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First Ever Multi-Conjugate Adaptive Optics System Achieves First Light at the VLT

Cerro Paranal, Chile – March 30, 2007:   A multi-conjugate adaptive optics system has been successfully demonstrated on ESO's Very Large Telescope, the first time that multi-conjugate AO has been demonstrated on-sky.
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  On the evening of 25 March 2007, the Multi-Conjugate Adaptive Optics Demonstrator (MAD) achieved First Light at the Visitor Focus of Melipal, the third Unit Telescope of the Very Large Telescope (VLT). MAD allowed the scientists to obtain images corrected for the blurring effect of atmospheric turbulence over the full 2×2 arcminute field of view. This world premiere shows the promises of a crucial technology for Extremely Large Telescopes.

Telescopes on the ground suffer from the blurring effect induced by atmospheric turbulence. However, with adaptive optics (AO) techniques, this major drawback can be overcome so that the telescope produces images that are as sharp as theoretically possible. Adaptive optics systems work by means of a computer-controlled deformable mirror (DM) that counteracts the image distortion induced by atmospheric turbulence.

The concept is not new. In 1989, the first adaptive optics system ever built for astronomy (named
Science With Adaptive Optics, Brandner
"COME-ON") was installed on the 3.6-m telescope at the ESO La Silla Observatory, as the early fruit of a highly successful continuing collaboration between ESO and French research institutes (ONERA and Observatoire de Paris). Ten years ago, ESO initiated an adaptive optics program to serve the needs of its frontline VLT project. Today, the Paranal Observatory has one of the most advanced complements of adaptive optics instrumentation, with no less  
than 7 systems currently installed (NACO, SINFONI, CRIRES and four MACAO AO systems for the interferometric mode of the VLT).

Present AO systems can only correct the effect of atmospheric turbulence in a relative small region of the sky – typically 15 arcseconds, the correction degrading very quickly when moving away from the central axis. Engineers have therefore developed new techniques to overcome this limitation, one of which is multi-conjugate adaptive optics (MCAO). At the end of 2003, ESO, together with partners in Italy and Portugal, started the development of a MCAO Demonstrator, named MAD.

"The aim of MAD is to prove the feasibility and performances of new adaptive optics techniques, such as MCAO, meant to work on large fields of view and to serve as a very powerful test tool in understanding some of the critical issues that will determine the development of future instruments, for both the VLT and the Extremely Large Telescopes," said Norbert Hubin, head of the AO group at ESO.

MAD is an advanced generation adaptive optics system, capable of compensating for the atmospheric turbulence disturbance on a large field of view (FoV) on the sky. It can successfully correct a 1–2 arcmin FoV, much larger than the ~15 arcsec typically provided by the existing adaptive optics facilities.

MAD was fully developed and extensively characterized by ESO using a dedicated
MCAO Concept In the Multi-Conjugate Adaptive Optics concept, several Guide Stars located in the Field of View are used simultaneously to perform a tomography of the atmospheric turbulence volume cone above the telescope by means of wavefront sensors. The measured wavefronts are combined in real-time to compute the commands applied to the deformable mirrors (two in the case of MAD) optically conjugated at different altitudes above the telescope. These deformable mirrors commands are optimized such as to homogeneously maximize the correction over the scientific field of view.
  Image:  © ESO
turbulence generator (MAPS, Multi Atmospheric Phase screens and Stars) able to reproduce in the laboratory the temporal evolution and the vertical structure of the turbulence observed at the Observatory.

MAD was then disassembled and shipped to Paranal for re-integration at the Nasmyth Visitor focus of UT3. The integration took about 1 month,
after which the system was ready for daylight testing and further characterization.

"On the night of 25 March, we could successfully close the first MCAO loop on the open cluster NGC 3293," said Enrico Marchetti, the MAD Project Manager. "The system behaviour was very stable and the acquisition and closed loop operations were fast and smooth."

After routine checks on the closed loop stability and preliminary scans of the system parameters, the telescope was pointed to Omega Centauri, a very crowded area in the sky, and an optimal test case for extracting accurate measurements on AO correction performance with good spatial resolution on the FoV. Three 11 magnitude stars within a circle of ~1.5 arcmin diameter were selected as the baseline for wavefront sensing and the MCAO loop was closed successfully. Omega Centauri will be observed for several nights more, in order to test the AO correction in different seeing conditions.

"This is a tremendous achievement that opens new perspectives in the era of extremely large telescopes," said Catherine Cesarsky, ESO's Director General. "I am very proud of the ESO staff and wish to congratulate all involved for their prowess," she added.
Omega Centauri A mosaic of images covering the central parts of Omega Centauri, the most luminous globular cluster as seen from Earth, taken with the MAD multi-conjugate adaptive optics demonstrator. The images were taken with MAD and the CAMCAO infrared camera in the Br-γ spectral line. The pixel scale is 0.028 arcsec/pixel. The stars in the 2 arcmin field of view have a FWHM between 0.08 and 0.10 arcsec.
  Image:  © ESO
The MAD images perfectly show the validity of the concept. The image quality was almost uniform over the whole field of view and beautifully corrected for some of the atmospheric turbulence.

Strehl Map Maps of Strehl ratio (a measure of light concentration) in the case of single conjugated adaptive optics, i.e. as used in present AO systems (SCAO; left) and multi-conjugated (MCAO; right), as measured in images of Omega Centauri.  
Different colours correspond to different Strehl ratios, from 0 (black) to red (35%). The SCAO map illustrates how inhomogeneous the correction is across the field of view. The peak Strehl ratio is about 30% close to the guide star, to the right, and decreases with the distance to the guide star due to atmospheric anisoplanatism. The MCAO map, however, is much more uniform across the field of view, and peaks close to the location of the three guide stars shown by crosses. A comparison between the two images clearly shows the advantage of MCAO.
  Image:  © ESO

Further Information: 
The Multi-Conjugate Adaptive Optics (MCAO) Demonstrator MAD was built by ESO in collaboration with the Astronomical Observatories of Arcetri and Padova (Italy) and the Faculdade de Ciências da Universidade de Lisboa (Portugal), as a pathfinder for 2nd generation VLT instrumentation and the European Extremely Large Telescope project.

The MCAO technique is based on probing the atmospheric turbulence on a large volume of atmosphere by means of several wavefront sensors (WFS), which point at different locations in the observed field of view, and by means of several deformable mirrors – optically conjugated at different altitudes on the atmosphere above the telescope – which correct for the atmospheric disturbance. The signals provided by the wavefront sensors are reconstructed to generate accurate information on the vertical structure of the atmospheric turbulence and then recombined in an optimal way to accomplish the best correction with the deformable mirrors located in the AO system. Since the wavefront sensors look at different directions in the field of view, the resulting correction is then optimized and homogeneously maximized across it. MAD makes use of two deformable mirrors, optically conjugated at 0 km and 8.5 km above the telescope.
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