First faint dual-field phase-referenced observations on the Keck interferometer

TL;DR

This paper reports the first successful faint dual-field phase-referenced observations with the Keck interferometer, enabling longer integrations on faint targets by correcting atmospheric turbulence effects in real-time.

Contribution

It introduces the implementation and on-sky performance of the Dual Field Phase Referencing instrument on the Keck interferometer, demonstrating improved sensitivity for faint targets.

Findings

Achieved observation of a K=11.5 target, fainter than previous limits.

Demonstrated real-time atmospheric turbulence correction.

Showed stable fringe contrast with Laser Guide Star AO.

Abstract

Ground-based long baseline interferometers have long been limited in sensitivity by the short integration periods imposed by atmospheric turbulence. The first observation fainter than this limit was performed on January 22, 2011 when the Keck Interferometer observed a K=11.5 target, about one magnitude fainter than its K=10.3 limit. This observation was made possible by the Dual Field Phase Referencing instrument of the ASTRA project: simultaneously measuring the real-time effects of the atmosphere on a nearby bright guide star, and correcting for it on the faint target, integration time longer than the turbulence time scale are made possible. As a prelude to this demonstration, we first present the implementation of Dual Field Phase Referencing on the interferometer. We then detail its on-sky performance focusing on the accuracy of the turbulence correction, and on the resulting fringe contrast stability. We conclude with a presentation of early results obtained with Laser Guide Star AO and the interferometer.