Real-time measurement of the Na layer profile for tomographic reconstruction: experimental results and its application to the E-ELT case
I. Montilla, J. P. Luke, J. G. Marichal-Hernández, M. Puga, J.M. Rodríguez-Ramos
Instituto de Astrofísica de Canarias, Canary Islands, Spain Universidad de La Laguna, Canary Islands, Spain
Extremely Large Telescopes are being designed with integrated AO modules and most of their instruments will rely on them for their optimum performance. To reconstruct the wavefront we need to use Guide Stars as references, but the absence of enough Natural GSs to have a good sky coverage make it necessary the use of Laser GSs. Several technical challenges have to be solved in order to perform a good wavefront reconstruction using LGSs. In the case of Na LGSs we need to know the height at which the LGS is focused and the profile and extension of the Na layer. We propose the use of a plenoptic camera to obtain this information. The plenoptic camera was originally created to allow the capture of the Light Field, a four-variable volume representation of all rays and their directions, that allows the creation by synthesis of a 3D image of the observed object. This 3D reconstruction make it possible to retrieve the distances at which the objects are, and for this reason it is especially adequate to measure the height variations of the LGS beacons. This novel approach provides real-time information on the Na layer profile that can be introduced in the reconstruction algorithm to solve the problems derived by the spot elongation. Also we can compute at which height is focused the LGS, overcoming therefore the two challenges mentioned before. We present in this paper the laboratory results obtained with a setup simulating the laser spot and the telescope equipped with the plenoptic camera that proof that the expected height of the layers is retrieved. We also present our plans to implement on-sky tests of our system using the Na LGS of the Optical Ground Station in the Observatorio de Tenerife, and the application of this advanced concept to the E-ELT.