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

Zalpha Challita; Emmanuel Hugot; Fabrice Madec; Marc Ferrari; David Le Mignant; Sébastien Vivès; Jean-Gabriel Cuby

doi:10.1117/12.896960

30 September 2011 Active optics: variable curvature mirrors for ELT laser guide star refocusing systems

Zalpha Challita, Emmanuel Hugot, Fabrice Madec, Marc Ferrari, David Le Mignant, Sébastien Vivès, Jean-Gabriel Cuby

Proceedings Volume 8172, Optical Complex Systems: OCS11; 81721A (2011) https://doi.org/10.1117/12.896960
Event: SPIE Optical Systems Design, 2011, Marseille, France

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.

Citation Download Citation

Zalpha Challita, Emmanuel Hugot, Fabrice Madec, Marc Ferrari, David Le Mignant, Sébastien Vivès, and Jean-Gabriel Cuby "Active optics: variable curvature mirrors for ELT laser guide star refocusing systems", Proc. SPIE 8172, Optical Complex Systems: OCS11, 81721A (30 September 2011); https://doi.org/10.1117/12.896960

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