Julien Borgnino

From the grating scale monitor to the generalized seeing monitor

An instrument named the grating scale monitor for measuring the outer scale L0 from the angle-of-arrival (AA) fluctuations of a perturbed wave front was developed a few years ago at Nice University. The AA is detected with a 5-ms time resolution by modulation of the stellar image in a small telescope with a grating. One uses the normalized covariance of AA fluctuations to estimate L0. A new version of this instrument, the generalized seeing monitor (GSM) is described. It consists of four identical modules for measuring the AA at four locations on the wave front. A spatiotemporal analysis of these data leads to the determination of seeing epsilon0, outer scale L0, and the wave-front speed. In addition, isoplanatic angle theta0 is determined from scintillation, making the characterization of turbulence with the GSM almost complete. We describe the instrument and make a detailed analysis of its performance and accuracy. Several site-testing campaigns have been conducted with the GSM: at La Silla (Chile), Oukaïmeden (Morocco), Maidanak (Uzbekistan), and Cerro Pachon and Cerro Paranal (Chile). The main results of these campaigns are presented and discussed.

Estimation of the spatial coherence outer scale relevant to long baseline interferometry and imaging in optical astronomy

A technique is proposed allowing estimation of the spatial coherence outer scale, from simultaneous measurements of Frieds parameter ro and of the variance of angle-of-arrival fluctuations. This optical parameter must be differentiated from the classical geophysical parameter called the outer scale of turbulence; its knowledge is important for long baseline interferometry and imaging in optical astronomy.

Effect of a finite spatial-coherence outer scale on the covariances of angle-of-arrival fluctuations

Abstract The transverse and longitudinal covariances of the angle-of-arrival fluctuations are theoretically calculated, taking into account the effect of a finite outer scale for the spatial coherence of the wavefronts perturbed by the atmospheric turbulence described successively by the Kolmogorov-Tatarski model and the Von Karman model. Estimations of this outer scale can thus be deduced from simultaneous measurements of these covariances and Frieds parameter r 0 . A numerical simulation supports this statement. The effect of a finite outer scale on differential seeing measurements is estimated for typical experimental conditions. For long baseline interferometry the statistical analysis of angle-of-arrival fluctuations appears to be a very efficient way to obtain the necessary information, as well for site testing as during the observations.

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.

Solar diameter measurements with Calern Observatory astrolabe and atmospheric turbulence effects

In order to deduce significant astrophysical results from solar diameter measurements it is necessary to take an accurate account of instrumental and atmospheric effects. This paper presents a comparison between visual and CCD camera measurements performed by means of the Calern Observatory solar astrolabe during the last 4 years; this allows us to evaluate visual measurements done previously (from 1975 to 1989). Then, a study of atmospheric effects is developed. From CCD measurements, the image quality, expressed by Frieds seeing parameter,r0, is estimated and related to the errors occurring in solar diameter measurements. A statistical analysis gives about 0.26 arc sec (or 0.13 arc sec for the semi-diameter) as the lowest value that this error may reach at Calern Observatory. One conclusion of this work is that it is important in the future to have image quality observations, obtained using a dedicated monitor, in order to evaluate and classify the measurements. A survey of the seeing might so lead to improve the precision of the results by weighting each diameter estimation and eventually to schedule the observations.

Contribution to the space-time study of stellar speckle patterns

The temporal behavior of stellar speckle patterns is statistically analyzed. The time-only power spectrum is shown to be the sum of two exponentially decreasing functions defining two characteristic time constants. The corresponding correlation is the sum of two Lorentzian functions. This is consistent with the first-order expansion of the power spectrum deduced from the multiple-layer model for atmospheric turbulence. However, this model fails to account for the experimental data that show a strong correlation between the spatial structure of a speckle pattern and its temporal behavior. This leads to the introduction of a new empirical model, called the randomly jittered speckle pattern model, which gives a preponderant place to image motion. The speckle lifetime then appears to be substantially longer than the corresponding measured time constant. As a consequence, a preliminary compensation of the image motion appears to be particularly interesting in speckle interferometry or active optics experiments.

First statistics of the turbulence outer scale at Dome C

Context. The outer scale of wavefronts is of interest for the dimensioning and the optimisation of the High Angular Resolution techniques such as Interferometry and Adaptive Optics, particularly for the new generation of telescopes such as the Extremely Large Telescopes (ELT). Aims. We aim to provide the first statistics of the outer scale at Dome C site in Antarctica to finalize the characterization of this site. Methods. A new version of the Generalized Seeing Monitor has been developed for extremely cold conditions. Two DIMMs (Differential Image Motion Monitor) were coupled to extract Angle-of-Arrival (AA) fluctuations using CCD detectors. Correlations of these AA fluctuations for different baselines lead to outer scale estimations. Results. For the first time, statistics of the outer scale at Dome C are provided leading to small values in the surface layer at Dome C. These small outer scale values compared to temperate sites considerably reduce the fringe excursion of interferometers and the low orders of the normalized amplitude of atmospheric Zernike coefficients, particularly the tip-tilt. Conclusions. The Dome C small outer scale combined with the large coherence time and large isoplanatic angle are very useful for the development of Adaptive Optics systems and long-baseline interferometers.

Determination of Fried's parameter r 0 prediction for the observed r.m.s. contrast in solar granulation

Frieds seeing parameter r0 can be determined from the observed r.m.s. contrast of solar granulation. The computation is based on results obtained from solar speckle interferometry. The influence of the telescope diameter on solar observations is shown graphically, and predictions for the observed contrast in solar granulation are made as a function of wavelength. The importance of seeing conditions is discussed, and a conclusion is made on the optimum telescope size to be used for given turbulence conditions

Wavefront outer scale deduced from interferometric dispersed fringes

In addition to site characterization, measurements of critical atmospheric parameters are required to design and to optimize future adaptive optic systems and long-baseline interferometers. It is possible to estimate seeing conditions by processing data obtained with existing High Angular Resolution instruments. We report the results of joint observations with the GI2T interferometer and the GSM site-testing monitor performed over a period of several nights. We compared estimates of the wavefront outer scale done at various baselines as well as the seeing (Fried’s parameter). We processed interferometric data by calculating power spectra of dispersed fringe images. Deduced measurements of the optical path difference lead to the estimates of the outer scale. We found that the outer scale values obtained from the GI2T data are mostly in the 5−30 m range, in good agreement with GSM measurements.

Measurements of the wavefront outer scale at Paranal: influence of this parameter in interferometry

The Generalized Seeing Monitor (GSM) has undertaken a campaign of measurement of the optical turbulent parameters: the Frieds parameter, the outer scale (L0) and the isoplanatic patch, at Paranal, the site of the VLT, in December 1998. The principle aim of this mission was to measure L0 values at Paranal. In a first part, we present the GSM, the results of the mission and particularly the statistics of L0. In a second part, we emphasize the effect of L0 on the optical path difference (OPD) values for the co-phasing of an interferometer. We derive the exact formulation of the OPD for the Von Karman model and we give the expected OPD at Paranal with L0 values measured by the GSM. We give also the expression of the residual OPD at the output of a fringe tracker system as a function of L0, of the exposure time of the fringe sensor and of the frequency of the closed-loop of the system.