  In this paper we report on the laboratory experiment we settled in the
Shanghai Astronomical Observatory (SHAO) to investigate the pyramid wavefront
sensor (WFS) ability to measure the differential piston on a sparse aperture.
The ultimate goal is to verify the ability of the pyramid WFS work in closed
loop to perform the phasing of the primary mirrors of a sparse Fizeau imaging
telescope. In the experiment we installed on the optical bench we performed
various test checking the ability to flat the wave-front using a deformable
mirror and to measure the signal of the differential piston on a two pupils
setup. These steps represent the background from which we start to perform full
closed loop operation on multiple apertures. These steps were also useful to
characterize the achromatic double pyramids (double prisms) manufactured in the
SHAO optical workshop.
