1 Introduction

It is well known that performances of high angular resolution techniques (HAR) are severely limited by the random optical effects induced by the turbulent atmosphere on the light propagation. A quantitative spatial and temporal characterization of the corrugated wavefront at the ground is necessary to optimize the operation of adaptive optics (AO) and long baseline interferometry (LBI) instruments and to guide the future development of these techniques.

The parameters which characterize the wavefront statistics from an optical point of view are the well known Fried's parameter $r_0(\lambda)$, the spatial coherence outer scale of the wavefront ${\mathcal L}_0$ (called outer scale hereafter), which becomes of importance when comparable to the telescope aperture, the isoplanatic angle $\theta _0$($\lambda$), and the characteristic time which determines the response time for the deformable mirror in AO or for the fringe tracking in LBI.

A specific instrument named GSM (Generalized Seeing Monitor) dedicated to the monitoring of these parameters has been built. It has been essentially developed (Martin et al. [1994]) to evaluate the outer scale statistics because no systematic measurements existed for it. The scattered values available in the literature were deduced from various techniques and were somewhat controversial (Mariotti et al. [1984]; Colavita et al. [1987]; Rigaut et al. [1991]; Ziad et al. [1994]; Buscher et al. [1995]; Takato et al. [1995]; Davis et al. [1995]; Beriot et al. [1997]).

The VLT system with its four 8 m apertures is a set of optical telescopes. It is located at the ESO Cerro Paranal observatory in Chile which is among the major world astronomical sites. The properties of the atmospheric turbulence at this site are of the utmost importance for the operation of this unique system.

A statistics of seeing and cloud coverage at Paranal over several years was obtained from the Differential Image MotionMonitor of ESO (Sarazin & Roddier [1990]) and shows it to be one of the lowest among those measured at several astronomical sites, with the long-term median seeing of 0.70''. However, the information on some other properties of atmosphere at Paranal was missing. The aim of our measurement campaign with GSM in December 1998 was to provide statistics of the outer scale ${\cal {L}}_0$and of the isoplanatic angle $\theta _0$. The corresponding results are presented in Sect. 3.

Previously, the GSM worked successfully at the ESO La Sillaobservatory (Martin et al. [1998b]), and in some otherobservatories over the world (Oukaïmeden in Morocco, Maydanak in Uzbekistan, and Cerro Pachon in Chile, thesouthern site for the Gemini project).

The schematic plan of the Paranal site is given in Fig. 1. The pillars with GSM telescopes were located near the ESO DIMM seeing monitor. A pre-existing mast was used to install three couples of temperature sensors for temperature micro-fluctuation measurements in the Surface Layer (SL).

Figure 1: The plan of Paranal site and the location of theequipment

All atmospheric parameters given in this report are calculated for a wavelength $\lambda= 500$ nm for observations at zenith. Seeing angle $\varepsilon$ is the full width at half maximum (FWHM) of a long-exposure stellar image in a large telescope, $\varepsilon = 0.98 \lambda / r_0$. Outer scale ${\cal {L}}_0$is a C_n²-weighted average of the local turbulence outer scale ${\cal {L}}_0$(h), as defined by Borgnino ([1990]). The isoplanatic angle $\theta _0$ is relevant for adaptive optics (Fried [1982]).