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Proceedings Paper

Active optics: variable curvature mirrors for ELT laser guide star refocusing systems
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Paper Abstract

The future generation of Extremely Large Telescopes will require a complex combination of technologies for adaptive optics (AO) systems assisted by laser guide stars (LGS). In this context, the distance from the LGS spot to the telescope pupil ranges from about 80 to 200 km, depending on the Sodium layer altitude and the elevation of the telescope. This variation leads to a defocusing effect on the LGS wave-front sensor which needs to be compensated. We propose an active mirror able to compensate for this variation, based on an original optical design including this active optics component. This LGS Variable Curvature Mirror (LGS-VCM) is a 120 mm spherical active mirror able to achieve 820 μm deflection sag with an optical quality better than 150 nm RMS, allowing the radius of curvature variation from F/12 to F/2. Based on elasticity theory, the deformation of the metallic mirror is provided by an air pressure applied on a thin meniscus with a variable thickness distribution. In this article, we detail the analytical development leading to the specific geometry of the active component, the results of finite element analysis and the expected performances in terms of surface error versus the range of refocalisation. Three prototypes have been manufactured to compare the real behavior of the mirror and the simulations data. Results obtained on the prototypes are detailed, showing that the deformation of the VCM is very close to the simulation, and leads to a realistic active concept.

Paper Details

Date Published: 30 September 2011
PDF: 8 pages
Proc. SPIE 8172, Optical Complex Systems: OCS11, 81721A (30 September 2011); doi: 10.1117/12.896960
Show Author Affiliations
Zalpha Challita, Lab. d'Astrophysique de Marseille (France)
Emmanuel Hugot, Lab. d'Astrophysique de Marseille (France)
Fabrice Madec, Lab. d'Astrophysique de Marseille (France)
Marc Ferrari, Lab. d'Astrophysique de Marseille (France)
David Le Mignant, Lab. d'Astrophysique de Marseille (France)
Sébastien Vivès, Lab. d'Astrophysique de Marseille (France)
Jean-Gabriel Cuby, Lab. d'Astrophysique de Marseille (France)


Published in SPIE Proceedings Vol. 8172:
Optical Complex Systems: OCS11
Gérard Berginc, Editor(s)

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