  The use of laser guide stars in astronomical adaptive optics results in
elongated Shack-Hartmann wavefront sensor image patterns. Image correlation
techniques can be used to determine local wavefront slope by correlating each
sub-aperture image with its expected elongated shape, or reference image. Here,
we present a technique which allows the correlation reference images to be
updated while the adaptive optics loop is closed. We show that this can be done
without affecting the resulting point spread functions. On-sky demonstration is
reported. We compare different techniques for obtaining the reference images,
and investigate performance over a wide range of adaptive optics system
parameters. We find that image correlation techniques perform better than the
standard centre-of-gravity algorithm and are highly suited for use with
open-loop multiple object adaptive optics systems.
