Multiconjugate adaptive optics with plenoptic cameras and the Fourier Transform Reconstructor I. Montilla a , M. Reyes a , B. Femen´ ıa a and J. M. Rodr´ ıguez-Ramos b a Instituto de Astrof´ ısica de Canarias, Santa Cruz de Tenerife 38205, Spain; b Universidad de La Laguna, Tenerife, Spain ABSTRACT Multi-Conjugate Adaptive Optics (MCAO) will play a key role in future astronomy. Every Extremely Large Telescope (ELT) is being designed with its MCAO module, and most of their instruments will rely on that kind of correction for their optimum performance. Many technical challenges have to be solved in order to develop MCAO systems. One of them, related to its use on ELT’s, is to find fast algorithms to perform the reconstruction at the required speed. For that reason we have been studying the application of the Fourier Transform Reconstructor (FTR) to MCAO. We use the Fourier Slice Theorem in order to reconstruct the atmospheric volume. The process consists on reconstructing ”slices” of atmosphere, taking 1D-FFT’s of the different projections to build a 2D Fourier space that is inverse-transformed to build the reconstructed slice. The advantage of using the FTR is that this algorithm gives us directly the Fourier Transform of the projections, speeding up the process. To do a good reconstruction it is necessary to know the height at which the laser guide star is focused, and we propose to use a plenoptic camera to get this information, that we use together with the available information relative to the atmosphere we are reconstructing, C 2 n , to weight the inverse-transforms and obtain a better estimate. The height is obtained in real-time, a very important advantage for the reconstruction. We present the preliminary results of our MCAO simulations and the configuration of the plenoptic camera that could be applied to an ELT Keywords: wavefront reconstruction, adaptive optics 1. INTRODUCTION Multiconjugate adaptive optics was first proposed by Beckers 1 in 1988 and further studied by Tallon and Foy 2 in 1990. Their idea was to overcome the limitation of having a limited corrected field of the size of the isoplanatic patch. At visible wavelengths it amounts to a few arcseconds, clearly not enough for many observations. Conju- gating several mirrors at different heights was the solution they proposed. Using several guide stars the sky above the telescope is illuminated and the phase distribution in the few layers corresponding to the conjugated heights is computed. The three-dimensional turbulence cannot be reconstructed because the number of unknowns is larger than the number of measurements, but a good estimate can be found using tomographical techniques. In this paper we address the use of a fast algorithm, the FTR, to do MCAO. The performance of the FTR compared to other reconstruction methods to do SCAO, has been already reported in previous works. 3 This paper is organized in two parts: Section 2, that explains how we have used the FTR to do tomographic reconstruction of the atmosphere, and Section 3 where it is shown what it is a plenoptic camera and how can it be used to do AO. 2. MCAO WITH THE FTR First proposed by Freischlad and Koliopoulos 4 and later adapted by Poyneer et al. 5 for use on the Fried geometry, the wavefront reconstruction method using a Discrete Fourier Transform basically consists in reconstructing the wavefront φ in Fourier space, by applying a certain filter to the Fourier transform of the gradients of the subapertures sampled by the detector. Our idea is to use this reconstruction method in combination with the Fourier Slice Theorem 6 in order to perform a fast calculation of the commands that have to be send to each Further author information: (Send correspondence to I. Montilla) I. Montilla: E-mail: imontilla@iac.es, Telephone: +34 922 605 730