ELT system performance optimization at the 50% completion milestone

The ELT construction programme some months ago passed the 50% completion in terms of earned value. In the last couple of years, the detailed design phase of the ELT was finalised and many subsystems are already under construction (some close to be delivered to ESO). As part of the final design consolidation, there was still room for optimizing the diffractionlimited performance of the telescope. A significant effort has been devoted to investigating potential areas of improvement in the as-designed ELT system. The most prominent cases are local-seeing reduction, pupil fragmentation mitigation and vibration rejection. Although the work in these areas started years ago when stating requirements on the concerned subsystems to arrive to the as-specified telescope, in the last two years we have been able to run system-level simulations with the as designed subsystems providing unvaluable feedback for optimizing the ELT performance. This paper presents the several system-level activities that have been undertaken and describes the objectives, the work done, as well as the results that have been obtained so far.