Segment phasing experiments on the High Order Test bench

The segmented primary mirror of the E-ELT imposes particular requirements on an Extreme Adap- tive Optics (XAO) system. At present, there are already several AO systems working on segmented telescopes but the achieved performances are too low to draw conclusions for XAO systems aiming at some 90% Strehl ratio in the NIR. On other hand, several analytical studies and simulations were done, but laboratory studies are required to confirm the corrections expected. The goal of the present study is to determina the capability of XAO systems to deal with segmentation piston errors. In particular, the e ffects on the AO performance and the ability of the AO system to correct the segmentation piston errors were studied. The experiments were carried out on the High Order Test Bench at ESO (Munich) using a Shack-Hartmann wave front sensor and under most realistic condi- tions with phase screens simulating atmospheric turbulence and segmentation piston errors. Segment geometry was chosen such that about 6 actuators of the XAO DM cover one segment representing the design of EPICS at the EELT. Extreme Adaptive Optics (XAO) at the E-ELT requires dealing with a primary segmented mirror. The wave front control of the ELT includes three main units: adaptive optics, active optics and phasing camera. Each wave front control is affected to some extend by the total wavefront, but, on the other hand the different control units can "help" each other. The wavefront sensor of the AO unit will see the telescope aberrations and segments misalignments; hence the AO deformable mirror will try to compensate for them. The goal of the present study is to determina the capability of the adaptive optics to deal with and correct for the segmentation piston errors. At present, there are several segmented telescope working with AO systems but the experience from them could not be use for extrapolation to a XAO system. Several analytical studies and simulations were done (2). From these studies, the ability of the AO system to correct for the segmentation errors strictly depends on the design of the DM and the wave front sensor. The density of the actuators, the coupling factor and the geometry of actuators distribution have directly impact on the performance. The ELT primary mirror will be composed by around 1000 hexagonal segments of 1.5 m with a gap of 10 mm. 5-7 actuators per segment are expected considering a deformable mirror of 200x200 actuators. The experiments were carried out on the HOT Bench using a Shack-Hartman WFS and taking into account the ELT geometry and the requirements for a XAO instrument. They will also be run under realistic atmospheric conditions.