The ELT will have a pioneering five-mirror optical design that will allow it to unveil the Universe in unprecedented detail. The mirrors all have different shapes, sizes and roles but will work together seamlessly. The primary, M1, is the most spectacular: a giant 39-metre concave mirror that will collect light from the night sky and reflect it to the secondary mirror. The convex M2, the largest secondary mirror ever employed on a telescope, will hang above M1 and will reflect light back down to M3, which in turn will relay it to an adaptive flat mirror (M4) above it. This fourth mirror will adjust its shape a thousand times a second to correct for distortions caused by atmospheric turbulence, before sending the light to M5, a flat tiltable mirror that will stabilise the image and send it to the ELT instruments.
The optical solution for the ESO ELT is a 39-m aperture folded three-mirror anastigmat (M1, M2, M3). Folding is provided by two flat mirrors (M4, M5) sending the beams to either Nasmyth foci (prefocal stations) along the elevation axis of the telescope.
The focal ratio of the primary mirror (M1) is f/0.87. The total Nasmyth field of view is 10 arc minutes, limited by the dimensions of the way-through hole in one of the flat folding mirrors (M4). As-designed optical quality is diffraction limited up to the edge of the curved field at all wavelengths. Field curvature is 9 88 m and the centre of field curvature is before the focus. The Nasmyth focal ratio is rather large (f/17.75, focal length 684022 mm). The folding arrangement (flat M4 and M5 mirrors) is conceived to provide conveniently located flat surfaces for an adaptive mirror (M4, conjugated to 621 m in front of the telescope, from M1) and field stabilisation (M5, with a minor off-pupil effect). The M4 mirror is inclined by 7.75°.
The telescope is highly dependent on integrated wavefront control, the purpose of which is to maintain the wavefront and the tracking within tolerances. This allows for limited drifts of the state and alignment of the optical surfaces, provided that mutual compensations hold the telescope to tolerances.
All mirrors bar M5 have shape adjustment capability to some extent; mirrors M2 and M3 can be recentred, while mirrors M4 and M5 can steer the beams to dynamically adjust the line of sight according to surfaces misalignment, jitter and residues from the telescope kinematics.