3 minutes). Usually, heterodyne differential pathtechniques are used for nanometer precision measurements, but with thesemethods it is difficult to track the full beam size and to follow the lightpath up to the primary mirror of the telescope. Here, we present thepreliminary design of a differential path metrology system, developed withinthe GRAVITY project. It measures the instrumental differential path over thefull pupil size and up to the entrance pupil location. The differential phaseis measured by detecting the laser fringe pattern both on the telescopes'secondary mirrors as well as after reflection at the primary mirror. Based onour proposed design we evaluate the phase measurement accuracy based on a fullbudget of possible statistical and systematic errors. We show that thismetrology design fulfills the high precision requirement of GRAVITY.

The Fringe Detection Laser Metrology for the GRAVITY Interferometer at the VLTI