AO4ELT 2

Vibration control of ELTs

Jörg-Uwe POTT — 2011-09-16T19:00:20Z

Submitted by Jörg-uwe POTT

Authors

J.-U. Pott and the LINC-NIRVANA team

Affiliations

Max-Planck-Institute of Astronomy (MPIA), Heidelberg, Germany

Abstract

MPIA is the PI institute of the MCAO-supported Fizeau imager LINC-NIRVANA at the LBT, and a partner of the E-ELT first light NIR imager MICADO (both SCAO and MCAO assisted). LINC-NIRVANA is a true pathfinder for future ELT-AO imagers both in terms of size and technology. I will present our vibration control strategies, involving accelerometer based real-time vibration measurements, feedforward and feedback optical path control, predictive filtering, vibration sensitive active control of actuators, and the development of a dynamical model of the entire telescope. Our experiences, made with LINC-NIRVANA, will be fed into the MICADO structural AO design to reach highest on-sky sensitivity.

Using artificial neural networks (ANN) for open-loop tomography

Richard MYERS — 2009-02-28T23:00:00Z

Submitted by James OSBORN

Authors

James Osborn, Francisco Javier De Cos Juez, Dani Guzman, Timothy Butterley, Richard Myers, Andres Guesalaga and Jesus Laine

Affiliations

Universidad Catolica, Univeristy of Durham and Universidad de Oviedo

Abstract

The next generation of adaptive optics (AO) systems require tomographic techniques in order to correct for atmospheric turbulence along lines of sight separated from the guide stars. Multi-object adaptive optics (MOAO) is one such technique. Here, we present a method which uses an artificial neural network (ANN) to reconstruct the target phase given off-axis references sources. This method does not require any input of the turbulence profile and is therefore less susceptible to changing conditions than some existing methods. We compare our ANN method with a standard least squares type matrix multiplication method (MVM) in simulation and find that the tomographic error is similar to the MVM method. In changing conditions the tomographic error increases for MVM but remains constant with the ANN model and no large matrix inversions are required.

Real time control zonal algorithm for adaptive optical system

Irina SERGIEVSKAYA — 2009-02-28T23:00:00Z

Submitted by Irina SERGIEVSKAYA

Authors

I.Sergievskaya, S.Royo, J.Riu, M.Ares

Affiliations

CD6-UPC

Abstract

An attempt was made to develop a zonal real time control algorithm for adaptive optical system based on block tridiagonal control matrix. Efficiency of a zonal approach for AOS with large number of actuators and local nature of response functions was studied using mathematical modeling and compared to classical modal least square solution and iterative methods. Calculation efficiency and residual wavefront error are examined. Turbulent media aberration was simulated using Von Karman model. Experimental realization using available 37 actuators piezoelectric DM and Shack-Hartmann wavefront sensor was performed for mathematical model validation.

The effect of the instrument environment on the Altair AO system

Julian CHRISTOU — 2009-02-28T23:00:00Z

Submitted by Julian CHRISTOU

Authors

Julian Christou, Benoit Neichel, Francois Rigaut

Affiliations

Gemini Observatory

Abstract

The Altair AO system at Gemini North has been in regular operation for many years. Over the past few years we have been routinely monitoring its performance looking at the telemetry circular buffers from the nightly tuning, in particular the residual tip-tilt (TT). We present analysis of these data which illustrate how the instrument support structure (ISS) environment affects the residual TT and how different components of the TT signal is affected by the instrument compliment.

LINC-NIRVANA Derotators

Frank KITTMANN — 2009-02-28T23:00:00Z

Submitted by Frank KITTMANN

Authors

Frank Kittmann, Thomas Bertram, Al Conrad, Jan Trowitzsch, Florian Briegel, Juergen Berwein, Lars Mohr

Affiliations

MPIA

Abstract

The near infrared interferometer LINC-NIRVANA combines the beams coming from the two primary mirrors of the Large Binocular Telescope to increase the resolution of the science camera image. LINC-NIRVANA is using layer-oriented multi-conjugate adaptive optics (MCAO) to reduce the influence of the atmospheric turbulence. The deformable mirrors of the MCAO systems are conjugated to the ground layer and a second layer in the upper atmosphere, respectively. Ground layer wavefront sensors and high layer wavefront sensors measure the wavefront in these two layers. Due to geometrical constraints unique to this type of instrument, it is not possible to provide a single derotator at the entrance of each incoming beam. Due to that fact, field derotation has to be applied for each wavefront sensor and for the science detector separately. The fields of the high layer wavefront sensors are derotated by the use of K-Mirrors, whereas the ground layer wavefront sensors and the science detector rotate themselves to compensate the field rotation. The derotation axes of the derotators will not perfectly match the field rotation center and projected derotation axis. These resulting effects have to be considered in the tip/tilt control strategy. In this paper we will describe the degree of accuracy required for the field derotation and compare that requirement with our test results. This includes timing and positioning accuracy over the complete derotation trajectory.

End-To-End performance test of the LINC-NIRVANA Wavefront-Sensor system.

Juergen BERWEIN — 2009-02-28T23:00:00Z

Submitted by Juergen BERWEIN

Authors

Juergen Berwein, Thomas Bertram, Al Conrad, Florian Briegel, Frank Kittmann, Xiangyu Zhang, Lars Mohr

Affiliations

Max Planck Institute for Astronomy

Abstract

LINC-NIRVANA is an imaging Fizeau interferometer, for use in near infrared wavelengths, being built for the Large Binocular Telescope. Multi-conjugate adaptive optics (MCAO) increases the sky coverage and the field of view over which diffraction limited images can be obtained. For its MCAO implementation, Linc-Nirvana utilizes four total wavefront sensors; each of the two beams is corrected by both a ground-layer wavefront sensor (GWS) and a high-layer wavefront sensor (HWS). The GWS controls the adaptive secondary deformable mirror (DM), which is based on an DSP slope computing unit. Whereas the HWS controls an internal DM via computations provided by an off-the-shelf multi-core Linux system. Using wavefront sensor data collected from a prior lab experiment, we have shown via simulation that the Linux based system is sufficient to operate at 1kHz, with jitter well below the needs of the final system. Based on that setup we tested the end-to-end performance and latency through all parts of the system which includes the camera, the wavefront controller, and the deformable mirror. We will present our loop control structure and the results of those performance tests.

Disturbance rejection analysis of LQG control for an adaptive optics system

Jean-pierre FOLCHER — 2009-02-28T23:00:00Z

Submitted by Jean-pierre FOLCHER

Authors

Jean-Pierre FOLCHER and Marcel CARBILLET

Affiliations

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

Abstract

We analyse the performance of an LQG based controller for an adaptive optics (AO) system. In many AO applications wind velocities and the strength of the distortion of the wavefront can change rapidly, rendering the disturbance prediction far from optimal and degrading the disturbance rejection property of the AO loop. Special attention is given to the choice of the disturbance model to ensure satisfactory disturbance rejection performance despite turbulence variations (wind velocities, strength of distortion wavefront). he proposed approach also serves the trade-off between the accuracy of the prediction and the model complexity to limit the computational cost of the LQG control. The achieved performances are evaluated through numerical experiments using the Software Package CAOS.

Real-sky adaptive optics experiments on optimal control of tip-tilt modes

Niek DOELMAN — 2009-02-28T23:00:00Z

Submitted by Niek DOELMAN

Authors

Niek Doelman (1) Rufus Fraanje (2) Remco den Breeje (1)

Affiliations

(1) TNO Delft (2) Delft University of Technology

Abstract

In recent years various researchers have concentrated on control performance improvement for adaptive optics systems by using more sophisticated design methods. These approaches account for the inherent spatial and temporal correlations in the wavefront sensor data. Several control schemes have been proposed, of which the common essence is the minimization of a criterion function, yielding so-called ‘optimal' or LQG control solutions. These are in some cases also referred to as ‘predictive control'. Following the H2-optimal control design approach proposed by Hinnen [JOSA A Vol. 24, 2007], a real-sky experiment has been carried out on the McMath-Pierce solar telescope on Kitt Peak, Arizona. The purpose of the experiment was to validate the favourable results of optimal control, as obtained in simulations and laboratory experiments, on a real-time AO system on a telescope with real-sky turbulence. During the experimental week, it appeared that the deformable mirror did not have sufficient stroke to cope with the strong wavefront aberrations as measured by the AO wavefront sensor. Therefore, it was decided to focus on optimal control of the lower aberration modes tip and tilt only (using the separate TT-mirror). The control experiments demonstrate that for the particular AO system and seeing conditions (Nov 14, 2010) real-time optimal control can reduce the tip and tilt amplitudes by an additional factor of about 2 (RMS), compared to the common integrator control of the tip and tilt modes. For the low frequency band the improvement ranges from 10 to 20 dB. This performance agrees reasonably well with the predicted performance which is based on off-line analysis of the WFS data. The paper will discuss the experimental results in detail and also address important aspects like the non-stationarity of the wavefront aberrations, coupled versus decoupled tip-tilt control and measures to increase the robustness of the controller.

Optimal woofer tweeter control demonstration

Brice LE ROUX — 2009-02-28T23:00:00Z

Submitted by Brice LE ROUX

Authors

B. LeRoux, K. ElHadi, M. NDiaye, M. Gray

Affiliations

Laboratoire d'astrophysique de Marseille (LAM)

Abstract

Large aperture telescope adaptive optics incorporates several deformable and active mirrors. Several options have been proposed for several DM adaptive optics systems. We study an optimal control approach for these woofer tweeter systems based on a Kalman filtering method. This approach allows to share out the spatial energy of correction between the mirrors and to deal with different temporal response time. The approach is presented and a validation of the control method is carried out in a numerical simulation. We finally present the experimental validation of such control solutions for woofer-tweeter systems. The validation bench and the optical components are presented and the first experimental results are shown.

MOAO LQG control for CANARY: theory and first laboratory results

Gaetano SIVO — 2009-02-28T23:00:00Z

Submitted by Gaetano SIVO

Authors

G. Sivo, C. Kulcsár, H.-F. Raynaud, J.-M. Conan, É. Gendron, F. Vidal

Affiliations

G. SIVO : L2TI Université Paris 13 & Onera DOTA/HRA C. Kulcsár : L2TI Université Paris 13 H.-F. Raynaud : L2TI Université Paris 13 J.-M. Conan : Onera DOTA/HRA É. Gendron : LESIA Observatoire de Paris F. Vidal : LESIA Observatoire de Paris

Abstract

Single Conjugated Adaptive Optics (SCAO) is a proven technique used in order to correct the effect of atmospheric turbulence and vibrations of the WaveFront (WF). The corrected field of view (FoV) is however limited by the anisoplanetism effect. Many concepts of Wide Field AO (WFAO) systems are under development, especially for the design of Extremely Large Telescopes (ELTs) instruments. Multi-Object Adaptive Optics (MOAO) is one of these WFAO concepts that is particularly suited to high redshifts galaxies observations in very wide FoV. The E-ELT instrument EAGLE will use this approach. CANARY is the on-sky pathfinder for MOAO. It obtained the first compensated images on Natural Guide Stars (NGSs) at the William Herschel Telescope in September 2010.

The control and performance optimization of such complex system are a key issue. Linear Quadratic Gaussian (LQG) control is an appealing strategy that provides optimal control for an explicit minimum variance performance criterion. It also provides a unified formalism that allows accounting for specific multi WF Sensing (WFS) channels, both for Laser Guide Stars (LGSs) and NGSs, and for various disturbance sources (turbulence, vibrations). Furthermore, preliminary simulation results suggest that performance can be significantly improved with tomographic LQG control compared to MMSE static reconstruction. Our objective is to obtain a first on-sky demonstration of tomographic LQG control during CANARY Phase B, featuring LGS and NGS WFS.

We show how the specific MOAO CANARY configuration can be embedded in a state-space framework. The state-space model includes: stochastic auto regressive models of order 2 for the turbulent phase in each layer and for vibrations affecting the telescope; LGS and NGS measurement equations; DM model and delays in the loop. Model identification and off-line calculations necessary for a robust on-sky operation are discussed. First laboratory results and on-sky test plan for the coming observing run are presented.