89 research outputs found
Optical turbulence simulations at Mt Graham using the Meso-NH mode
The mesoscale model Meso-NH is used to simulate the optical turbulence at Mt
Graham (Arizona, USA), site of the Large Binocular Telescope. Measurements of
the CN2-profiles obtained with a generalized scidar from 41 nights are used to
calibrate and quantify the model's ability to reconstruct the optical
turbulence. The measurements are distributed over different periods of the
year, permitting us to study the model's performance in different seasons. A
statistical analysis of the simulations is performed for all the most important
astroclimatic parameters: the CN2-profiles, the seeing {\epsilon}, the
isoplanatic angle {\theta}0 and the wavefront coherence time {\tau}0. The model
shows a general good ability in reconstructing the morphology of the optical
turbulence (the shape of the vertical distribution of CN2) as well as the
strength of all the integrated astroclimatic parameters. The relative error
(with respect to measurements) of the averaged seeing on the whole atmosphere
for the whole sample of 41 nights is within 9.0 %. The median value of the
relative error night by night is equal to 18.7 %, so that the model still
maintains very good performances. Comparable percentages are observed in
partial vertical slabs (free atmosphere and boundary layer) and in different
seasons (summer and winter). We prove that the most urgent problem, at present,
is to increase the ability of the model in reconstructing very weak and very
strong turbulence conditions in the high atmosphere. This mainly affects the
model's performances for the isoplanatic angle predictions, for which the
median value of the relative error night by night is equal to 35.1 %. No major
problems are observed for the other astroclimatic parameters. A variant to the
standard calibration method is tested but we find that it does not provide
better results, confirming the solid base of the standard method.Comment: 12 pages, 12 figures. The definitive version can be found at:
http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2966.2010.18097.x/abstrac
A different glance to the site testing above Dome C
Due to the recent interest shown by astronomers towards the Antarctic Plateau
as a potential site for large astronomical facilities, we assisted in the last
years to a strengthening of site testing activities in this region,
particularly at Dome C. Most of the results collected so far concern
meteorologic parameters and optical turbulence measurements based on different
principles using different instruments. At present we have several elements
indicating that, above the first 20-30 meters, the quality of the optical
turbulence above Dome C is better than above whatever other site in the world.
The challenging question, crucial to know which kind of facilities to build on,
is to establish how much better the Dome C is than a mid-latitude site. In this
contribution we will provide some complementary elements and strategies of
analysis aiming to answer to this question. We will try to concentrate the
attention on critical points, i.e. open questions that still require
explanation/attention.Comment: 3 figures, EAS Publications Series, Volume 25, 2007, pp.5
Wind speed vertical distribution at Mt. Graham
The characterization of the wind speed vertical distribution V(h) is
fundamental for an astronomical site for many different reasons: (1) the wind
speed shear contributes to trigger optical turbulence in the whole troposphere,
(2) a few of the astroclimatic parameters such as the wavefront coherence time
(tau_0) depends directly on V(h), (3) the equivalent velocity V_0, controlling
the frequency at which the adaptive optics systems have to run to work
properly, depends on the vertical distribution of the wind speed and optical
turbulence. Also, a too strong wind speed near the ground can introduce
vibrations in the telescope structures. The wind speed at a precise pressure
(200 hPa) has frequently been used to retrieve indications concerning the tau_0
and the frequency limits imposed to all instrumentation based on adaptive
optics systems, but more recently it has been proved that V_200 (wind speed at
200 hPa) alone is not sufficient to provide exhaustive elements concerning this
topic and that the vertical distribution of the wind speed is necessary. In
this paper a complete characterization of the vertical distribution of wind
speed strength is done above Mt.Graham (Arizona, US), site of the Large
Binocular Telescope. We provide a climatological study extended over 10 years
using the operational analyses from the European Centre for Medium-Range
Weather Forecasts (ECMWF), we prove that this is representative of the wind
speed vertical distribution at Mt. Graham with exception of the boundary layer
and we prove that a mesoscale model can provide reliable nightly estimates of
V(h) above this astronomical site from the ground up to the top of the
atmosphere (~ 20 km).Comment: 12 pages, 9 figures (whereof 3 colour), accepted by MNRAS May 27,
201
Study of the winter 2005 Antarctica polar vortex
During winter and springtime, the flow above Antarctica at high altitude
(upper troposphere and stratosphere) is dominated by the presence of a vortex
centered above the continent. It lasts typically from August to November. This
vortex is characterized by a strong cyclonic jet centered above the polar high.
In a recent study of our group (Hagelin et al., 2008) of four different sites
in the Antarctic internal plateau (South Pole, Dome C, Dome A and Dome F), it
was made the hypothesis that the wind speed strength in the upper atmosphere
should be related to the distance of the site to the center of the Antarctic
polar vortex. This high altitude wind is very important from an astronomical
point of view since it might trigger the onset of the optical turbulence and
strongly affect other optical turbulence parameters. What we are interested in
here is to localize the position of the minimum value of the wind speed at high
altitude in order to confirm the hypothesis of Hagelin et al. (2008).Comment: 3rd ARENA conference, 11-15 May 2009 EAS Publication Serie
Mt. Graham: Optical turbulence vertical distribution at standard and high vertical resolution
A characterization of the optical turbulence vertical distribution and all
the main integrated astroclimatic parameters derived from the CN2 and the wind
speed profiles above Mt. Graham is presented. The statistic includes
measurements related to 43 nights done with a Generalized Scidar (GS) used in
standard configuration with a vertical resolution of ~1 km on the whole 20-22
km and with the new technique (HVR-GS) in the first kilometer. The latter
achieves a resolution of ~ 20-30 m in this region of the atmosphere.
Measurements done in different periods of the year permit us to provide a
seasonal variation analysis of the CN2. A discretized distribution of the
typical CN2 profiles useful for the Ground Layer Adaptive Optics (GLAO)
simulations is provided and a specific analysis for the LBT Laser Guide Star
system ARGOS case is done including the calculation of the 'gray zones' for J,
H and K bands. Mt. Graham confirms to be an excellent site with median values
of the seeing without dome contribution equal to 0.72", the isoplanatic angle
equal to 2.5" and the wavefront coherence time equal to 4.8 msec. We provide a
cumulative distribution of the percentage of turbulence developed below H*
where H* is included in the (0,1 km) range. We find that 50% of the whole
turbulence develops in the first 80 m from the ground. The turbulence
decreasing rate is very similar to what has been observed above Mauna Kea.Comment: 12 pages, 6 figures, Proc. SPIE Conference "Ground-based and Airborne
Telescopes III", 27 June 2010, San Diego, California, US
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