  At present, the principal limitation on the resolution and contrast of
astronomical imaging instruments comes from aberrations in the optical path,
which may be imposed by the Earth's turbulent atmosphere or by variations in
the alignment and shape of the telescope optics. These errors can be corrected
physically, with active and adaptive optics, and in post-processing of the
resulting image. A recently-developed adaptive optics post-processing
technique, called kernel phase interferometry, uses linear combinations of
phases that are self-calibrating with respect to small errors, with the goal of
constructing observables that are robust against the residual optical
aberrations in otherwise well-corrected imaging systems. Here we present a
direct comparison between kernel phase and the more established competing
techniques, aperture masking interferometry, point spread function (PSF)
fitting and bispectral analysis. We resolve the alpha Ophiuchi binary system
near periastron, using the Palomar 200-Inch Telescope. This is the first case
in which kernel phase has been used with a full aperture to resolve a system
close to the diffraction limit with ground-based extreme adaptive optics
observations. Excellent agreement in astrometric quantities is found between
kernel phase and masking, and kernel phase significantly outperforms PSF
fitting and bispectral analysis, demonstrating its viability as an alternative
to conventional non-redundant masking under appropriate conditions.
