J. J. Fuensalida

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.

Processing of turbulent-layer wind speed with Generalized SCIDAR through wavelet analysis

We describe a new method involving wavelet transforms for deriving the wind velocity associated with atmospheric turbulence layers from Generalized SCIDAR measure- ments. The algorithm analyses the cross-correlation of a series of scintillation patterns separated by lapses oft, 2�t, 3�t, 4�t and 5�t using wavelet transforms. Wavelet analysis provides the position, direction and altitude of the different turbulence layers detected in each cross-correlation. The comparison and consistency of the turbulent layer displacements in consecutive cross-correlations allow the determination of their velocities and avoid misidentifications associated with noise and/or overlapping layers. To validate the algorithm, we have compared the velocity of turbulence layers derived on four nights with the wind vertical profile provided by balloon measurements. The software is fully automated and is able to analyse huge amounts of Generalized SCI- DAR measurements.

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.

Adaptive optics parameters connection to wind speed at the Teide Observatory

Current projects for large telescopes demand a proper knowledge of atmospheric turbulence to design efficient adaptive optics systems in order t reach large Strehl ratios. However, the proper characterization of the turbulence above a particular site requires long-term monitoring. Due to the lack of long-term information on turbulence, highaltitude winds (in particular winds at the 200 mbar pressure level) were proposed as a parameter for estimating the total turbulence at a particular site, with the advantage of records of winds going back several decades. We present the first complete study of atmospheric adaptive optics parameters above the Teide Observatory (Canary Islands, Spain) in relation to wind speed. On-site measurements of C 2 N (h) profiles (more than 20200 turbulence profiles) from G-SCIDAR observations and wind vertical profiles from balloons have been used to calculate the seeing, the isoplanatic angle and the coherence time. The connection of these parameters to wind speeds at ground and 200 mbar pressure level are shown and discussed. Our results confirm the well-known high quality of the Canary Islands astronomical observatories. The presence of optical turbulence in the Earth’s atmosphere drastically affects ground-based astronomical observations. The wavefront of the light coming from astronomical objects is distorted when passing through the turbulence layers, the wavefront being aleatory when reaching the entrance pupil of telescopes. The result is a degradation of the angular resolution of ground-based astronomical instruments. Several techniques have been developed to compensate for the effects of the atmosphere on astronomical images trying to reach the diffraction limit, the most popular being adaptive optics (AO hereafter) systems. The larger the telescope diameter, the more difficult the proper correction of the atmospheric turbulence becomes. The excellent image quality requirements of current large and future extremely large telescopes needs the design of adaptive optic systems with the capacity of adaptability to the prevailing turbulence conditions at the observing site. A proper knowledge of the statistical behaviour of the parameters describing the atmospheric turbulence at any site is crucial for the design of efficient systems. There are three basic parameters relevant to AO design and operation: Fried’s parameter (r0), the isoplanatic angle (�0), and the coherence time (�0). These

Ground to space optical communication characterization

Since the European Space Agency (ESA) geostationary data-relay satellite ARTEMIS started its operation in February 2003, ESA and the Instituto de Astrofisica de Canarias (IAC) have carried out routinely bidirectional optical link experiments between ARTEMIS and the Optical Ground Station (OGS), installed in the Teide Observatory of the IAC in the Canary Islands, Spain. The experiments aimed at characterizing statically and dynamically the performance of the optical downlinks and uplinks in different atmospheric turbulence conditions, together with the development and testing of appropriate theoretical models to predict the link performance. An overview of the OGS and additional facilities on top the IAC Teide Observatory is given, as well as a summary of the statistical results on propagation and communication for different experimental configurations, including different number of transmitting subapertures and divergence in the uplink. Key parameters, like scintillation and fade and surge statistics, are correlated with theoretical predictions and an analysis of the far field pattern is included. The results of the deep space uplink experiments between the OGS and ESA satellite SMART-1 are also presented. Finally ESA free space optical communication programs are summarised, including optical payloads on board different satellites.

Astronomical site selection: On the use of satellite data for aerosol content monitoring

The main goal of this work is to analyse new approaches to the study of the properties of astronomical sites. In particular, satellite data measuring aerosols have recently been proposed as a useful technique for site characterization and searching for new sites to host future very large telescopes. Nevertheless, these data need to be critically considered and interpreted in accordance with the spatial resolution and spectroscopic channels used. In this paper, we have explored and retrieved measurements from satellites with high spatial and temporal resolutions and concentrated on channels of astronomical interest. The selected data sets are the Ozone Monitoring Instrument on board the NASA Aura satellite and the Moderate Resolution Imaging Spectroradiometer on board the NASA Terra and Aqua satellites. A comparison of remote-sensing and in situ techniques is discussed. As a result, we find that aerosol data provided by satellites up to now are not reliable enough for aerosol site characterization, and in situ data are required.

Astronomical site ranking based on tropospheric wind statistics

High-altitude wind speeds have been adopted as a parameter for astronomical site selection based on the relationship found at the Paranal and Cerro Pachon sites between the average velocity of the turbulence (V0) and winds at the 200-mbar pressure level (V200). Although this relationship has not been checked at any other site in the world and a connection between image quality and V200 has not been proven anywhere, high-altitude wind speed (V200) is a parameter for checking the suitability of sites for adaptive optics and surveying potential sites for extremely large telescopes. We present comprehensive and reliable statistics of high-altitude wind speeds and the tropospheric flows at the location of five important astronomical observatories. We have used the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis data base to collect wind data at nine tropospheric pressure levels for the five selected sites. For comparison and validation of the data from the climate diagnostic model, we have also obtained wind profiles from radiosonde stations. The degrees of correlation found indicate a high level of significance between NCEP/NCAR Reanalysis and balloon data bases, pointing to NCEP/NCAR Reanalysis as a useful data base for site characterization. Statistical analysis exclusively of high-altitude winds points to La Palma as the most suitable site for adaptive optics, with a mean value of 22.13 m s−1 at the 200-mbar pressure level. La Silla is at the bottom of the ranking, with the largest average value 200 mbar wind speed (33.35 m s−1). We have found a clear annual periodicity of high-altitude winds for the five sites under study. We have also explored the connection of high- to low-altitude atmospheric winds as a first approach of the linear relationship between the average velocity of the turbulence and high-altitude winds. We may conclude that high- and low-altitude winds show good linear relationships at the five selected sites. The highest correlation coefficients correspond to Paranal and San Pedro Martir, while La Palma and La Silla show similar high- to low-altitude wind connection. Mauna Kea shows the smallest degree of correlation, which suggests a weaker linear relationship. Our results support the idea of high-altitude winds as a parameter for ranking astronomical sites in terms of their suitability for adaptive optics, although we have no evidence for adopting the same linear coefficient at different sites. The final value of this linear coefficient at a particular site could drastically change the interpretation of high-altitude wind speeds as a direct parameter for site characterization.

Extinction over the Canarian observatories: the limited influence of Saharan dust

Abstract We present a study of the atmospheric extinction over the observatories of the Canary Islands, based mainly on photometric measurements of the V -band extinction coefficient obtained during 2850 nights at the Carlsberg Automatic Meridian Circle at the Roque de los Muchachos Observatory (La Palma), and complemented with visible and infrared data obtained at the Teide Observatory (Tenerife). The median value of the extinction coefficient in the V -band is found to be 0.113 mag airmass −1 , which compares favourably with the extinction found at other first-quality astronomical sites. Long-term extinction variations due to the effects of the volcanic eruptions of El Chichon (1982) and Mt. Pinatubo (1991) are also described in this study. A detailed description of the seasonal variations provides the opportunity to study the influence of Saharan dust episodes, mostly concentrated in the summer period. This allowed us to estimate how Saharan dust affects astronomical observations. It is concluded that more than 75% of summer nights are not affected by dust, while this number rises over 90% for the rest of the year.

Preliminary design and plans for the GTC adaptive optics system

The 10m Gran Telescopio Canarias (GTC) is currently being installed in the Observatorio del Roque de los Muchachos (ORM) on the island of La Palma. An adaptive optics (AO) system will be installed at one of the Nasmyth foci of the telescope within a year of the telescope being commissioned. The preliminary design of the adaptive optics system is presented here. The system will initially be operated in single-conjugate mode using a natural guide star, but provisions are made for upgrade to dual-conjugate operation and the use of laser guide stars. The main system requirements and the optical and mechanical design solutions are outlined here. It is planned to employ a piezo-stack deformable mirror having approximately 350 actuators and a Shack-Hartmann wavefront sensor. The tip-tilt correction will be provided by the secondary mirror of the GTC which is a lightweighted Beryllium mirror with a drive system capable of fast tip-tilt and chopping. In preparation for dual-conjugate operation we have studied the optimal altitude of the second deformable mirror (the first will be conjugate to the telescope pupil) using numerical simulations and measurements of turbulence obtained at the ORM. We have used the GSC II catalogue to determine sky-coverage for multi-natural guide star wavefront sensing, as required for dual-conjugate operation. In addition we have investigated a novel approach to multi-object wavefront sensing based on curvature sensing.

Statistics of atmospheric parameters for multiconjugated adaptive optics for the Observatorio del Roque del los Muchachos

We present the statistical results of the optical-turbulence profiles at the Observatorio del Roque de los Muchachos over a period of six consecutive months. The data were obtained using the generalized SCIDAR technique at the 1m Jacobous Kaptein Telescope. In general, most of the turbulence is concentrated close to the observatory level (2400 m above sea level) with no more than two turbulent layers at higher altitudes. The temporal evolution along six consecutive months indicates that the turbulence is concentrated at lower altitude layers during winter. Large isoplanatic angles are also reached in winter compared to the values in spring. For the turbulence profiles measured in February, March and April we have analized the statistical position of demorfable mirrors in an ideal Multi-Conjugate Adaptive Optics system (with two or three deformable mirrors) and the improvements in the isoplanatic angles.