AO4ELT 2

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.

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.