Real-time correlation reference update for astronomical adaptive optics
  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. 
Alastair Basden
astro-ph.IM