  Shack-Hartmann wavefront sensors for both solar and laser guide star adaptive
optics (with elongated spots) need to observe extended objects. Correlation
techniques have been successfully employed to measure the wavefront gradient in
solar adaptive optics systems and have been proposed for laser guide star
systems. In this paper we describe a method for synthesising reference images
for correlation Shack-Hartmann wavefront sensors with a larger field of view
than individual sub-apertures. We then show how these supersized reference
images can increase the performance of correlation wavefront sensors in regimes
where large relative shifts are induced between sub-apertures, such as those
observed in open-loop wavefront sensors. The technique we describe requires no
external knowledge outside of the wavefront-sensor images, making it available
as an entirely "software" upgrade to an existing adaptive optics system. For
solar adaptive optics we show the supersized reference images extend the
magnitude of shifts which can be accurately measured from 12% to 50% of the
field of view of a sub-aperture and in laser guide star wavefront sensors the
magnitude of centroids that can be accurately measured is increased from 12% to
25% of the total field of view of the sub-aperture.
