Laboratory validation of the CiaoCiao wavefront sensor for phase discontinuity detection in fragmented-pupil optical telescopes

Abstract

The next generation of ground-based optical telescopes will feature fragmented pupils, such as the six-petal geometry of the Extremely Large Telescope (ELT), introducing differential piston errors between disconnected pupil regions. These phase discontinuities are challenging for conventional adaptive optics wavefront sensors and can lead to degraded correction performance. The CiaoCiao wavefront sensor (WFS) was developed as a rotational shearing interferometer specifically designed to measure differential piston in fragmented pupils. In this work, we present an improved laboratory implementation of the CiaoCiao concept developed at the INAF–Arcetri Astrophysical Observatory. End-to-end simulations are used to assess the sensor performance under realistic adaptive optics conditions, while a compact four-sided Sagnac interferometer is employed for the experimental validation. A six-petal segmented mirror reproducing the ELT fragmented pupil is used to generate controlled differential piston patterns while an ALPAO DM88 deformable mirror introduces global pupil aberrations for sensitivity characterization. Experimental measurements demonstrate a reconstructed piston repeatability of approximately 15nm peak to-peak under static conditions and a linear response for injected differential piston amplitudes up to approximately 200nm. The modal reconstruction successfully retrieves global aberrations with limited modal cross-talk, validating the capability of the CiaoCiao WFS for fragmented-pupil wavefront sensing. Finally, a compact optical architecture is proposed as a step towards future telescope implementation.