MAD onboard VLT: the Layer Oriented WaveFront Sensor 

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Figure 1:MAD at the VLT

 In 2002 ESO decided to make an experiment on sky at VLT to prove that the new techniques conceived in the last years concerning Multi Conjugate Adaptive Optics are working properly, which means that these kind of systems should provide, using several reference sources, a corrected FoV, possibly diffraction limited, much bigger than the ones provided by classical single star Adaptive Optics systems, of the order of 1-2 arc minutes. The name of this on-sky experiment is Multi conjugate Adaptive optics Demonstrator

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Figure 2:Object imaged with MAD

In this way, the light of the different references can be optically co-added on the detector, thus giving the possibility to use fainter stars compared the Star Oriented technique, where each detector is looking at a single reference. Furthermore, the detector can be directly conjugated to a specific layer and gives the optimum information to drive the correspondent deformable mirror, conjugated at the same altitude of the wavefront sensor. In this way, the different loops act completely independently, giving the possibility to tune the spatial and temporal frequency of the detectors to the characteristics of the turbulent layers close to the conjugation altitude of the detectors. Any pupil plane wavefront sensor is particularly suitable for such a task, but the pyramid wavefront sensor, conceived in 1995, offers a practical and compact solution to the layer-oriented design with the advantage of an increased sensitivity with respect to other commonly used sensors, such as Curvature and Shack-Hartmann. The more efficient usage of the light done by using the Layer-Oriented technique lead, as a possible result, to the possibility of using solely Natural Guide Stars as references for the MCAO system, at least for the reddest portion of the visible spectrum and for a limited, but significant, fraction of sky. So, the Layer Oriented wavefront sensor has been designed in Padova, assembled, integrated and tested in open loop in Arcetri Observatory and shipped to ESO-Garching at the end of 2005. The Instrument Control Software of the whole MAD instrument has been developed entirely in Padova Observatory. In 2006 the Layer Oriented wavefront sensor has been integrated at ESO Garching with the rest of the MCAO system, developed by ESO (but the infrared camera, developed by a consortium of Portuguese Institutes), and extensively tested in the laboratory in closed loop using a turbulence simulator. The test have been concluded in December 2006 giving very good results in term of stability and reliability of the system, and moreover in term of achieved performance: diffraction limited images over the scientific 2 arc minutes field of view have been routinely achieved even using faint references. The system has been shipped to Paranal in February 2007, and in September 2007 it has been mounted and aligned at the VLT telescope (UT3, Melipal) where it successfully had first light on the sky in the night of September 22nd. In this occasion has been achieved the first pyramid wavefront sensor closed loop on an 8m telescope and the first Ground Layer and Multi Conjugated Adaptive Optics in the sky in the layer-oriented mode.  

People: C. Arcidiacono, A. Baruffolo, R. Falomo, J. Farinato, A. Moretti, R. Ragazzoni

Collaboration: INAF OA Arcetri, INAF OA Bologna, ESO, Lisbona Univ.

Publications: Arcidiacono et al. (2007), Mem. SAIt 78,708; Marchetti et al. (2007), Messenger 129,8 ; Marchetti et al. (2008), SPIE 7015,7 ; Mignani et al. (2008), A&A 488,267; Moretti et al. (2009), A&A 493,539

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