P-REx II. Off-line Performance on VLTI/GRAVITY

TL;DR

This paper evaluates the P-REx method's off-line performance on VLTI/GRAVITY data, showing its potential to significantly improve optical path difference measurements and enhance fringe tracking sensitivity if noise levels are reduced.

Contribution

The study applies P-REx off-line to VLTI/GRAVITY data, demonstrating its potential to reduce OPD noise and improve interferometric sensitivity in optical astronomy.

Findings

Good agreement between simulated and actual P-REx data.

Telescope and instrumental noise currently dominate over atmospheric turbulence.

Potential to reduce OPD RMS by up to a factor of 10 if noise is minimized.

Abstract

For sensitive optical interferometry, it is crucial to control the evolution of the optical path difference (OPD) of the wavefront between the individual telescopes of the array. The OPD between a pair of telescopes is induced by differential optical properties such as atmospheric refraction, telescope alignment, etc. This has classically been measured using a fringe tracker that provides corrections to a piston actuator to account for this difference. An auxiliary method, known as the Piston Reconstruction Experiment (P-REx) has been developed to measure the OPD, or differential 'piston' of the wavefront, induced by the atmosphere at each telescope. Previously, this method was outlined and results obtained from LBT adaptive optics (AO) data for a single telescope aperture were presented. P-REx has now been applied off-line to previously acquired VLT's GRAVITY CIAO wavefront sensing data to estimate the atmospheric OPD for the six VLTI baselines. Comparisons with the OPD obtained from the VLTI GRAVITY fringe tracker were made. The results indicate that the telescope and instrumental noise of the combined VLTI and GRAVITY systems dominate over the atmospheric turbulence contributions. However, good agreement between simulated and on-sky P-REx data indicates that if the telescope and instrumental noise were reduced to atmospheric piston noise levels, P-REx has the potential to reduce the OPD root mean square of piston turbulence by up to a factor of 10 for frequencies down to 1 Hz. In such conditions, P-REx will assist in pushing the sensitivity limits of optical fringe tracking with long baseline interferometers.