The fringe detection laser metrology for the GRAVITY interferometer at the VLTI

Abstract

Interferometric measurements of optical path length differences of stars over large baselines can deliver extremely accurate astrometric data. The interferometer GRAVITY will simultaneously measure two objects in the field of view of the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO) and determine their angular separation to a precision of 10 µas in only 5 minutes. To perform the astrometric measurement with such a high accuracy, the differential path length through the VLTI and the instrument has to be measured (and tracked since Earth's rotation will permanently change it) by a laser metrology to an even higher level of accuracy (corresponding to 1 nm in 3 minutes). Usually, heterodyne differential path techniques are used for nanometer precision measurements, but with these methods it is difficult to track the full beam size and to follow the light path up to the primary mirror of the telescope. Here, we present the preliminary design of a differential path metrology system, developed within the GRAVITY project. It measures 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 both on the telescopes' secondary mirrors as well as after reflection at the primary mirror. Based on our proposed design we evaluate the phase measurement accuracy based on a full budget of possible statistical and systematic errors. We show that this metrology design fulfills the high precision requirement of GRAVITY.

Slowly rotating black holes in Einstein-Dilaton-Gauss-Bonnet gravity: Quadratic order in spin solutions

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“…the separation of 2 objects, at a level of the order of 10 micro-arcseconds on sky. To achieve this, a sophisticated metrology system has been designed [5]. The basic idea of the metrology system is to sample the optical path of the incoming telescope beams as complete as possible, with a precision of about 1nm (rms differential optical path difference of 2 objects separated by about 1" on sky).…”

GRAVITY: metrology

“…The basic working principle is the same as described for the preliminary design 4,5 . A laser beam is injected backwards at the two beam combiners, in a way that one of the two paths includes a phase shifter by which the relative delay between the two beams can be adjusted.…”