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(abridged) Extreme adaptive optics (XAO) encounters severe difficulties tocope with the high speed (>1kHz), high accuracy and high order requirements forfuture extremely large telescopes. An innovative high order adaptive opticssystem using a self-referenced Mach-Zehnder wavefront sensor (MZWFS) allowscounteracting these limitations. This sensor estimates very accurately thewavefront phase at small spatial scale by measuring intensity differencesbetween two outputs, with a $\lambda /4$ path length difference between its twolegs, but is limited in dynamic range due to phase ambiguity. During the pastfew years, such an XAO system has been studied by our team in the framework of8-meter class telescopes. In this work, we report on our latest results withthe XAO testbed recently installed in our lab, and dedicated to high contrastimaging with 30m-class telescopes (such as the E-ELT or the TMT). Afterreminding the principle of a MZWFS and describing the optical layout of ourexperiment, we will show the results of the assessment of the woofer-tweeterphase correctors, i.e., a Boston Micromachine continuous membrane deformablemirror (DM) and a Boulder Nonlinear Systems liquid crystal spatial lightmodulator (SLM). In particular, we will detail the calibration of the DM usingZygo interferometer metrology. Our method consists in the precise measurementof the membrane deformation while applying a constant deformation to 9 out of140 actuators at the same time. By varying the poke voltage across the DMoperating range, we propose a simple but efficient way of modeling the DMinfluence function using a Gaussian model. Finally, we show the DM flatteningon the MZWFS allowing to compensate for low order aberrations.

Development of an ELT XAO testbed using a Mach-Zehnder wavefront sensor: calibration of the deformable mirror