  Small inner working angle coronagraphs are essential to benefit from the full
potential of large and future extremely large ground-based telescopes,
especially in the context of the detection and characterization of exoplanets.
Among existing solutions, the vortex coronagraph stands as one of the most
effective and promising solutions. However, for focal-plane coronagraph, a
small inner working angle comes necessarily at the cost of a high sensitivity
to pointing errors. This is the reason why a pointing control system is
imperative to stabilize the star on the vortex center against pointing drifts
due to mechanical flexures, that generally occur during observation due for
instance to temperature and/or gravity variations. We have therefore developed
a technique called QACITS (Quadrant Analysis of Coronagraphic Images for
Tip-tilt Sensing), which is based on the analysis of the coronagraphic image
shape to infer the amount of pointing error. It has been shown that the flux
gradient in the image is directly related to the amount of tip-tilt affecting
the beam. The main advantage of this technique is that it does not require any
additional setup and can thus be easily implemented on all current facilities
equipped with a vortex phase mask. In this paper, we focus on the
implementation of the QACITS sensor at Keck/NIRC2, where an L-band AGPM has
been recently commissioned (June and October 2015), successfully validating the
QACITS estimator in the case of a centrally obstructed pupil. The algorithm has
been designed to be easily handled by any user observing in vortex mode, which
is available for science in shared risk mode since 2016B.
