  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.
