510 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
Optical turbulence forecast in the Adaptive Optics realm
(35-words maximum) In this talk I present the scientific drivers related to
the optical turbulence forecast applied to the ground-based astronomy supported
by Adaptive Optics, the state of the art of the achieved results and the most
relevant challenges for future progresses.Comment: 1 figure, Orlando, Florida United States, 25 - 28 June 2018, ISBN:
978-1-943580-44-6,Turbulence & Propagation, JW5I.1 Adaptive Optics: Analysis,
Methods and System
A dedicated tool for a full 3D Cn2 investigation
We present in this study a mapping of the optical turbulence (OT) above
different astronomical sites. The mesoscale model Meso-NH was used together
with the Astro-Meso-Nh package and a set of diagnostic tools allowing for a
full 3D investigation of the Cn2. The diagnostics implemented in the
Astro-Meso-Nh, allowing for a full 3D investigation of the OT structure in a
volumetric space above different sites, are presented. To illustrate the
different diagnostics and their potentialities, we investigated one night and
looked at instantaneous fields of meteorologic and astroclimatic parameters. To
show the potentialities of this tool for applications in an Observatory we ran
the model above sites with very different OT distributions: the antarctic
plateau (Dome C, Dome A, South Pole) and a mid-latitude site (Mt. Graham,
Arizona). We put particular emphasis on the 2D maps of integrated astroclimatic
parameters (seeing, isoplanatic angles) calculated in different slices at
different heights in the troposhere. This is an useful tool of prediction and
investigation of the turbulence structure. It can support the optimization of
the AO, GLAO and MCAO systems running at the focus of the ground-based
telescopes.From this studies it emerges that the astronomical sites clearly
present different OT behaviors. Besides, our tool allowed us for discriminating
these sites.Comment: 7 pages, 5 figures, SPIE 2010 conferenc
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