In the framework of the Active Phasing Experiment (APE), four different phasing techniques are tested. The
ZErnike Unit for Segment phasing sensor (ZEUS) is integrated on the APE bench. APE has been tested in
the laboratory before it will be installed on one of the Nasmyth platform of a Very Large Telescope (VLT)
Unit Telescope to perform on sky tests. The ZEUS phasing sensor concept has its origins in the Mach-Zehnder
interferometer equipped with a spatial filter in its focal plane. In this paper, the ZEUS phasing sensor is
described together with its theoretical background and deployment within the APE experiment. The algorithms
and its elements used to reconstruct the wavefront are described. Finally, the preliminary results obtained in
the laboratory are presented.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.