Current projects for large telescopes demand a proper knowledge of atmospheric turbulence to design efficient adaptive optics systems in order to 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, high-altitude 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 CN2(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.
Deep Dive into 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 to 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, high-altitude 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 CN2(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 sp
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 (θ0), and the coherence time (τ0). These ⋆ E-mail: bgarcia@iac.es parameters can be defined in terms of the refractive index structure constant profile (C 2 N (h)) and the vertical wind profile (V (h)) (Roddier, Gilli & Lund 1982):
(1)
(2)
where ζ is the zenith angle, k is the optical wave number and V0 is the average velocity of the turbulence given by:
(4)
These parameters are convenient measurements of the strength, distribution and variation of the turbulence (see Hardy 1998 for a detailed introdution to adaptive optics for astronomical telescopes).
Monitoring programs of turbulence structure at astronomical sites are therefore mandatory for obtaining the input parameters for the design and operation of efficient AO systems providing high Strehl ratios. Nevertheless, data should be obtained over decades to obtain sufficient statistical significance. In order to overcome the lack of long-term information on turbulence structure at astronomical sites, winds at the 200 mbar pressure level (V200 hereafter) were proposed as a parameter for estimating the total turbulence at any particular site. This proposal is based on the hypothesis that the integrated C 2 N profile is strongly related to the peak of the atmospheric wind vertical profile, which used to be at around the altitude of the 200 mbar pressure level (Vernin 1986). The V200 proposal as a parameter for site AO capabilities was supported by the similar seasonal trend of the seeing and V200 at Mauna Kea and La Silla (Vernin 1986), and the results found at Cerro Pachón and Paranal (Sarazin & Tokovinin 2002,S&T02 hereafter), where V0 was found proportional to V200: V0 = 0.4 ×V200 (S&T02). In addition, a good correlation-of the form V0 = 0.56 ×V200was also found above San Pedro Mártir (Mexico) using an atmospheric model to simulate a large dataset of C 2 N profiles (Masciadri & Egner 2006, M&E06 hereafter).
Such a linear connection between V0 and V200 at any site-an assumption that is currently under discussion and being tested-would simplify the calculation of the input parameters for AO design. Henceforward, the problem could be reduced to determining statistics for the existing worldwide long-term high-altitude winds data in climatological databases. Indeed, V200 statistics has been already used as a parameter for ranking astronomical sites for their suitability for AO (Ilyasov, Tillayev & Ehgamberdiev 2000;Sarazin 2002;Carrasco & Sarazin 2003;Chueca et al. 2004;Carrasco et al. 2005;García-Lorenzo et al. 2005;Bounhir, Benkhaldoun, & Sarazin 2008).
Despite poor empirical results connecting seeing and V200 (Vernin 1986), the idea of a relation between image quality and high-altitude wind speed is increasingly widespread among those of the astronomical community interested in AO.
Different meteorological processes are responsible for generating turbulence in the atmosphere. The turbulence in the first kilometre above the ground level (the boundary layer) is caused by local factors (Lee, Stull & Irvine 1984). These factors are buoyant convection processes, such as thermals rising produced by surface solar heating, and mechanical processes, such as wind shear produced by the surface friction in the wind speed or by the lee waves formed by mountains or other geographic effects (Stull 1988). In the free atmosphere, turbulence generators are connected to the synoptic scales conditions (Erasmus 1986). The combination of both contributions will determine the quality of sites for astronomical observations. Site testing studies have reported a connection between ground layer winds and image quality (Chonis, Claver & Sebag 2009;Lombardi et al. 2007;Varela, Muñoz-Tuñón & Gurtubai 2001;Muñoz-Tuñón, Varela & Mahoney 1998;Erasmus
…(Full text truncated)…
This content is AI-processed based on ArXiv data.
