ELT METIS wavefront control strategy

METIS is the European Extremely Large Telescope (ELT) 1st-generation Mid-Infrared ELT Imager and Spectrograph. It will offer spectroscopic, imaging and coronagraphic capabilities from 3 up to 13 microns with Adaptive-Optics correction. With its Final Design Review due late 2022 we report on the wavefront control strategy devised to meet the METIS science and technological requirements. Such strategy addresses challenging aspects as i) the appearance of differential petal piston modes in the presence of secondary mirror support struts caused either by numerical processing or the actual, physical low-wind effect, ii) the numerical pupil derotation and mis- reg compensation, iii) the adaptation to transient disturbance signals such as telescope-to-instrument handover control and iv) the compliance with constrained modal control of the pre-focal beam corrector mirrors (M4/M5). The overall METIS wavefront control strategy consists in a split approach cemented in a sequence of steps: 1) Tikhonov-regularised spatial wavefront estimation/reconstruction on a zonal Cartesian coordinate system tied to the pyramid (P-WFS) sampling pixel grid, 2) the regularised projection onto a global modal control space including correction of mis-registrations and rotation between the P-WFS coordinate grid and the ELTs M4/M5, and 3) the time-filtering through the application of proportional-integral control before converting to actuator commands readied for the ELTs collaborative TT off-loading scheme whilst avoiding hitting the mirrors constraints in amplitude, speed and force. We present physical-optics simulation results of the whole AO system obtained with prototyped instances of the real-time and soft-real-time computers including sensitivity analysis with respect to observational, atmospheric, non-atmospheric (telescope-intrinsic such as wind-induced low-order modes comprising tip-tilt) and instrument-specific conditions and disturbances. An error budget is put together that meets the METIS science requirements in terms of wavefront error with reassuring margins thus endorsing the strategy devised.