The way to Xao: A system to correct NCP and TT aberrations tested in lab for SHARK-NIR

SHARK-NIR is the coronagraphic camera for direct imaging and spectroscopy designed for LBT, currently in its AIV phase. Exo-planets are the main scientific targets of the instrument as well as galactic jets and disks. Exploiting SOUL, the upgraded LBT AO system, will push the sensitivity in the faint end regime, allowing observations of extragalactic AGNs and QSOs. LTB AO system feeds SHARK-NIR with an already corrected wavefront, which enters the instrument channel when reflected by a dichroic: the transmitted light goes to the wavefront sensor. The AO correction residuals are of the order of 50 nm, moreover, there is a contribution (~100 nm) called Non Common Path Aberrations (NCPA), mainly due to manufacturing and alignment issues, and to flexures and temperature variations affecting the optical elements, from the dichroic along the instrument. To reach very high contrast with direct imaging, the coronagraphs need a smaller wavefront residual (not exceeding 40 nm), and a reduced residual jitter: from ~15 mas to 3-5 mas. The current configuration foresees an internal Deformable Mirror (DM) to compensate both NCPA and residual tip-tilt. The strategy consists in evaluating the NCPA before each observation, adequately adapting the DM, finally closing the tip-tilt loop in real time, maintaining the NCPA shape. We set up a test-bench to measure the DM performance, applying a realistic NCPA shape and then closing the Tip-Tilt loop using the technical camera foreseen for the instrument. Tip-Tilt corrections are retrieved by centroid analysis comparing the rms of a time history introduced in the system, i.e. a typical Tip-Tilt residual over a certain amount of time, when closing the loop. We check the DM shape using a Shack-Hartmann sensor. In this paper, we report the results for different simulated magnitudes, time history amplitudes and loop frequencies, choosing these parameters according to SHARK-NIR science cases.