Analysis of the island effect for ELT MICADO MAORY SCAO mode

The presence of 51-cm wide spiders supporting the secondary mirror of the Extremely Large Telescope breaks the wave-front continuity of the incoming light and suggests to consider the telescope pupil as 6 independent petals, each of them having a di erent piston value. It is therefore necessary to co-phase them in order to return to a single continuous wave-front. Not being corrected, these phase steps in the pupil would severely decrease the optical performance. As part of MICADO instrument Single Conjugate Adaptive Optics (SCAO), a Pyramid wave-front Sensor (P-WFS) is being used to provide a measurement of the aberrated wave-front. Di ractive optics enables to describe phase steps accross the pupil as a signal in the wave-front sensor focal plane: a piston applied on one of the 6 segments will create a speci c pattern on the detector plane of the P-WFS so that it should in principle be possible to detect it. However, in practice, the sensor is not sensitive enough to di erential piston due to other wave-front errors and it drives M4 to take unintended piston values on its six segments. This e ect is known as petalling. The adaptive optics end-to-end simulation platform COMPASS allows to perform numerical simulations in closed loop and quantify the impact of IE on the SCAO performance. Speci cally, we investigate the P-WFS di erential piston sensing capability under atmospheric turbulence, with remaining closed-loop residuals. We show that these residuals are read as spurious di erential piston : this error being additive throughout the time, the di erential piston varies rapidly and reaches values up to a few microns. To cope with this e ect, we set up a minimum mean-square error (MMSE) reconstructor dedicated to di erential piston estimation control and we compare the results with the already implemented slaved actuators method.