E-ELT M4 - Extremely Large Telescope

Location Cerro Armazones, Chile
Customer ESO
Primary mirror:

39 m

Secondary mirror:

2,6 m

# Actuators:


The European Extremely Large Telescope (E-ELT), is a revolutionary new ground-based telescope concept with a 39 meters primary mirror.

The adaptive optics is mainly based on two corrective surfaces, a tilting mirror (M5) and a high-order adaptive mirror (M4).

In late 2007 ESO granted two competitive Preliminary Design Studies of the M4 adaptive unit, one of the most critical and complex parts of the telescope. One contract has been assigned to Microgate (acting as Prime Contractor) in team with ADS, SAFRAN/SAGEM and INAF.

In 2012 the AdOptica consortium, made by Microgate and ADS, obtained an advanced preliminary design study aimed to consolidate the previous study while adapting to the evolution of the telescope design.

The adaptive unit is a 2.4 x 2.5m elliptic mirror controlled by 5352 contactless actuators.

Our design exploits the concept adopted in all previous units, but several implementation details were changed to deal with the challenges imposed by the size of the M4 unit.

In particular we adopted a modular concept based on replaceable control bricks that allows easy interfacing and system maintainability. Differently to the previous systems, the mechanical support structure acts also as reference for controlling the optical surfaces.

The baseline concept adopts a flat segmented thin shell made of six identical petals.

Microgate tasks cover control system design and simulation, electronic design, electronic manufacturing and test of Demonstration Prototype.


Sample Rendering of how it works the technology behind the E-ELT Adaptive Quaternary Mirror (M4). The "contactless" adaptive mirror is based on a continuous, thin mirror (~1.6mm) actively controlled in position and shape by a large number of force actuators (voice coil motors). During operation, the thin mirror "levitates" controlled by the electromagnetic field generated by the actuators. There is no mechanical contact between actuators and thin mirror. Each actuator is made by a fixed coil wound on a cold finger and a moving magnet, glued to the rear surface of the thin adaptive mirror.


  • 640 mm
    Mirror diameter
  • 2 mm
    Thin mirror thickness
  • 5352
    Number of actuators
  • 5 nm rms
    Capacitive sensors reading noise
  • 40 KHz
    Control loop frequency
  • 1.3 ms
    Typical response time (any mode)
  • 3
    Number of control electronics crates
Project status

After the competitive Phase B design, we completed an advanced preliminary design (Phase 1 study) in early 2015. This study comprehended the development, construction and test of an advanced Demonstration Prototype that has been fully tested, both for dynamic and optical performances, demonstrating that the concept developed in the preliminary design was fulfilling the strict M4 specifications. The control bricks embedded in the prototype were already at pre-production level.

In June 2015 the AdOptica consortium between Microgate and ADS International has signed the contract for the Final Design, Manufacturing, Delivery and On-Site Integration of the M4 Adaptive Unit of the E-ELT. The activity will cover all activities until delivery and system re-integration and test in Chile, planned for June 2023.

ESO Announcements:

Read the Article "ELT M4 — The Largest Adaptive Mirror Ever Built" published on ESO's Messenger magazine