With a 39-metre diameter primary mirror, the ELT will be equipped with five mirrors in total. Two of them — M4 and M5 — form part of the adaptive optics system of the telescope. Their unique synergy will allow the ELT to take extremely high-quality, sharp images. The M5 mirror is hosted in a special cell that includes a fast tip-tilt system for image stabilisation that will compensate perturbations caused by the telescope mechanisms, wind vibrations, and atmospheric turbulence. This implies that the mirror must be simultaneously very light and very rigid. 

When complete, M5 will be a flat, elliptical mirror measuring 2.7 by 2.2 metres constructed from six lightweight silicon-carbide segments brazed together. ESO has signed contracts for the manufacture of M5 with the French companies Safran Reosc and Mersen Boostec. Safran Reosc will supply the M5 mirror along with the auxiliary equipment required for its handling, transport, operation and maintenance. Mersen Boostec will supply the mirror’s lightweight substrate to Safran Reosc’s specifications, and will also be responsible for the supply of the M5 replacement blanks if necessary. The Spanish company SENER Aeroespacial will carry out the design, construction and verification of the cell for the M5 mirror, as well as its control system and auxiliary equipment. 

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The M5 unit folds the optical beams towards each Nasmyth focus along the telescope elevation axis. It includes a switching mechanism, part of the telescope structure that rotates to allow the unit to feed different Nasmyth platforms and instruments. The unit also stabilises the image movements, which are induced by telescope mechanisms and wind-shaking vibrations, by adjusting the mirror’s tip and tilt angles to a few tens of milli-arcsec accuracy, within a 10 Hz bandwidth.  

The M5 unit consists of the M5 cell and the M5 mirror assembly, in addition to auxiliary equipment (integration, handing, transport, storage, and maintenance equipment). The M5 cell includes the tip-tilt stage, which consists of a base frame and tip-tilt actuators, and the M5 cell structure.

The M5 cell is fixed to the main structure of the ELT. It is a stiff structure that provides interface to the M5 tip-tilt stage at an accurate location in the telescope and allows for handling the M5 mirror assembly and tip-tilt stage as a single component (the M5 subunit), when installing it into, or removing it from, the telescope for integration and maintenance. The tip-tilt stage provides mounting interfaces to the mirror assembly and drives it along tip and tilt. 

The three tip-tilt actuators drive the M5 mirror assembly by changing their length along the mirror’s normal axis. The base frame holds the tip-tilt actuators and provides interface to the mirror’s lateral support.

The M5 mirror assembly includes the mirror itself and the mirror support, which is made of the M5 axial support and the M5 lateral supports. The mirror is flat and has an elliptical shape of about 2.7 by 2.2 metres. The silicon-carbide stiff and lightweight structure allows the mirror to achieve high stiffness-to-weight performance, and provides an optical surface that can be polished to the required optical quality.  

The axial support holds the M5 mirror along its optical axis and transmits the tip-tilt displacements generated by the actuators to it. The axial support also constrains the mirror axial degrees of freedom along the optical axis and decouples the other ones to minimise the spurious forces transmitted to the mirror.  

The lateral support holds the mirror along its in-plane directions. It constrains its in-plane degrees of freedom and decouples the other ones to allow tip-tilt displacements by means of a membrane connection at the mirror centre.