ESO Awards Contract for E-ELT Adaptive Mirror Design Study
22 maj 2012
ESO has taken a further step towards the construction of the European Extremely Large Telescope (E-ELT) by awarding the preliminary design study contract for the adaptive fourth mirror (M4) of the E-ELT to the AdOptica consortium made up of ADS International (Italy) and Microgate (Italy) . This mirror will be a major milestone in adaptive optics technology, becoming the largest adaptive mirror ever made for a telescope. It will be crucial for exploiting the E-ELT’s unprecedented potential, allowing astronomers to achieve major scientific breakthroughs during the next decades.
The M4 mirror is part of the E-ELT’s adaptive optics system, which will be responsible for correcting the blurring effects produced both by turbulence in the atmosphere, and the effects of wind on the telescope structure. When installed this remarkable deformable mirror will allow the E-ELT to reach the theoretical maximum resolution possible in its observations .
The M4 flat mirror will be about 2.5 metres in diameter but just 2 millimetres thick, allowing it to be deformed like a flexible film. More than five thousand voice-coil actuators  will flex the shape of the reflecting surface of the mirror up to a thousand times per second, precisely cancelling out the distorting effect of the atmosphere. This technology has been successfully used to develop similar though smaller deformable thin-shell mirror systems such as the 1.1-metre secondary mirror for ESO’s Very Large Telescope (VLT) (ann12015).
The construction and testing of the E-ELT M4 unit and its control system is a major technological challenge. It will be more than twice the size of the VLT secondary mirror — itself the largest adaptive mirror manufactured to date — and will be controlled by around five times as many actuators. Each of these will need to be controlled with unprecedented accuracy.
Apart from the extraordinary difficulty of making this system meet the demanding technical specifications, the M4 unit will also have to be reliable and easy to maintain.
This mirror will also be vital for the correct alignment of the telescope: huge telescopes like the E-ELT need to have their optics frequently realigned during the night. The M4 unit — the E-ELT's fastest optical component — is a crucial part of the system for correcting misalignments between all the other mirrors in the telescope.
The contract recently awarded covers a preliminary design study phase over the next 18 months. During the following eight years, the companies involved in the project will have the challenging task of constructing and testing this state-of-the-art adaptive optics device that will allow the E-ELT to achieve an image quality never previously reached at optical/infrared wavelengths.
 The design and construction of the M4 unit will be, in total, a project spanning nine years. In the past four years, two contracts have been awarded, one each to CILAS/AMOS/Onera (France/Belgium) and Microgate/ADS/Sagem (Italy/France), for producing M4 unit prototypes and firm fixed offers to design and build the final productions units.
 The resolution of an astronomical observation is — in theory — only limited by the size of a telescope’s primary mirror, with larger mirrors producing better observations: this maximum possible resolution is known as the telescope’s diffraction limit. However, ground-based telescopes usually only achieve a much lower resolution because of the distortion produced by the passage of light through the Earth’s atmosphere, known to astronomers as “seeing”. This effect can be reduced, but not eliminated, by locating telescopes at sites with excellent atmospheric conditions such as ESO’s Paranal Observatory in Chile. To remove the effect altogether, and hence to allow them to reach their full potential, large telescopes need optical systems that counteract the distortion produced by the atmosphere. Adaptive optics systems work by rapidly deforming one of the telescope’s mirrors in real-time, precisely counteracting the effect of the atmosphere. For 40-metre-class telescopes such as the E-ELT, working at the diffraction-limit will be crucial to exploit the E-ELT’s full potential. The E-ELT’s adaptive optics system will provide an improvement of about a factor 500 compared to the best seeing conditions achieved so far without adaptive optics.
 Voice-coil actuators are a special form of electric motor, capable of moving an object with an extremely high acceleration, while relocating with precision of millionths of a millimetre over a limited range of travel. Their name comes from their common use in loudspeakers.
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ESO, La Silla, Paranal, E-ELT and Survey Telescopes Public Information Officer
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