Amokrane Berdja

Multi-instrument measurement campaign at Paranal in 2007 Characterization of the outer scale and the seeing of the surface layer

Aims. Within the framework of site qualification for the future European large telescope E-ELT, a campaign of measurements was carried out for ten nights in December 2007 at Paranal using six independent instruments. Methods. To characterize the optical turbulence, two techniques were used: the statistical analysis of the fluctuations of the angle of arrival and the scintillation of the observed objects which are, in this case, a single star for DIMM, GSM, and MASS, a double star for Cute-SCIDAR, and Moon limb for MOSP and LuSci. Results. The optical parameters measured in this campaign and presented here are the seeing, the isoplanatic angle, the coherence time, and the outer scale. We obtain a good agreement with the value measured in previous campaigns. We also extracted the vertical profile of the turbulence given by C 2(h), and the profiles of the outer scale for the first time at Paranal. A comparison of the different results that we present here allows the determination of the energy distribution in the free atmosphere, on the ground layer as well as in the first meters above ground. This reveals a significant contribution of the surface layer to the degradation of the global seeing.

Fresnel diffraction and polychromatic effects on angle-of-arrival fluctuations

Seeing monitoring in astronomy is widely based on the statistical analysis of angle-of-arrival (AA) fluctuations, which are usually modelled in the framework of the near-field approximation where diffraction through turbulence is ignored. They are consequently believed to be completely independent of wavelength. We discuss in this paper the influence of Fresnel diffraction from distant turbulence layers on multi-wavelength (polychromatic) AA fluctuations. For this purpose we propose a model for polychromatic AA fluctuations in weak turbulence conditions and derive an analytical model for their variance in the case where scintillation is ignored. We also present a numerical simulation that includes scintillation and justifies that this latter may be neglected in the analytical model.

Simulation of pupil-plane observation of angle-of-arrival fluctuations in daytime turbulence

High angular resolution observations of the sun are limited by atmospheric turbulence. The MISOLFA seeing monitor (still under construction) is developed to obtain spatial and temporal statistical properties of optical turbulence by analyzing local motions observed on solar edge images. The solar flying shadows used for angle-of-arrival spatio-temporal analysis are observed in the pupil plane image by mean of a rectangular thin slit positioned on the solar edge image. A numerical simulation of the light propagation in both the atmospheric turbulence medium and the MISOLFA optical system is carried out studying the relation of the measured intensity variations in the pupil plane to angle-of-arrival fluctuations in the non-isoplanatic case. First results are presented and discussed.

LOTUCE2: a dome-seeing instrument for the E-ELT

Free-atmosphere, and surface-layer optical-turbulence have been extensively monitored over the years. The optical-turbulence inside a telescope enclosure en the other hand has yet to be as fully characterized. For this latest purpose, an experimental concept, LOTUCE (LOcal TUrbulenCe Experiment) has been developed in order to measure and characterise the so-called dome-seeing. LOTUCE2 is an upgraded prototype whose main aim is to measure optical turbulence characteristics more precisely by minimising cross-contamination of signals. This characterisation is both quantitative (optical turbulence strength) and qualitative (assessing the optical turbulence statistical model). We present the new opto-mechanical design, with the theoretical capabilities and limitations to the actual models.

Experimental results on using artificial neural networks for accurate centroiding in Shack-Hartmann wavefront sensors with elongated spots

We introduced the use of Artificial Neural Networks (ANN) for centroiding in Shack-Hartmann wavefront sensors in the presence of elongated spots, as it will occur in Extremely Large Telescopes. We showed in simulation that ANNs can outperform existing techniques, such as the Matched Filter. The main advantage of our technique is its ability to cope with changing conditions, as real atmospheric turbulence behaves. Here we present experimental results from the laboratory that confirm the findings in our original article, while at the same time they are useful to refine the ANN-based techniques.

Using DARC in a multi-object AO bench and in a dome seeing instrument

The Durham adaptive Optics Real Time Controller (DARC)1 is a real-time system for astronomical adaptive optics systems originally developed at Durham University and in use for the CANARY instrument. One of its main strengths is to be a generic and high performance real-time controller running on an off-the-shelf Linux computer. We are using DARC for two different implementations: BEAGLE,2 a Multi-Object AO (MOAO) bench system to experiment with novel tomographic reconstructors and LOTUCE2,3 an in-dome turbulence instrument. We present the software architecture for each application, current benchmarks and lessons learned for current and future DARC developers.

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

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 C2n(h) and the outer scale L0(h). New techniques based on the moon limb observation for the monitoring of the C2n(h) and L0(h) profiles with high vertical resolution will be presented.