Marc S. Sarazin

MASS: a monitor of the vertical turbulence distribution

The MASS (Multi-Aperture Scintillation Sensor) instrument consists of a 14-cm off-axis reflecting telescope and a detector unit which measures the scintillations of single stars in four concentric zones of the telescope pupil using photo-multipliers. Statistical analysis of these signals yields information of the vertical turbulence profile with a resolution of dh/h=0.5. We describe the instrument and present the results of its first field tests, including comparisons with DIMM seeing monitor and generalized SCIDAR. MASS will be used to obtain the extensive statistics of turbulence profiles at potential sites of future giant telescopes, as needed to predict the quality of adaptive seeing compensation.

Isoplanatism in a multiconjugate adaptive optics system

Turbulence correction in a large field of view by use of an adaptive optics imaging system with several deformable mirrors (DMs) conjugated to various heights is considered. The residual phase variance is computed for an optimized linear algorithm in which a correction of each turbulent layer is achieved by applying a combination of suitably smoothed and scaled input phase screens to all DMs. Finite turbulence outer scale and finite spatial resolution of the DMs are taken into account. A general expression for the isoplanatic angle thetaM of a system with M mirrors is derived in the limiting case of infinitely large apertures and Kolmogorov turbulence. Like Frieds isoplanatic angle theta0,thetaM is a function only of the turbulence vertical profile, is scalable with wavelength, and is independent of the telescope diameter. Use of angle thetaM permits the gain in the field of view due to the increased number of DMs to be quantified and their optimal conjugate heights to be found. Calculations with real turbulence profiles show that with three DMs a gain of 7-10x is possible, giving the typical and best isoplanatic field-of-view radii of 16 and 30 arcseconds, respectively, at lambda = 0.5 microm. It is shown that in the actual systems the isoplanatic field will be somewhat larger than thetaM owing to the combined effects of finite aperture diameter, finite outer scale, and optimized wave-front spatial filtering. However, this additional gain is not dramatic; it is less than 1.5x for large-aperture telescopes.

New challenges for adaptive optics: extremely large telescopes

ABSTRA C T The performance of an adaptive optics (AO) system on a 100-m diameter ground-based telescope working in the visible range of the spectrum is computed using an analytical approach. The target Strehl ratio of 60 per cent is achieved at 0.5mm with a limiting magnitude of the AO guide source near R magnitude , 10; at the cost of an extremely low sky coverage. To alleviate this problem, the concept of tomographic wavefront sensing in a wider field of view using either natural guide stars (NGS) or laser guide stars (LGS) is investigated. These methods use three or four reference sources and up to three deformable mirrors, which increase up to 8-fold the corrected field size (up to 60 arcsec at 0.5mm). Operation with multiple NGS is limited to the infrared (in the J band this approach yields a sky coverage of 50 per cent with a Strehl ratio of 0.2). The option of open-loop wavefront correction in the visible using several bright NGS is discussed. The LGS approach involves the use of a faintOR , 22U NGS for low-order correction, which results in a sky coverage of 40 per cent at the Galactic poles in the visible.

DYNAMICAL RECURRENT NEURAL NETWORKS — TOWARDS ENVIRONMENTAL TIME SERIES PREDICTION

Dynamical Recurrent Neural Networks (DRNN) (Aussem 1995a) are a class of fully recurrent networks obtained by modeling synapses as autoregressive filters. By virtue of their internal dynamic, these networks approximate the underlying law governing the time series by a system of nonlinear difference equations of internal variables. They therefore provide history-sensitive forecasts without having to be explicitly fed with external memory. The model is trained by a local and recursive error propagation algorithm called temporal-recurrent-backpropagation. The efficiency of the procedure benefits from the exponential decay of the gradient terms backpropagated through the adjoint network. We assess the predictive ability of the DRNN model with meterological and astronomical time series recorded around the candidate observation sites for the future VLT telescope. The hope is that reliable environmental forecasts provided with the model will allow the modern telescopes to be preset, a few hours in advance, in the most suited instrumental mode. In this perspective, the model is first appraised on precipitation measurements with traditional nonlinear AR and ARMA techniques using feedforward networks. Then we tackle a complex problem, namely the prediction of astronomical seeing, known to be a very erratic time series. A fuzzy coding approach is used to reduce the complexity of the underlying laws governing the seeing. Then, a fuzzy correspondence analysis is carried out to explore the internal relationships in the data. Based on a carefully selected set of meteorological variables at the same time-point, a nonlinear multiple regression, termed nowcasting (Murtagh et al. 1993, 1995), is carried out on the fuzzily coded seeing records. The DRNN is shown to outperform the fuzzy k-nearest neighbors method.

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.

Testing turbulence model at metric scales with mid-infrared VISIR images at the VLT

We probe turbulence structure from centimetric to metric scales by simultaneous imagery at mid-infrared and visible wavelengths at the Very Large Telescope (VLT) and show that it departs significantly from the commonly used Kolmogorov model. The data can be fitted by the von Karman model with an outer scale of the order of 30 m and we see clear signs of the phase structure function saturation across the 8-m VLT aperture. The image quality improves in the infrared faster than the standard λ -1/5 scaling and may be diffraction limited at 30-m apertures even without adaptive optics at wavelengths longer than 8 μm.

European Extremely Large Telescope Site Characterization I: Overview

ABSTRACT.The site for the future European Extremely Large Telescope (E-ELT) is already known to be Armazones, near Paranal (Chile). The selection was based on a variety of considerations, with an important one being the quality of the atmosphere for the astronomy planned for the ELT. We present an overview of the characterization of the atmospheric parameters of candidate sites, making use of standard procedures and instruments as carried out within the Framework Programme VI (FP6) of the European Union. We have achieved full characterization of the selected sites for the parameters considered. Further details on adaptive optics results and climatology will be the subject of two forthcoming articles. A summary of the results of the FP6 site-testing campaigns at the different sites is provided.

The eye of the beholder: designing the OWL

Preliminary requirements and possible technological solutions for the next generation of ground-based optical telescopes were laid down at ESO in 1998. Since then, a phase A study has been commissioned, the objective of which is to produce a conceptual design compatible, to the maximum possible extent, with proven technology, and establish realistic plans for detailed design, site selection, construction and operation for a 100-m class optical, diffraction-limited telescope. There was no doubt about how daunting such a challenge would be, but, somewhat surprisingly, it turns out to be firmly confined to adaptive optics concepts and technologies. The telescope itself appears to be feasible within the allocated budget and without reliance on exotic assumptions. Fabrication of key subsystems is fully within the reach of a properly engineered, industrialized process. A consolidated baseline is taking shape, and alternative system and subsystem solutions are being explored, strengthening the confidence that requirements could be met. Extensive development of wavefront measurement techniques enlarges the palette of solutions available for active wavefront control of a segmented, active telescope. At system level, ESO is developing enabling experiments to validate multi-conjugate adaptive optics (MAD for Multi-conjugate Adaptive optics Demonstrator) and telescope wavefront control (APE, for Active Phasing Experiment).

Development of a portable SLODAR turbulence profiler

We report on the development of a prototype portable monitor for profiling of the altitude and velocity of atmospheric optical turbulence. The instrument is based on the SLODAR Shack-Hartmann wave-front sensing technique, applied to a portable telescope and employing an electron-multiplication (EM) CCD camera as the wave-front sensor detector. Constructed for ESO by the astronomical instrumentation group at the University of Durham, the main applications of the monitor will be in support of the ESO multi-conjugate adaptive optics demonstrator (MAD) project, and for site characterization surveys for future extremely large telescopes. The monitor can profile the whole atmosphere or can be optimized for profiling of low altitude (0-1km) turbulence, with a maximum altitude resolution of approximately 150m. First tests of the system have been carried out at the La Palma observatory.

Atmospheric and internal turbulence measured on the Very Large Telescope Interferometer with VINCI

In March 2001, the commissioning instrument of the VLTI, VINCI, succeeded in obtaining its first fringes by linking two 40 cm aperture siderostats on a 16 m baseline. During the first year of operation, thousands of interferometric observations on different baselines were carried out, with the technical goal of characterizing this complex system. We report in this paper these first measurements and estimate the main parameters of the atmospheric and internal turbulence along the complete light path. We first illustrate the degradation of the visibility accuracy caused by the differential piston and evaluate the contribution of the internal optical path fluctuations with respect to the atmospheric ones. The stability of the VLTI complex is demonstrated, which enabled us to record easily fringes with Unit Telescopes (UTs) on baselines as long as 102.5 m (November 2001). In the last part, infrared measurements of the atmospheric differential piston are reported. They were obtained with the siderostats on two different baselines ranging from 16m to 66m. Estimations of the coherence time at Cerro Paranal are derived from these commissioning data and compared to the values predicted by the Astronomic Site Monitor (ASM). Finally, constraints on the outer scale length are discussed.