AO4ELT 2

DAYTIME OBSERVATIONS WITH ELTs IN THE THERMAL INFRARED USING LASER GUIDE STAR ADAPTIVE OPTICS

Jacques BECKERS — 2011-08-19T10:06:24Z

Submitted by J. M. Beckers

Authors

J. M. Beckers

Affiliations

retired

Abstract

Using Magneto-Optical Filters (MOFs; also called FADOFs = Faraday Anomalous Dispersion Optical Filters) it is possible to clearly see Sodium Laser Guide Stars in the daytime sky. This makes it possible to use ELT Adaptive Optics systems for diffraction limited observations 24 hours/day. Because of the bright daytime sky this LGS AO application is only of astronomical interest in the mid-infrared wavelength region (4 – 25 microns wavelengths) where the thermal radiation of the atmosphere-telescope system dominates the scattering of sunlight thus making the day- and night- sky background comparable. Incorporating MOFs in the LGS wavefront sensor thus would more than double the ELT observing time for mid-infrared astronomy and would make sources in almost the entire sky available for observation at any time of the year. Even though the AO would increase the brightness of point-sources, it would not compete with the James Webb Space Telescope in terms of detectability. The gain with respect to the JWST lies in the 5 to 6 times better linear angular resolution. The contrast gain in brightness at near-IR wavelengths is sufficient to give sufficient natural guide stars there for tip-tilt control. MOFs have been shown to function with Na lasers in LIDAR applications (see Beckers and Cacciani, Experimental Astronomy 11, 133, 2001). The main complication associated with incorporating MOFs in ELT AO system is likely the requirement to make the telescope and its enclosure robust in the daytime environment. I refer to SPIE Proceedings 6986 (2008) for a recent reference on this topic.

ELT LGS-AO: Optimizing the LGS return flux

Domenico BONACCINI CALIA — 2011-06-06T20:37:22Z

Submitted by D. Bonaccini

Authors

D.Bonaccini Calia, I.Guidolin, S.Lewis, W.Hackenberg, R.Holzlohner, G.Lombardi

Affiliations

European Southern Observatory (ESO)

Abstract

For the ELTs the use of AO with multiple sodium LGS will be routine. We think it is important to study the LGS generation in order to optimize the LGS-AO systems. It is in this context that we are working on the understanding and optimization of the LGS parameters. In this paper we report on our LGS return flux studies, aimed at identifying the optimal laser formats for CW and pulsed lasers. We have done recently numerical simulations on the LGS return flux for different laser formats, solving the Bloch equations for the interaction of the mesospheric sodium atoms with the laser radiation, which need now to be validated. We will report on the ESO Wendelstein transportable 20W LGS unit system, recently built and tested to make systematic field studies on the LGS, which will be described including the validation of the return flux simulations. The first experimental results are due this summer and will be reported as available.

Opportunities for AO learnt from meterology and microscopy

Nazim BHARMAL — 2009-02-28T23:00:00Z

Submitted by Nazim BHARMAL

Authors

Nazim Bharmal

Affiliations

CfAI, Durham University

Abstract

Imaging enhancement is common in microscopy and LIDARs are used extensively in meteorology. The lessons learnt from these two disciplines in regard to AO, with emphasis on ELT-scale adaptation, is discussed. Several particular themes are explored in detail: resolution enhancement of laser guide stars in order to enhance spatial coherence; homodyne detection of Shack-Hartmann images for attaining (optically) fast, variable gain; and guide star backscatter from below their focus for in-situ turbulence profiling with high spatial resolution.

Variable Curvature Mirrors for ELT Laser Guide Star refocusing systems

Zalpha CHALLITA — 2009-02-28T23:00:00Z

Submitted by Zalpha CHALLITA

Authors

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

Affiliations

Laboratoire d'Astrophysique de Marseille (LAM)

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, LGS defocusing is one of the system issues that can be tackled using active refocusing mirrors such as Variable Curvature Mirrors (VCM). Indeed, 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, and induces a large defocusing at the LGS wave-front sensor focal plane. To compensate for that, we propose an original concept including a VCM specifically designed to keep a focused spot on the wave-front sensor: the mirror is made of a thin meniscus bend using a pressure applied on its back face. Due to the large defocusing, the LGS-VCM must be able to change its shape from F/12.5 to F/5, leading to more than 1 mm sag. The VCM benefits of a specific shape with a variable radial thickness distribution, allowing keeping an optical quality better than λ/5 over this very large range of deformation. The work presented here details 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. Two prototypes have been manufactured to compare the real behaviour of the mirror and the simulations data. Results obtained on the prototypes show that the deformation of the VCM is very close to the simulation, and leads to a realistic concept.