M4 is a ~2.6 meters adaptive mirror mounted on a positioning system providing a first stage large stroke low frequencymechanical tilt, a two dimensions rigid body decentering degrees of freedom and a focus selector. The M4 Adaptive Unit has 4 sub-units:

  • The 2.6 meters M4 Adaptive Mirror
  • The M4 Positioning System
  • The M4 Mounting Structure
  • The M4 Control System

The main objective of the M4 Adaptive Mirror is to correct for real-time high order wavefront distortion produced by atmosphere, telescope wind shake and low spatial frequency errors. It has to compensate as well small amplitude fast residual tip-tilt correction left after the M5 field stabilization.The M4 Positioning System provide a first stage large amplitude low frequency tip-tilt correction and a pupil lateral correction to compensate for telescope gravity and thermal loads plus adapter tracking wooble and run-out errors. It provides finally the Nasmyth foci selector.

M4 adaptive mirrors Main requirements
  Conditions Requirements
Fitting error
Median seeing: 0.85" 145 nm rms (goal 110)
Temporal error
Median Seeing: tau0=2.5 ms at 1kHz wavefront sensor sampling frequency 60 nm rms
Telescope optical errors
Lowest 10 Zernike modes (excl tip, tilt, piston) Maximum mechanical PV: 20 micron;Maximum update frequency: 1Hz;Maximum step amplitude PV: 1.5 micron
Maximum average power
  8 kW


M4 Positioning System Main Requirements
Pupil lateral correction
Range (mm)
± 20
Min Continuous speed (mm/s)
Absolute accuracy (�m)
Resolution (�m)
Mechanical Tilt Correction
Range (")
± 120 (± 14" on sky)
Speed ("/s)
5 (0.6"/s on sky)
Absolute accuracy (")
0.5 (0.06" on-sky)
Resolution (")
0.27 (0.03" on-sky)

Possible Technologies

Two competitive Studies have been launched in parallel in 2008. The project obtained from both companies a Demonstrator prototype fully tested and the M4 Adaptive Unit Preliminary Design document package in June-July 2010.

The design based on voice coil actuators technology was selected and a Preliminary design contract with the 39 meter EELT was kick-off in 2012. Preliminary design was successfully passed in April 2015.


M4 rendering (PDR design) Demonstration prototype


More information on Microgate design is available on Microgate webpage

Vernet E. et al , SPIE 2014

Vernet E. et al , AO for ELT conference, 2013