A prototype of the Laser Guide Stars wavefront sensor for the E-ELT multi-conjugate adaptive optics module
INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, I-40127 Bologna,
2 Universita degli Studi di Bologna - Dipartimento di Astronomia, via Ranzani 1, I-40127 Bologna, Italy
3 ONERA, BP72 - 29 avenue de la Division Leclerc, FR-92322 Chatillon Cedex, France
4 ESO, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei Munchen, Germany
a e-mail: firstname.lastname@example.org
The future 42m diameter European Extremely Large Telescope (E-ELT) is foreseen to use Sodium Laser Guide Stars (LGS) in order to increase sky coverage and correction uniformity. The E-ELT MCAO module, MAORY, will use 6 LGSs to measure the Wavefront (WF) aberrations by means of Shack-Hartmann (SH) sensors. However the LGSs are affected by peculiar problems related to the atmospheric Sodium layer. The non-negligible thickness of the layer, for instance, causes the artificial reference source to look elongated, especially in WF sensor sub-apertures at the edge of the pupil. The WF sensor performance are limited by the elongation itself as well as by the fluctuations of the Sodium layer altitude, thickness and density. In particular these fluctuations have dramatic effects on the robustness of the algorithms used for the WF measurement. Moreover the defocus measurement is compromised by a strong non-atmospheric component. Another important aspect is the impact of the low-order aberrations introduced on the LGS images by the MCAO module optics that are optimized for infinite conjugate imaging, while the LGS is at a finite distance. We present in this paper a laboratory prototype that simulates an elongated source that can trace out some LGS features such as the arbitrary profile and altitude variation. In the optical path of the prototype three pupil images are foreseen. High order and low order aberrations can be introduced in the first two pupil planes by means of static screens and a Deformable Mirror (DM) respectively. The WF is then measured by a 40x40 sub-apertures SH Sensor. The main aim of the prototype is to check performance and robustness of WF Sensor algorithms for different illumination conditions, varying aberrations and pixel scale.
© Owned by the authors, published by EDP Sciences, 2010