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T. Berkefeld (Max-Planck-Institut f\"ur Astronomie)
The small corrected field of view (FOV) of present Adaptive Optics (AO) systems can be enlarged by correcting the individual turbulent layers instead of the integrated wavefront distortion. However, this so-called multi-conjugate adaptive optics (MCAO) setup requires multiple guide stars for wavefront sensing and the separation of the influence of the different turbulent layers. A modal approach of calculating the wavefront error of angular anisoplanatism in MCAO systems allows the separation of the error contributions of natural (tip/tilt) guide stars from those of the laser guide stars. Different laser guide star projection geometries and their influence on the Strehl-ratio are discussed. Minimizing the anisoplanatism delivers the optimal position of the deformable mirrors (DM). For a 3.5-m-telescope and the Cn2-profile at the Calar Alto Observatory, Spain, two DMs corresponding to low and high altitude layers and seven guide stars would suffice to achieve a good correction in K-Band over a field of three arc\-minutes. The individual wavefront distortion of the two layers can be measured with only one Shack-Hartmann sensor situated in the conjugate plane of the upper turbulent layer. Based on an equation connecting the amount of scintillation with the Cn2-profile and the telescope aperture, this simple optical setup distinguishes the wavefront distortions of the two layers using the intensity fluctuations caused by the low-altitude layer. The separation error is calculated as a function of the Cn2-profile, the light level and the telescope- and AO-parameters. It is shown that the separation error is small for typical cases. MCAO systems are especially useful for solar telescopes working at visible wavelengths because the granules on the solar surface can be used as wavefront sensing objects, making laser guide stars obsolete. As a result, the corrected FOV increases from a few arcseconds (present AO systems) to the limits set by the DMs and the wavefront sensor. The author gratefully acknowledges the funding of this work by the Max Planck Society.
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