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In the framework of the Multi Conjugate Adaptive Optics technique several new ideas have been proposed in the last new years to improve the quality of the correction by using in the most possible efficient way the light coming from the references. The Layer Oriented approach, based on the usage of pupil plane wavefront sensors such as the pyramid, is using the superimposition of the light coming from several references at the level of the detector in order to increase the SNR and is using just the signal which is needed to drive loops indipendently tuned (in term of spatial and temporal sampling) to different altitudes. The Multiple Field of View technique proposes to increase the field of view of the detectors conjugated to altitudes close to the ground (where the pupil superimposition is high), thus increasing the probability to find suitable references and consequently the equivalent photon density on the metapupil.
The Multiple Resolution technique is instead based on the idea of somehow increasing the depth of focus of the detectors in order to maximize the volume of atmosphere sensed by each detector even by using large Field of Views.
Ground layer MCAO has also been proposed as a technique to accomplish only partial correction over a wide field of view and some on sky experiments are going to be exploited on the sky. From time to time there might be the impression that we arrived to a kind of a limit for the efficiency of the systems, considering that the light is limited by the FoV that cannot be increased indefinitely, but this might be a by product of the fact to look at the wavefront reconstruction in a linear fashion. In fact, it has been already shown in the past that it is possible to drive additional DMs by using the signal coming from detectors not conjugated to them, but this is accomplished by using a quadratic reconstruction. Furthermore, more recently, the idea of using the signal coming from an anular region surrounding the scientific field of view to improve the correction in the centre has also been suggested and again the signal has to be treated in a quadratic way. All these considerations suggest that there is still room for improvements at the expense of giving up on some conventional approaches to treat these problematics.
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