MAORY/MORFEO @ ELT: preliminary design of the adaptive optics subsystem

Multiconjugate adaptive Optic Relay For ELT Observations (MORFEO, formerly known as MAORY) is the multiconjugate adaptive optics system of the ELT, providing a diffraction limited correction over a 60" field to the near infrared spectroimager MICADO; it recently completed the preliminary design phase. MORFEO's tomographic sensing is based on 12 WaveFront Sensors (WFS) working with 6 Laser Guide Stars (LGS) and 3 Natural Guide Star (NGS) and the wavefront correction is realized through the ELT's adaptive M4 and up to 2 post-focal deformable mirrors. Under median conditions, MORFEO will provide about 35% SR in K band with a 50% sky coverage at the galactic pole, and 55% SR in optimal conditions with a SR variation of <= 10% over a 60" corrected field. In this work we briefly summarize the key aspects of the AO system, focusing on the analysis that motivates the main design choices for wavefront sensors, deformable mirrors, real-time control and the many auxiliary loops envisaged to maintain an optimal and stable correction during the observation.