The final position actuator design (PACT) is based on a piezo / servo hybrid and is shown below. ESO signed the contract with PI in June 2017.
More information on the actuator selection criteria, and comparison between a hard (piezo) and soft (voice coil) solution can be found in this SPIE paper “Getting ready for serial production of the segmented 39-meter ELT primary: status, challenges and strategies”.
In order to achieve the optical performance of a single 39m diameter mirror (it is not feasible to one single telescope mirror of that size), all segments have to be aligned to each other to within nanometers all the time to overcome disturbances from temperature fluctuations, varying gravity effects, wind buffeting and vibration from the telescope structure during tracking motions of the telescope. Smallest alignment errors between the mirror segments will degrade the optical performance of the telescope.
The aperture of a telescope directly determines the light-collecting area, which in turn defines the exposure time. More area means more photons collected per exposure time enabling a deeper look into the universe.
With its total surface area of close to 1000 square meters, the ELT is supposed to collect around 100 million times more light than the human eye and boasts about 250 times the light gathering area of the Hubble Space Telescope.
The ELT is currently scheduled to see its first light in 2024.
More information is available on the ESO website