  The Multiconjugate adaptive Optic Relay For ELT Observations (MORFEO,
formerly known as MAORY) is the adaptive optics (AO) module for the Extremely
Large Telescope (ELT) aimed at providing a 1 arcmin corrected field to the
Multi-AO Imaging CamerA for Deep Observations (MICADO) and to a future client
instrument. It should provide resolution close to the diffraction limit on a
large portion of the sky and in a wide range of atmospheric conditions. Its
ability to provide a flat wavefront must face the known aspect of the
atmospheric turbulence and telescope environment, but also the final
characteristic of a telescope still to be fully developed and built. In this
work we focused on issues related to the segmentation of the telescope pupil
(like low wind effect, residual phasing error at handover and control related
issues), that could limit the system performance. MORFEO currently does not
foresee a dedicated sensor to measure the phase step between adjacent mirror
segments: in this work we study the possibility to use the low order wavefront
sensors designed to sense and correct tip-tilt and focus as phasing sensors
thanks to the linearized focal-plane technique (LIFT).
