AO4ELT 2

Analysis of Ground Layer Turbulence Profiles at CTIO and Mauna Kea

Ronald GAGNE — 2011-08-19T10:09:58Z

Submitted by R. Gagné

Authors

Ronald Gagné, Paul Hickson, Thomas Pfrommer, Masen Lamb, Marc Baril

Affiliations

University of British Columbia

Abstract

For the past 4 years high-resolution ground layer turbulence measurements have been carried out on Cerro Tololo Inter-American Observatory (CTIO) with a lunar scintillometer. We present C_N2-profiles in the range up to 1 km and seeing statistics from fall 2007 to spring 2011. Comparisons with weather parameter links local topography to optical turbulence. The observations in Chile are compared to a ground-layer study on Mauna Kea at the Canada France Hawaii Telescope by the use of median seeing values as well as scale heights derived from the turbulence integral.

ELT Site Characterization for AO, the Tools and the Results

Marc SARAZIN — 2011-06-15T08:39:38Z

Submitted by M. Sarazin

Authors

M. Sarazin

Affiliations

ESO

Abstract

With the choice of the sites of the three main ELT projects worldwide, an unprecedentedly large site characterization effort is coming to an end. During the past decade more than 20 summits have been studied by the site survey teams of E-ELT, GMT and TMT projects. Other institutions have provided support or funding (NOAO, EU-FP6) so that close to one hundred scientists, engineers and students have been involved in this search for top quality observing conditions. For the first time also, the various project have deployed a very uniform instrumentation suite, often using similar measurement methods (DIMM) and even in some cases identical instruments (MASS). The consequence is that the core of the collected database is directly usable and could be made available to the community in its original state. The various teams have also maintained close contact during the whole process and new instruments were developed on the fly to solve the remaining unknowns. After sharing the tools and ideas, the time of sharing data has come and a review is proposed of what has been achieved and what is now available.

First results on a Cn2 profiler for GeMS

Angela CORTES — 2011-05-16T20:00:50Z

submitted by A. Guesalaga

Authors

Angela Cortes (1), Benoit Neichel (2), Francois Rigaut (2), Dani Guzman (1), Andres Guesalaga (1)

Affiliations

(1)Pontificia Universidad Católica de Chile, 4860 Vicuna Mackenna, Casilla 7820436, Santiago, Chile ; (2)Gemini Observatory Southern Operations Center, Colina el Pino s/n, Casilla 603, La Serena, Chile

Abstract

GeMS (the Gemini MCAO System) is a facility instrument for the Gemini-South telescope, currently in its final commissioning phases. The system includes 5 laser guide stars, 3 natural guidestars, 3 deformable mirrors optically conjugated at 0, 4.5 and 9km and 1 tip-tilt mirror. Among these unique features, a Cn2 Slodar Profiler is being implemented. The Cn2 profile is reconstructed from the slopes seen by the 5 high order WFS, each one pointing in a LGS direction. Residuals from the 16x16 subapertures WFSs and DM commands are used to obtain pseudo-open loop data for SLODAR, allowing us to reconstruct up 16 layers. The first part of this paper describes the algorithms used for this purpose and the results obtained from simulations and from artificially generated turbulence, resulting from exiting the 3 DMs. These latter have been extensively used to calibrate the method in a controlled environment. The second part of the paper presents the first on-sky results obtained during commissioning nights. In particular, we compare these results with external data obtained from a MASS/DIMM instrument.

The vibration compensation system for ARGOS

Diethard PETER — 2009-02-28T23:00:00Z

Submitted by Diethard PETER

Authors

D.Peter, W. Gaessler, J. Borelli, M. Kulas

Affiliations

Max-Planck-Institut fuer Astronomie

Abstract

For every adaptive optics system telescope vibrations can strongly reduce the performance. This is true for the receiver part of the system i.e. the telescope and wave front sensor part as well as for the transmitter part in the case of a laser guide star system. Especially observations in deep fields observed with a laser guide star system without any tip-tilt star will be greatly spoiled by telescope vibrations. The ARGOS GLAO system actually being built for the LBT aims to implement this kind of mode where wave front correction will rely purely on signals from the laser beacons. To remove the vibrations from the uplink path a vibration compensation system will be installed. This system uses accelerometers to measure the vibrations and corrects their effect with a small fast tip-tilt mirror. The controller of the system is built based on the assumption that the vibrations take place at a few distinct frequencies. Here I present a lab set-up of this system and show first results of the performance.

Monitoring of the atmospheric turbulence profiles for thespecification of ELTs adaptive optics systems

Aziz ZIAD — 2009-02-28T23:00:00Z

Submitted by Aziz ZIAD

Authors

Aziz Ziad, Julien Borgnino, François Martin, Jérôme Maire, Erick Bondoux, Jean-Baptiste Daban, Richard Douet, Yan Fanteï-Caujolle et Alex Robini

Affiliations

Laboratoire H. Fizeau-UMR 6525 Université de Nice Sophia Antipolis

Abstract

The futures large telescopes will be certainly equipped with Multi-Conjugate Adaptive Optics systems. The optimization of the performances of these techniques requires a precise specification of the different components of these systems. Major of these technical specifications are related to the atmospheric turbulence particularly the structure constante of the refractive index Cn2 and the outer scale L0. New techniques for the monitoring of the Cn2 and L0 profiles with high vertical resolution will be presented.

Cn2 profilometry from Shack-Hartmann data: model and experiment

Juliette VOYEZ — 2009-02-28T23:00:00Z

Submitted by Juliette VOYEZ

Authors

J. Voyez, C. Robert, N. Védrenne, B. Fleury, V. Michau, T. Fusco

Affiliations

ONERA

Abstract

The design phase for the Wide Field Adaptive Optics (WFAO) systems for the ELTs has started. LTAO (ATLAS), MCAO (MAORY) and MOAO (EAGLE) approaches have been studied for the E-ELT. All these systems have in common a need for a precise tomographic reconstruction of the turbulent volume. In that frame, the Cn2 structure, representing the turbulence strength, becomes a critical parameter for the final tomographic reconstruction performance. Getting of high-resolution Cn2 profiles is then a crucial point for the design of E-ELT AO systems. In this context, we have proposed a new profilometry method using Shack-Hartmann (SH) data. Slopes and scintillation indexes being recorded simultaneously with a Shack-Hartmann wavefront sensor (SHWFS), their correlation is exploited in order to retrieve the Cn2 profile. This method, named CO-SLIDAR (COupled Slope and scIntillation Detection And Ranging), uses correlation of SHWFS data from two separated stars. CO-SLIDAR has been validated in numerical simulations in a precedent work. The next step is an on-sky validation with a full-dedicated SHWFS, to measure a Cn2 profile with 40 layers, up to 20 km of altitude, with a 500m altitude resolution. The system will be set up on a 1.5m telescope. Here, we first expound the theoretical background of SH data and CO-SLIDAR processing. The Cn2 profile is estimated by minimizing a maximum likelihood criterion under positivity constraint. Then, we describe the experimental system and the future on-sky experiment. Determination of outer scale L0 with CO-SLIDAR, taking advantage of the large telescope diameter, is investigated.

Optical turbulence forecast with non-hydrostatical mesoscale models

Elena MASCIADRI — 2009-02-28T23:00:00Z

Submitted by Elena MASCIADRI

Authors

Elena Masciadri, Franck Lascaux, Susanna Hagelin

Affiliations

INAF

Abstract

At the new generation ground-based facilities (ELTs) all observing operations will be done in Service Mode. It follows that, to optimize the flexible-scheduling of scientific observations, the optical turbulence prediction is mandatory, particularly when observations concerns AO facilities. Without such a tool the risk is that any potential advantage provided by an AO facility would be neutralized. In this contribution we will review the principle of the technique of optical turbulence prediction with non-hydrostatic mesoscale models as well as its most important challenges. Besides we will present the progress we recently obtained applying these models to top class astronomical sites.

Producing Large Synthetic Turbulence Plates using MRF Polishing

Jean-Pierre VERAN — 2009-02-28T23:00:00Z

Submitted by Jean-pierre VERAN

Authors

John Pazder (1), Jean-Pierre Véran (1), Chris Hall (2), Glen Herriot (1), Scott Roberts (1)

Affiliations

(1) Herzberg Institute of Astrophysics, National Research Council, Victoria, BC, Canada (2) QED Technologies, Rochester, NY, USA

Abstract

In order to exercise real-time AO correction without being on the sky or even on the telescope, most AO systems include a calibration unit that simulates a number of sky sources as well as atmospheric turbulence. The synthetic atmospheric turbulence can be produced either by physically mixing air masses at different temperatures; or by using one or several moving phase plates (in reflection or in transmission), on which the turbulence has been encoded.

The transmitting phase plate design is usually preferred, since it minimizes the space envelope while ensuring that the turbulence has known pre-determined characteristics. Several techniques exist to manufacture such phase plates, including micro-machining using semi-conductor technology (Silios); and applying acrylic paint to a transparent substrate (UCSC). However, none of these techniques appear to be able to produce plates larger than 6 inches or 150mm. This is significantly too small for ELT size AO systems, such as NFIRAOS, which requires a phase plate of dimensions exceeding 600x300mm.

We have explored the feasibility of using the MRF polishing technique to produce large phase screens such as the one required for NFIRAOS. We present the measurements that we have obtained on a 200x200mm pathfinder prototype, manufactured by QED Technologies on a BK7-n substrate. We find that the synthetic turbulence has exactly the prescribed structure down to a spatial scale of 5mm. For scales smaller than 5mm and down to 0.5mm, we measured less structure than prescribed, but only by about 20%, making the generated turbulence useful, even at those scales. Based on these results, we conclude that MRF polishing appears to be a very promising technique for producing large turbulence phase screens for ELT-class AO systems.

Impact of the Cn² description on Wide Field AO performance

Anne COSTILLE — 2009-02-28T23:00:00Z

Submitted by Anne COSTILLE

Authors

Anne Costille (1), Thierry Fusco (2)

Affiliations

(1) IPAG, (2) ONERA

Abstract

In the frame of the design of new instruments for the Extremely Large Telescopes (ELT), new techniques of Adaptive Optics have been developed. These new techniques, generically called Wide Field Adaptive Optics (WFAO), are based on a tomographic reconstruction of the turbulent volume followed by a projection onto DM(s) in order to ensure a good correction either in dedicated directions (LTAO or MOAO schemes) or in a large Field of View (GLAO or MCAO schemes). All these systems require a 3D phase reconstruction and thus a statistical representation of the turbulent volume through the knowledge of the Cn² profile. It matters both for an accurate simulation of the input perturbations in the case of performance analysis and system design, but also for an efficient model description in the tomographic reconstruction process. Firstly, we discuss and analyze the impact of the structure and the complexity of the real Cn² profile onto the WFAO performance. We demonstrate that a classical integrated parameter such as θ_0 is not sufficient and that a more complex criterion including information on Cn² distribution is mandatory. Then, we focus on the impact of Cn² model error in the tomographic reconstruction process with respect to the input profile. We demonstrate that number and position of layers are two critical parameters but we also highlight the fact that in a regularized (MMSE-like) scheme, increasing the number of layers in the model will relax the requirements on the knowledge of their absolute positions. In conclusion, we demonstrate that it is critical to have access to high resolution Cn² profile with few hundred meters resolution to ensure a good performance evaluation of a WFAO system. We also give a first quantification of the number and accuracy of the prior information on Cn² to have an efficient tomographic reconstruction process.

Vibration characterization and mitigation at the Gemini-South telescope

Andres GUESALAGA — 2009-02-28T23:00:00Z

Submitted by Andres GUESALAGA

Authors

Ignacio Rodriguez(1), Benoit Neichel(2), Markus Hartung(2), Thomas Haywards(2), Julian Christou(3), Francois Rigaut(2), Dani Guzman(1), Andres Guesalaga(1)(*)

Affiliations

(1)Pontificia Universidad Católica de Chile, 4860 Vicuna Mackenna, Casilla 7820436, Santiago, Chile (2)Gemini Observatory Southern Operations Center, Colina el Pino s/n, Casilla 603, La Serena, Chile (3)Gemini Observatory Northern Operations Center, 670 N. A'Ohoku Place, Hilo HI 96720, USA (*) corresponding author: aguesala@ing.puc.cl

Abstract

This paper describes a vibration analysis carried out at the Gemini South telescope for several observation instruments, namely: Canopus, NICI, GSAOI and P2 (a peripherical WFS used for telescope guiding). The aim is to find the origins of these disturbances and possible ways to mitigate them. The analysis has shown that common vibration patterns can be identified; however they differ in terms of spike broadness, direction and modes affected. Based on these data, two types of controllers (Kalman and infinity) were designed with a particular emphasis on stability and robustness, especially under uncertainty in some of the loop components. From an analysis of the real open-loop data obtained from the instruments, we demonstrate that these controllers would provide a substantial improvement compared to standard integrator controllers in terms of vibration rejection and Strehl ratio. We recently implemented a Kalman controller for the Canopus Tip/Tilt loop. The first results obtained with this controller will be presented as well.