Cophasing implies precise, active control of three degrees of freedom of each individual segment mirror: translation along the optical axis (piston) and rotation about two axes perpendicular to the optical axis (tip-tilt).
[1] Each segment of the segmented telescope is a solid body having 6 degrees of freedom exposed to the gravitation force, wind blowing, and other mechanical forces.
To achieve a resolution commensurable with that of a monolithic telescope of the same diameter the segmented surface must be controlled with a precision better than
Projects for future extremely large telescopes (ELTs) generally depend on the use of a segmented primary mirror.
While the basic technologies required for segmented telescopes have been demonstrated for the 10m Keck telescope or GTC telescope, ELTs of diameters form 50 to 100 m represent a qualitative change with respect to wave front control related to segmentation in comparison with the current 10 meters technology.