165 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
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
Optical turbulence vertical distribution with standard and high resolution at Mt. Graham
A characterization of the optical turbulence vertical distribution (Cn2
profiles) and all the main integrated astroclimatic parameters derived from the
Cn2 and the wind speed profiles above the site of the Large Binocular Telescope
(Mt. Graham, Arizona, US) is presented. The statistic includes measurements
related to 43 nights done with a Generalized Scidar (GS) used in standard
configuration with a vertical resolution Delta(H)~1 km on the whole 20 km and
with the new technique (HVR-GS) in the first kilometer. The latter achieves a
resolution Delta(H)~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 Cn2 useful for the Ground
Layer Adaptive Optics (GLAO) simulations is provided and a specific analysis
for the LBT Laser Guide Star system ARGOS (running in GLAO configuration) 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 epsilon = 0.72", the isoplanatic angle theta0 = 2.5"
and the wavefront coherence time tau0= 4.8 msec. We find that the optical
turbulence vertical distribution decreases in a much sharper way than what has
been believed so far in proximity of the ground above astronomical sites. We
find that 50% of the whole turbulence develops in the first 80+/-15 m from the
ground. We finally prove that the error in the normalization of the
scintillation that has been recently put in evidence in the principle of the GS
technique, affects these measurements with an absolutely negligible quantity
(0.04").Comment: 11 figures. MNRAS, accepte
Towards an automatic system for monitoring of CN2 and wind speed profiles with GeMS
Wide Field Adaptive Optics (WFAO) systems represent the more sophisticated AO
systems available today at large telescopes. A critical aspect for these WFAO
systems in order to deliver an optimised performance is the knowledge of the
vertical spatiotemporal distribution of the CN2 and the wind speed. Previous
studies (Cortes et al., 2012) already proved the ability of GeMS (the Gemini
Multi-Conjugated AO system) in retrieving CN2 and wind vertical stratification
using the telemetry data. To assess the reliability of the GeMS wind speed
estimates a preliminary study (Neichel et al., 2014) compared wind speed
retrieved from GeMS with that obtained with the atmospherical model Meso-Nh on
a small sample of nights providing promising results. The latter technique is
very reliable for the wind speed vertical stratification. The model outputs
gave, indeed, an excellent agreement with a large sample of radiosoundings (~
50) both in statistical terms and on individual flights (Masciadri et al.,
2013). Such a tool can therefore be used as a valuable reference in this
exercise of cross calibrating GeMS on-sky wind estimates with model
predictions. In this contribution we achieved a two-fold results: (1) we
extended analysis on a much richer statistical sample (~ 43 nights), we
confirmed the preliminary results and we found an even better correlation
between GeMS observations and the atmospherical model with basically no cases
of not-negligible uncertainties; (2) we evaluate the possibility to use, as an
input for GeMS, the Meso-Nh estimates of the wind speed stratification in an
operational configuration. Under this configuration these estimates can be
provided many hours in advanced with respect to the observations and with a
very high temporal frequency (order of 2 minutes or less).Comment: 12 pages, 7 figures, Proc. SPIE 9909 "Adaptive Optics Systems V",
99093B, 201
Meteorologic parameters analysis above Dome C made with ECMWF data
In this paper we present the characterization of all the principal
meteorological parameters (wind speed and direction, pressure, absolute and
potential temperature) extended over 25 km from the ground and over two years
(2003 and 2004) above the Antarctic site of Dome C. The data set is composed by
'analyses' provided by the General Circulation Model (GCM) of the European
Center for Medium Weather Forecasts (ECMWF) and they are part of the catalog
MARS. A monthly and seasonal (summer and winter time) statistical analysis of
the results is presented. The Richardson number is calculated for each month of
the year over 25 km to study the stability/instability of the atmosphere. This
permits us to trace a map indicating where and when the optical turbulence has
the highest probability to be triggered on the whole troposphere, tropopause
and stratosphere. We finally try to predict the best expected isoplanatic angle
and wavefront coherence time employing the Richardson number maps, the wind
speed profiles and simple analytical models of CN2 vertical profiles.Comment: 28 pages, 14 figures, pdf file, to be published on July 2006 - PASP,
see also http://www.arcetri.astro.it/~masciad
Towards an automatic wind speed and direction profiler for Wide Field AO systems
Wide Field Adaptive Optics (WFAO) systems are among the most sophisticated AO
systems available today on large telescopes. The knowledge of the vertical
spatio-temporal distribution of the wind speed (WS) and direction (WD) are
fundamental to optimize the performance of such systems. Previous studies
already proved that the Gemini Multi-Conjugated AO system (GeMS) is able to
retrieve measurements of the WS and WD stratification using the SLODAR
technique and to store measurements in the telemetry data. In order to assess
the reliability of these estimates and of the SLODAR technique applied to such
a kind of complex AO systems, in this study we compared WS and WD retrieved
from GeMS with those obtained with the atmospherical model Meso-Nh on a rich
statistical sample of nights. It has been previously proved that, the latter
technique, provided an excellent agreement with a large sample of
radiosoundings both, in statistical terms and on individual flights. It can be
considered, therefore, as an independent reference. The excellent agreement
between GeMS measurements and the model that we find in this study, proves the
robustness of the SLODAR approach. To by-pass the complex procedures necessary
to achieve automatic measurements of the wind with GeMS, we propose a simple
automatic method to monitor nightly WS and WD using the Meso-Nh model
estimates. Such a method can be applied to whatever present or new generation
facilities supported by WFAO systems. The interest of this study is, therefore,
well beyond the optimization of GeMS performance.Comment: 9 figures, 2 tables, MNRAS accepte
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