AO4ELT 2

Latest Ground Layer Adaptive Optics results and advancements in Laser Tomography implementation at the 6.5m MMT telescope

Eduardo BENDEK — 2011-05-20T07:26:54Z

Submitted by E. Bendek

Authors

Eduardo Bendek, Michael Hart, Keith Powell, Vidhya Vaitheeswaran, Don McCarthy, Craig Kulesa

Affiliations

University of Arizona

Abstract

Laser tomography capability using the Multi Laser Guide Star system is being implemented at the 6.5 m MMT telescope on Mt. Hopkins AZ. The system uses five range gated and dynamically refocused Rayleigh laser beacons to perform the tomographic sampling of the atmosphere. Corrections are then applied to the wavefront using the 336-actuator adaptive secondary mirror of the telescope. We present the latest results obtained using the GLAO mode, using for the first time, a plate scale on our IR science camera that samples the diffraction scale at the Nyquist limit. We find a reduction in the width of the on-axis point-spread function from 1.1” to <0.2” in H band. In addition, progress toward Laser Tomography is presented using an approach in which a minimum mean square reconstructor matrix is computed from simultaneous measurements recorded by the LGS wavefront sensor and a ground-truth NGS sensor. This paper also discusses our approach to estimate the turbulence intensity distribution Cn2, which is essential to our goal to build tomographic reconstructor matrices with an analytic forward model, to be updated on the fly during regular observing.

Vibration Suppression Algorithms for NFIRAOS on TMT

Carlos CORREIA — 2011-05-17T15:01:54Z

Submitted by C. Correia

Authors

C. Correia (a) , J.-P. Véran (a) , G. Herriot (a) , B. Ellerbroek (b), L. Wang (b), L. Gilles (b)

Affiliations

a – Herzberg Institute of Astrophysics; b – Thirty Meter Telescope

Abstract

Vibration suppression in Astronomical Adaptive Optics (AO) systems has gathered great attention in the context of next-generation instrumentation for current telescopes and future Extremely Large Telescopes (ELTs). This paper focus on the application of a novel multi-rate algorithm formulated in [Correia et al 2011][1] to the 1st-light Multi-Conjugate Adaptive Optics Facility (NFIRAOS) for the Thirty-Meter Telescope (TMT).

Numerical results cover the case of several vibration peaks with variable peak width based on telemetry from the Keck observatory. This algorithm is compared to other two solutions at sampling frequencies in the range [20–800] Hz, required to sense stars with magnitudes from mv=12 − 22 in H-band. Efficient methods to identify vibrations from closed-loop telemetry data are also discussed.

The Slope-Oriented Hadamard scheme for in-lab or on-sky interaction matrix calibration

Serge MEIMON — 2011-05-16T20:00:22Z

submitted by S. Meimon

Authors

Serge Meimon, Thierry Fusco, Cyril Petit

Affiliations

ONERA

Abstract

The correct calibration of the interaction matrix affects the performance of an adaptive optics system. In the case of high-order systems, when the number of mirror modes is worth a few thousands, the calibration strategy is critical to reach the maximum interaction matrix quality in the minimum time. This is all the more true for the E-ELT, for which on sky calibration procedures have to be considered. Here, we first build a tractable interaction matrix quality criterion. We then propose the Slope-Oriented Hadamard scheme which optimizes this quality criterion.

We demonstrate that for a given level of quality, the calibration time needed using the Slope-Oriented Hadamard method is ten times less than with a classical Hadamard scheme. These analytical and simulation results are confirmed experimentally both on the ONERA AO bench (BOA) and on the SPHERE XAO system (SAXO).

Last, we show how to use this method in an on-sky calibration scheme, and we quantify the gain in time compared to other calibration strategies.

Performance of MCAO on the E-ELT using the Fractal Iterative Method for fast atmospheric tomography

Michel TALLON — 2009-02-28T23:00:00Z

Submitted by Michel TALLON

Authors

Michel Tallon, Clémentine Béchet, Isabelle Tallon-Bosc, Miska Le Louarn, Éric Thiébaut, Richard Clare, Enrico Marchetti

Affiliations

1, 3, 5: Centre de Recherche Astrophysique de Lyon, France

2, 4, 6, 7: European Southern Observatory, Germany

Abstract

Adaptive optics (AO) on Extremely Large Telescopes (ELTs) must overcome the difficulty of solving a huge number of equations in real time, especially when atmospheric tomography is involved. This is particularly the case for multi-conjugate or multi-objects AO systems. The Fractal Iterative Method (FrIM) has been introduced as a fast iterative algorithm for minimum variance wavefront reconstruction and control on ELTs. In particular, it includes an accurate fast computation of turbulence priors by using the so-called fractal operator.

We present the first results obtained with FrIM in closed-loop in the context of atmospheric tomography. The method has been tested on Octopus, the end-to-end AO simulator at ESO, by considering MAORY, the multi-conjugate AO module planed for the E-ELT. This module aims at correcting a 2 arcmin field-of-view, by using 3 deformable mirrors, 6 Sodium laser guide stars, and 3 natural guide stars for low-order wavefront sensing.

We show the performance obtained in different conditions and analyze the effect of some parameters of FrIM, like the weight of the priors, or the number of conjugate gradient iterations for solving the reconstruction. We show how the duration of the simulations can be shortened on such a large aperture, with the introduction of artificial vibrations in the simulation. The results are also compared to a more classical approach using matrix-vector multiplication.

Real-time control developments for the CANARY MOAO instrument at Durham

Alastair BASDEN — 2009-02-28T23:00:00Z

Submitted by Alastair BASDEN

Authors

Alastair Basden, Richard Myers, Nigel Dipper, Eddy Younger, Tim Morris, Deli Geng, Sofia Dimoudi

Affiliations

Durham University

Abstract

The CANARY instrument is an MOAO demonstrator instrument for the E-ELT EAGLE instrument, fielded on the William Herschel Telescope. We present the CANARY real-time control system, both as used on-sky for the NGS only phase in 2010, and also as will be used on-sky in November 2011 with tomographic LGS capability and four LGSs. This system is based on the Durham AO Real-time Controller (DARC), for which we provide details of the main features. The challenges that we encountered will be presented as a learning experience, including camera synchronisation issues, and problems with a reliable telemetry on a loaded network. We will also present recent progress made in implementing a real-time control system for EAGLE in the laboratory in Durham, and the implications that this design has. By using a system comprised of a small number of high end GPU accelerators, theoretical performance is enough to perform matrix-vector based wavefront reconstruction for EAGLE, and we investigate whether this is a practical solution.

Identification of system misregistrations during AO-corrected observations

Clémentine BECHET — 2009-02-28T23:00:00Z

Submitted by Clementine BECHET

Authors

C. Béchet, É. Thiébaut, M. Tallon, J. Kolb, P-Y. Madec

Affiliations

1,4,5: European Southern Observatory, Germany 2,3: Centre de Recherche astrophysique de Lyon, France

Abstract

The E-ELT will be equipped with a deformable mirror inside the telescope. The performance of reconstruction and control depends on the calibration of the interaction matrix- or a model of the interaction matrix- , which characterizes the system and the relationship between the commands sent to the deformable mirrors (DM) and the wavefront sensors (WFS) slopes. Such a calibration will be more complex than for the current systems at the VLT since it will have to be at least partly measured on sky and for a much larger number of degrees of freedom (more than 5000). In addition, gravity or temperature variations for instance are likely to introduce slow evolution of the matching between the M4 Deformable mirror and the WFS geometry. This can occur during observations and therefore degrade the adaptive optics (AO) correction. To relax the need of frequent painful calibrations and to prevent a loss of performance due to misregistrations, we investigate how to track the evolution of the interaction matrix errors in closed-loop without introducing any degradation in the observations. This is done thanks to identification methods and optimization theory.

First, we formally describe the problem and the difficulties of such an identification in closed-loop configuration. Then, we present 2 solutions, based on the optimization of the error of estimates of the WFS slopes, at the output of the closed-loop AO. The performance of the methods and their limitations are discussed formally and thanks to numerical simulations of a high order AO system. We finally explore to which extent these methods currently studied for the Adaptive Optics Facility (AOF) at the VLT can be applied to the E-ELT.

Experimental comparison of Wide Field AO control schemes using the Homer AO bench.

Amélie PARISOT — 2009-02-28T23:00:00Z

Submitted by Amélie PARISOT

Authors

Parisot Amélie, Petit Cyril, Fusco Thierry

Affiliations

Onera, BP72, 92322 CHATILLON CEDEX

Abstract

Wide Field Adaptive Optics (WFAO) concepts, such as Laser Tomography AO (LTAO) or Multi-Conjugate AO (MCAO) have been developed in order to overcome the anisoplanatism limit of classical AO. Most of the future AO-assisted instruments of ELTs rely on such concepts which have raised critical challenges such as tomographic estimation and from laser and natural guide star combined with efficient DM(s) control. In that context, the experimental validation of the various clever control solutions proposed by several teams in the past years is now essential to reach a level of maturity compatible with their implementation in future WFAO developments for ELT. The ONERA wide field AO facility (HOMER bench) has been developed for these very issues. Gathering a 3D turbulence generator, laser and natural guide stars, two deformable mirrors with variable altitude positions and a PC-based flexible and user-friendly RTC , HOMER allows the implementation and comparison of control schemes from the simplest least-square to the optimal Linear Quadratic Gaussian solutions including Virtual DM and Pseudo-closed loop approaches. After a description of the bench internal calibrations and ultimate performance, all the control schemes are compared experimentally. Their evolutions as a function of wavefront sensors SNR as well as their robustness to calibration / model errors are particularly emphasised. Finally, we derive from the previous works some specific calibrations and identifications procedures ensuring both robustness and efficiency of WFAO systems and we extrapolate their applications to the future ELT AO systems.

Efficient control schemes with limited computation complexity for Tomographic AO systems on VLTs and ELTs

Cyril PETIT — 2009-02-28T23:00:00Z

Submitted by Cyril PETIT

Authors

C. Petit(1), M. Le Louarn(2), T. Fusco(1), P.-Y. Madec(2)

Affiliations

(1) Onera (2) ESO

Abstract

Various tomographic control solutions have been proposed during the last decades to ensure efficient or even optimal closed-loop correction to tomographic Adaptive Optics (AO) concepts such as Laser Tomographic AO (LTAO), Multi-Conjugate AO (MCAO). The optimal solution, based on Linear Quadratic Gaussian (LQG) approach, as well as suboptimal but efficient solutions such as Pseudo-Open Loop Control (POLC) require multiple Matrix Vector Multiplications (MVM). Disregarding their respective performance, these efficient control solutions thus exhibit strong increase of on-line complexity and their implementation may become difficult in demanding cases. Among them, two cases are of particular interest. First, the system Real-Time Computer architecture and implementation is derived from past or present solutions and does not support multiple MVM. This is the case of the AO Facility which RTC architecture is derived from the SPARTA platform and inherits its simple MVM architecture, which does not fit with LTAO control solutions for instance. Second, considering future systems such as Extremely Large Telescopes, the number of degrees of freedom is twenty to one hundred times bigger than present systems. In these conditions, tomographic control solutions can hardly be used in their standard form and optimized implementation shall be considered. Single MVM tomographic control solutions represent a potential solution, and straightforward solutions such as Virtual Deformable Mirrors have been already proposed for LTAO but with tuning issues.

We investigate in this paper the possibility to derive from tomographic control solutions, such as POLC or LQG, simplified control solutions ensuring simple MVM architecture and that could be thus implemented on nowadays systems or future complex systems. We theoretically derive various solutions and analyze their respective performance on various systems thanks to numerical simulation. We discuss the optimization of their performance and stability issues with respect to classic control solutions. We finally discuss off-line computation and implementation constraints.

A Kaczmarz type iterative reconstructor for Multi Conjugate Adaptive Optics

Ronny RAMLAU — 2009-02-28T23:00:00Z

Submitted by Ronny RAMLAU

Authors

R. Ramlau and M. Rosensteiner

Affiliations

Industrial Mathematics Institute, Kepler University Linz and Mathconsult GmbH, Linz

Abstract

Most of the large earthbound astronomical telescopes use Adaptive Optics technology (AO) in order to enhance the image quality. The degradation of the measured images is caused by atmospheric turbulences. Multi Conjugate Adaptive Optics (MCAO) is a technique that aims at a high imaging quality over a large field of view, which is achieved by using several (laser) guide stars, each assigned to a wavefront sensor, and several deformable mirrors that correct for turbulences in different atmospheric layers. Mathematically, the computation of a the optimal mirror deformations from wavefront measurements forms an inverse and ill posed problem. Therefore, regularization methods have to be used for a stable reconstruction. The solution to the MCAO problem involves three steps: reconstruction of the incoming wavefront, reconstruction of the turbulent layers (atmospheric tomography) and computation of the best mirror correction (fitting step). The standard approach collects these three operations into one matrix, leading to a large and ill conditioned matrix vector system. In our talk we present a method that solves the three subproblems subsequently. First, a fast wavefront reconstructor is used to reconstruct the incoming wavefronts from measurements of a Shack - Hartmann sensor. The atmospheric tomography problem is solved by a Kaczmarz type iterative reconstructor, which converges geometrically to a solution, meaning that only few iterations are needed for a sufficiently good reconstruction of the turbulent layers. The fitting step is finally achieved by a projection of the reconstructed turbulent layers to the mirrors. Due to the fast reconstructors for the subproblems the whole algorithm guarantees a fast reconstruction, which will be demonstrated with our numerical implementations. We wish to remark that the method can easily be adapted to Multi Object Adaptive Optics (MOAO), as only a different fitting step is needed.