The GRAVITY metrology system: narrow-angle astrometry via phase-shifting interferometry

TL;DR

The paper describes a novel phase-shifting interferometry metrology system for the GRAVITY instrument, achieving nanometer-level precision in measuring optical path differences to enable microarcsecond astrometry.

Contribution

It introduces a four-step phase-shifting method with a phase-step insensitive calibration algorithm for high-precision internal path difference measurements.

Findings

Achieved phase measurement accuracy of lambda/2000 in laboratory conditions.

Developed a robust ellipse fitting routine for phase calibration.

Demonstrated the system's capability to support 10 microarcsecond astrometry.

Abstract

The VLTI instrument GRAVITY will provide very powerful astrometry by combining the light from four telescopes for two objects simultaneously. It will measure the angular separation between the two astronomical objects to a precision of 10 microarcseconds. This corresponds to a differential optical path difference (dOPD) between the targets of few nanometers and the paths within the interferometer have to be maintained stable to that level. For this purpose, the novel metrology system of GRAVITY will monitor the internal dOPDs by means of phase-shifting interferometry. We present the four-step phase-shifting concept of the metrology with emphasis on the method used for calibrating the phase shifts. The latter is based on a phase-step insensitive algorithm which unambiguously extracts phases in contrast to other methods that are strongly limited by non-linearities of the phase-shifting device. The main constraint of this algorithm is to introduce a robust ellipse fitting routine. Via this approach we are able to measure phase shifts in the laboratory with a typical accuracy of lambda/2000 or 1 nanometer of the metrology wavelength.