M4 is at the heart of adaptive optics corrections on ESO’s ELT. This large deformable mirror, with a reflective surface 2.4 metres in diameter, will be used by all instruments on the telescope. The mirror surface is extremely thin, less than 2mm, and is maintained actively in levitation by 5000 magnets glued to its back side. These magnets play a key role in deforming the flexible M4 surface, making adjustments 1000 times per second to an accuracy of a few tens of nanometers. This is necessary for the mirror to correct for atmospheric disturbances, which are very short-lived and need to be adjusted for extremely quickly.

Besides turbulence, the adaptive mirror also helps correct for the wind shake of the telescope’s main structure and mirrors. More information on the M4 mirror can be found on a dedicated page.

Another essential component of ESO’s ELT adaptive optics system are the wavefront sensing cameras, which act as the “eyes” of the telescope by sensing the light from guide stars. ESO’s ELT will be equipped with three complementary types of wavefront sensing cameras, each with a distinct image sensor or detector, which will be used both by the telescope itself and by the science instruments. These wavefront sensors make extremely fast measurements of atmospheric distortions constantly, to optimally re-shape the surface of the deformable mirror.

These cameras are considered so critical for the functioning of the ELT adaptive optics that ESO decided to do much of the work on them in-house. Two types of cameras, called ALICE and LISA, are designed at ESO while the third camera type, FREDA, is an adaptation of a commercially available camera. In addition, ESO has developed, in collaboration with international company Teledyne, the detector for LISA.

Adaptive-optics real-time computers analyse the signals from the cameras in real-time — 500 times per second — to calculate how mirrors like M4 need to be deformed to correct for distortions caused by turbulence in the Earth’s atmosphere. More information on these control systems is available on a dedicated page.

There are multiple flavours of adaptive optics technology that have been developed, each of which is capable of achieving different goals.

Classical adaptive optics is performed in the Single Conjugated Adaptive Optics (SCAO) mode. In this mode, the turbulence corrected for is that in a cylinder projection of the telescope aperture towards a guide star. SCAO benefits from high performance, but across a small field of view. In addition, the amount of the night sky that can be observed in this mode is limited by the availability of sufficiently bright natural guide stars. The sky coverage can however be improved by using a laser guide star, thus turning SCAO into Single Laser Adaptive Optics (SLAO). This comes with some cost to performance due to the “cone effect”, because the lasers are projected at finite distance, and “anisoplanetism”, because only a single turbulence slice is sensed and corrected. The SCAO technology can be combined with coronography to achieve both extreme spatial resolution and image contrast, allowing, for instance, the detection of extremely faint exoplanets. The ELT instruments MICADO, HARMONI, METIS and ANDES are capable of operating using SCAO to observe the sky near tens of thousands of stars, bright enough to get good performance, but will benefit from the use of laser guide stars to enable the observation of the majority of the sky.

It is possible to go a step further and correct for both ground-level and higher-altitude turbulence, by using several deformable mirrors. This is achieved with the Multi-Conjugate Adaptive Optics (MCAO) mode. In this setup, multiple off-axis wavefront sensors are used to measure the turbulence in several directions. These measurements are fed to a tomographic reconstructor (using both multiple lasers and stars) that estimates the 3D turbulence, which is in turn projected onto the deformable planes to generate the control commands. This mode offers medium-to-high performance across a medium field-of-view. In the laser-guide-star-based MCAO mode there is high sky coverage, but also a need for several natural guide stars, to keep a handle on the 3D turbulence. MORFEO employs MCAO by using a deformable mirror (in addition to M4) that allows it to correct atmospheric turbulence not only directly on the line of sight of the telescope, but also in a wider field of view. Thanks to MCAO, MORFEO’s partner instrument MICADO is able to study large structures or multiple objects at once.

Another complementary approach is provided by Multi Object Adaptive Optics (MOAO). Here the goal is to obtain multiple small-field turbulence corrections within a larger overall field-of-view. Similarly to MCAO and LTAO, the 3D turbulence is tomographically reconstructed. But in this mode, the turbulence is corrected for along the specific lines of sight where the science objects are located. This is done using multiple natural- and/or laser-guide-star wavefront sensors across the field-of-view. A dedicated deformable mirror corrects each science channel. This mode can also be combined with a first common deformable mirror conjugated close to the ground level.

	SCAO Single Conjugated Adaptive Optics	LTAO Laser Tomography Adaptive Optics	GLAO Ground Layer Adaptive Optics	MCAO Multi-Conjugate Adaptive Optics
	click to enlarge	click to enlarge	click to enlarge	click to enlarge
Corrected FoV	Small (10-15 arcsec)	Small (10-15 arcsec)	Large (Several arcmin)	Medium (1-2 arcmin)
Performance/ Strehl ratio*	High	Medium/High	Low	Medium/High
Sky coverage	Small	Large	Large	Large
iInstrument	HARMONI, MICADO, METIS, ANDES	HARMONI	MOSAIC	MICADO, MORFEO

*The Strehl ratio provides a measure of how good the adaptive optics correction is. It is defined as the ratio between the peak intensity of the image corrected by adaptive optics compared to the maximum attainable intensity (i.e. if the system was perfect).