We have developed a new , adaptive cross - correlation ( ACC ) algorithm to estimate with high accuracy the shift as large as several pixels in two extended - scene images captured by a Shack - Hartmann wavefront sensor ( SH - WFS ) . It determines the positions of all of the extended - scene image cells relative to a reference cell using an FFT - based iterative image shifting algorithm . It works with both point - source spot images as well as extended scene images . We have also set up a testbed for extended0scene SH - WFS , and tested the ACC algorithm with the measured data of both point - source and extended - scene images . In this paper we describe our algorithm and present out experimental results .