Multi-instrument measurement campaign at Paranal in 2007

Abstract

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 n (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.

Recalibrated generalized SCIDAR measurements at Cerro Paranal (the site of the Very Large Telescope)

“…The absolute error of the median value of θ 0 is 0.13 arcsec. The recalibration of the GS data set performed in this paper suggests a revision of the results obtained in Dali Ali et al (2010) on the cross‐comparison between optical turbulence measurements obtained with different instruments during the PAR2007 campaign. A few of us are, at present, deeply involved in cross‐checking such measurements, with predictions performed with an atmospherical non‐hydrostatic numerical model.…”

“…In 2007 November/December, a site‐testing campaign (hereafter PAR2007) was organized at Cerro Paranal by the European Southern Observatory with the collaboration of several international teams (Dali Ali et al 2010). A set of different instruments ran simultaneously for a number of nights, varying between 10 and 20 nights, with the aim of quantifying different parameters that characterize optical turbulence: the seeing ɛ, the isoplanatic angle θ 0 , the wavefront coherence time τ 0 , the spatial coherence outer scale , the vertical profile of the spatial coherence outer scale , etc.…”

“…One of the main goals of the PAR2007 site‐testing campaign was the intercomparison of measurements, which is crucial not only for the characterization of a site but, even more, for a characterization of the accuracy, uncertainty and dispersion of each instrument with respect to others. Dali Ali et al (2010) have presented a summary of the analysis of all the measurements retrieved from all these instruments from 2007 December 17–26.…”

Angular correlation of the stellar scintillation for large telescopes

“…This effect resembles the central obscuration effect, which amplifies the high frequencies. The outer scale at high altitudes is large, with a characteristic length of about 20 to 50 m (Dali Ali et al ). This gives reason to assert with confidence that the obtained results are applicable for telescopes in the 2–4 m class.…”

Atmospheric optical turbulence at the Roque de los Muchachos Observatory: data base and recalibration of the generalized SCIDAR data

“…Classical SCIDAR (Vernin & Roddier 1973; Rocca et al 1974) is not sensitive to turbulence at the observatory level while generalized SCIDAR (Fuchs et al 1994; Avila et al 1997; Kluckers et al 1998; Johnston et al 2002) is able to measure this turbulence by locating the analysis plane in a virtual position a few kilometres below the pupil plane. Generalized SCIDAR has been extensively used at astronomical sites to study the atmospheric optical turbulence for applications in the development of adaptive optics systems and site characterization (Avila et al 1997, 2004; Fuchs, Tallon & Vernin 1998; Kluckers et al 1998; Avila, Vernin & Cuevas 1998; Kern et al 2000; Prieur‐L., Daigne & Avila 2001; Avila, Vernin & Sánchez 2001; Wilson et al 2003; Tokovinin et al 2005; García‐Lorenzo & Fuensalida 2006; Egner, Masciadri & McKenna 2007; Vernin et al 2007; Fuensalida, García‐Lorenzo & Hoegemann 2008; García‐Lorenzo et al 2009a; Chun et al 2009; Dali Ali et al 2010; Masciadri et al 2010; Mohr, Johnston & Cottrell 2010; García‐Lorenzo & Fuensalida 2011). From the early experimental implementations of the technique, it was noted that generalized SCIDAR data processing leads to an overestimation of the optical turbulence strength (Johnston et al 2002), which was assumed as negligible at any altitude.…”