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.

Design and Performance of Raman Fiber Amplifier Based 589-nm Guide Star Lasers for ESO VLT and Their Suitability for Future ELT AO Systems

Vladimir KARPOV — 2011-06-06T20:37:27Z

Submitted by W.R.L. Clements

Authors

V. Karpov (1), V. Protopopov (1), W. Clements (1), W. G. Kaenders (2), A .Friedenauer (2), B. Ernstberger (2), W. Hackenberg (3), D. Bonaccini Calia (3), St. A. Lewis (3)

Affiliations

(1) MPB Communications Inc., (2) TOPTICA Photonics AG, (3) Laser Systems Dept, European Southern Observatory (ESO)

Abstract

Large telescopes equipped with adaptive optics require 20-25W CW 589-nm light sources with emission linewidths of 5MHz. Towards this goal, ESO has been working for a number of years on the development of laser sources based on high-power narrow-band 1178-nm Raman fiber amplifiers (RFA) with subsequent frequency doubling to 589nm, demonstrating field tested lasers and powers beyond 50W CW. We present the design and performance of the guide star lasers being developed by industrial partners Toptica and MPBC, under contract from ESO, for deployment at the ESO VLT. The laser is designed and robustly engineered specifically for deployment on telescope facilities. The laser design is based on ESO's patented narrow-band RFA. The linearly-polarized, fiber-coupled emission of a Toptica CW diode laser, emitting 20mW at 1178nm, serves as master oscillator signal with stabilized emission frequency and controllable spectral linewidth up to a few MHz. The narrow-band seed signal is amplified in a polarization-maintaining (PM) Raman fiber amplifier developed by MPBC. The amplifier is pumped by a high-power 1120-nm PM fiber laser. With efficient suppression of stimulated Brillouin scattering, an unprecedented 40W of narrow-band RFA output has been obtained. The RFA output is then mode-matched into a resonant cavity doubler with a free spectral range exactly matching the sodium D2a to D2b separation. This allows simultaneous generation of an additional frequency component (D2b line) in the output beam to re-pump the electronic population of sodium atoms, thereby increasing the return flux. We have demonstrated doubling efficiencies >80%, resulting in CW output powers at 589nm easily exceeding the design goal of 20W. Fiber lasers provide excellent output beam quality and are turn-key, maintenance-free, reliable, ruggedized devices whose compactness allows installation directly on the launch telescope structure. They are therefore well suited for LGS applications, also considering the challenging environment of astronomical observatories.

The Four-Laser Guide Star Facility (4LGSF) for the ESO VLT Adaptive Optics Facility (AOF)

Domenico BONACCINI CALIA — 2011-06-06T20:37:24Z

submitted by D. Bonaccini

Authors

W. Hackenberg, D. Bonaccini Calia, S. Lewis, R. Holzlohner, B. Buzzoni, M. Comin, C. Dupuy, I. M. Guidolin, R. Guzman Collazos, L. Kern, M. Quattri, J. Quentin, R. Ridings, J. Argomedo, R. Arsenault, R. Conzelmann, B. Delabre, R. Donaldson, M. Downing, M. Duchateau, N. Hubin, G. Igl, L. Jochum, P. Jolley, A. Jost, M. Kiekebusch, J. Kolb, H. Kuntschner, J.-L. Lizon, M. Le Louarn, P.-Y. Madec, A. Manescau, J. Paufique, J.-F. Pirard, J. Reyes, A. Silber, C. Soenke, S. Stroebele, R. Stuik, S. Tordo, E. Vernet

Affiliations

European Southern Observatory (ESO)

Abstract

The 4LGSF is to be installed as a subsystem of the ESO Adaptive Optics Facility (AOF) on Unit Telescope 4 (UT4) of the VLT, to provide the AO systems GALACSI/MUSE and GRAAL/HAWK-I with four sodium laser guide stars. The 4LGSF will deploy four modular LGS Units at the UT4 Centrepiece. Two key aspects of the 4LGSF design are: (i) new industrial laser source (fibre lasers) with reduced volume, reduced need of maintenance, higher reliability, simpler operation and optimised spectral format for highly efficient sodium excitation, (ii) modular structure of the four LGS Units, composed of the laser and laser launch telescope, capable to operate independently of the others. The final design of the 4LGSF is now complete and the project has entered the manufacturing, assembly, integration and test phase. Furthermore, modular LGS units containing the laser emitter integrated on the launch telescope have already been demonstrated at ESO in the past years, and results will be presented. We believe that having the laser sources as an integral part of a modular unit together with the launching system offers many advantages at the system level, including the avoidance of beam relays, retaining the flexibility to use as many LGS as required independently, and the possibility of building redundancy into the system. We believe that many of these 4LGSF concepts can serve for ELT multi-LGS-assisted adaptive telescope designs and provide a valuable experience in advance of the E-ELT.

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.

Properties and dynamics of mesospheric sodium and the impact on sodium LGS AO systems

Thomas PFROMMER — 2009-02-28T23:00:00Z

Submitted by Thomas PFROMMER

Authors

T. Pfrommer and P. Hickson

Affiliations

UBC

Abstract

Adaptive optics systems correct for atmospheric distortion in real time in order to produce sharper images and enhance scientific capabilities. Sky coverage is greatly increased by the use of laser guide stars generated by resonant excitation of mesospheric sodium atoms. However, the performance of such systems is affected by physical process in the upper atmosphere. The extent and density of the sodium region is governed by competing processes of meteoric ablation that produce, and chemical reactions that remove, sodium atoms. The structure of this region is affected by gravity waves, wind shear and turbulence. Temporal and spatial variations in the mean sodium altitude produce focus-induced wavefront errors, degrading performance. Spatial density variations result in laser guide star image structure that reduces wavefront sensing accuracy. In order to better quantify these effects, we developed a high-performance sodium lidar system for the 6-m Large Zenith Telescope. With a power-aperture product more than two orders of magnitude greater than most atmospheric lidar systems, the facility is capable of recording sodium density profiles with sub-second and meter-scale resolution. Results from three years of observations will be presented. These reveal striking variability in the sodium region due to dynamical processes, strong nightly variations, and sporadic events. Meteor ablation trails produce strong spikes in sodium density on sub-second time scales. A recent upgrade now enables us to simulate laser asterisms and directly measure horizontal structure variations. Such variations can produce differential focus errors, an important aspect for multi-laser guide star facilities and multi-object adaptive optics.

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.

Rayleigh scattering, Fratricide effect and spot elongation: first on-sky results with GeMS

Benoit NEICHEL — 2009-02-28T23:00:00Z

Submitted by Benoit NEICHEL

Authors

B. Neichel F. Rigaut

Affiliations

Gemini Observatory

Abstract

GeMS, the Gemini MCAO system, is undergoing commissioning. Using 5 laser guide stars and a central launch, GeMS is particularly sensitive to Rayleigh backscatter and the fratricide effect. Using data covering a 5 months period, we will present results on:
Sodium return vs. Rayleigh background photons
mitigations and possible strategies to fratricide
Spot elongation measured
Benefits of fratricide for Laser Launch Telescope centering, fast acquisition procedure, cloud detection
Beam Transfer Optic design

Real-time measurement of the Na layer profile for tomographic reconstruction: experimental results and its application to the E-ELT case

Iciar MONTILLA — 2009-02-28T23:00:00Z

Submitted by Iciar MONTILLA

Authors

I. Montilla, J. P. Luke, J. G. Marichal-Hernández, M. Puga, J.M. Rodríguez-Ramos

Affiliations

Instituto de Astrofísica de Canarias, Canary Islands, Spain Universidad de La Laguna, Canary Islands, Spain

Abstract

Extremely Large Telescopes are being designed with integrated AO modules and most of their instruments will rely on them for their optimum performance. To reconstruct the wavefront we need to use Guide Stars as references, but the absence of enough Natural GSs to have a good sky coverage make it necessary the use of Laser GSs. Several technical challenges have to be solved in order to perform a good wavefront reconstruction using LGSs. In the case of Na LGSs we need to know the height at which the LGS is focused and the profile and extension of the Na layer. We propose the use of a plenoptic camera to obtain this information. The plenoptic camera was originally created to allow the capture of the Light Field, a four-variable volume representation of all rays and their directions, that allows the creation by synthesis of a 3D image of the observed object. This 3D reconstruction make it possible to retrieve the distances at which the objects are, and for this reason it is especially adequate to measure the height variations of the LGS beacons. This novel approach provides real-time information on the Na layer profile that can be introduced in the reconstruction algorithm to solve the problems derived by the spot elongation. Also we can compute at which height is focused the LGS, overcoming therefore the two challenges mentioned before. We present in this paper the laboratory results obtained with a setup simulating the laser spot and the telescope equipped with the plenoptic camera that proof that the expected height of the layers is retrieved. We also present our plans to implement on-sky tests of our system using the Na LGS of the Optical Ground Station in the Observatorio de Tenerife, and the application of this advanced concept to the E-ELT.

Gemini Multi-Conjugate Adaptive Optics (GeMS) Laser Guide Star Facility Commissioning Results

Celine DORGEVILLE — 2009-02-28T23:00:00Z

Submitted by Celine DORGEVILLE

Authors

Celine d'Orgeville et al.

Affiliations

Gemini Observatory

Abstract

The engineering and science commissioning phase of the Gemini Multi-Conjugate Adaptive Optics System (Gemini MCAO a.k.a. GeMS) project was kicked off in January 2011 when the Gemini South Laser Guide Star Facility (GS LGSF) propagated its 50W laser on the sky above the summit of Cerro Pachón, Chile. With 3 deformable mirrors, 3 Natural Guide Stars (NGS) and 5 sodium Laser Guide Stars (LGS), GeMS will be the first facility-class MCAO capability to be offered for normal science observations world-wide starting in 2012. This presentation will focus on the LGSF-side of the project and provide an overview of the LGSF subsystems, their top-level specifications, design, integration with the telescope, and current on-sky performance. Subsystems of the GS LGSF include a diode-pumped solid-state 1.06+1.32 micron sum-frequency laser producing over 50W of output power at the sodium wavelength (589nm), Beam Transfer Optics (BTO) that transport the 50W beam up the telescope, split the beam five-ways and configure the five 10W beams for projection by the Laser Launch Telescope (LLT) located behind the Gemini South 8m telescope secondary mirror, and a variety of safety systems to ensure safe laser operations for observatory personnel and equipment, neighbor observatories, as well as passing aircrafts and satellites.