Deformable lens for testing the performance of focal plane wavefront sensing using phase diversity

SHARK-NIR is a high-contrast camera for the LBT, it has been conceived and designed to fully exploit the high Strehl ratio adaptive optics correction delivered by the FLAO module, which is being upgraded to SOUL, and will implement different coronagraphic techniques, with contrast as high as 10(-6) down to 5 mas from the star. To maximize the achievable contrast, SHARK-NIR implements a couple of peculiar features, namely a fast internal TT loop to minimize the residual jitter and a local non-common path aberration correction, applied through an internal deformable mirror.To derive instrumental aberrations, one option is to use the phase diversity technique, which allows wavefront sensing by using two images in intrafocus and extrafocus positions. To calibrate this sensor, we initially assembled a dedicated optical bench equipped with a deformable mirror and a fast IR camera. To characterize phase diversity under more general conditions and independently from the SHARK-NIR setup, we focused then on a new simple test bench, implementing a new (multi-actuator) deformable lens able to reproduce low order aberrations up to 4th order of Zemike polynomials, as an aberration generator. The optical design is simpler and cheaper than using a deformable mirror, which requires a folded optical path and more optics. In this case we used an interferometer to characterize the linearity of the aberration generator and the phase diversity is used for sensing non-common path aberrations (NCPA) in SHARK-NIR.