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Optical Turbulence Characterization for Ground-Based Astronomy
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The optical turbulence, which creates perturbations of the wavefronts coming from the stars, is caused by small-scale fluctuations in the index of refraction of the atmosphere and is a problem for astronomers because it limits the maximum resolution of the ground-based telescopes. One way of identifying the best sites to build astronomical observatories, where the influence of the optical turbulence is as small as possible, is to use the standard meteorological parameters to get a first idea of the potential of a site. In the first part of this thesis the three sites on the Internal Antarctic Plateau that are the most interesting for astronomers (Dome A, Dome C and the South Pole) are investigated using the operational analyses of the ECMWF and a ranking of these three sites is presented.

The second part of this thesis focuses on the ability of the mesoscale model Meso-NH to simulate the optical turbulence as well as the wind speed at Mt Graham (AZ, USA). A rich sample of measurements of the vertical distribution of the optical turbulence, the largest sample used in this type of study so far, is used to calibrate the Meso-NH model and to quantify its ability to simulate the optical turbulence. The measurements are distributed over different periods of the year thus making it possible to evaluate the performance of the model in different seasons. Both the vertical distribution of the optical turbulence and the astroclimatic parameters (seeing, wavefront coherence time and isoplanatic angle) are investigated.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2010. , p. 68
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 778
Keywords [en]
site testing, atmospheric effects, turbulence, optical turbulence, applied meteorology, mesoscale models
National Category
Meteorology and Atmospheric Sciences Astronomy, Astrophysics and Cosmology
Research subject
Meteorology
Identifiers
URN: urn:nbn:se:uu:diva-132798ISBN: 978-91-554-7926-8 (print)OAI: oai:DiVA.org:uu-132798DiVA, id: diva2:359217
Public defence
2010-12-09, Axel Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Note
Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 708Available from: 2010-11-17 Created: 2010-10-27 Last updated: 2011-03-21Bibliographically approved
List of papers
1. Comparison of the atmosphere above the South Pole, Dome C and Dome A: first attempt
Open this publication in new window or tab >>Comparison of the atmosphere above the South Pole, Dome C and Dome A: first attempt
2008 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 387, no 4, p. 1499-1510Article in journal (Refereed) Published
Abstract [en]

The atmospheric properties above three sites (Dome C, Dome A and the South Pole) on the Internal Antarctic Plateau are investigated for astronomical applications using the monthly median of the analyses from ECMWF (the European Centre for Medium-Range Weather Forecasts). Radiosoundings extended on a yearly time-scale at the South Pole and Dome C are used to quantify the reliability of the ECMWF analyses in the free atmosphere as well as in the boundary and surface layers, and to characterize the median wind speed in the first 100 m above the two sites. Thermodynamic instability properties in the free atmosphere above the three sites are quantified with monthly median values of the Richardson number. We find that the probability to trigger thermodynamic instabilities above 100 m is smaller on the Internal Antarctic Plateau than on mid-latitude sites. In spite of the generally more stable atmospheric conditions of the Antarctic sites compared to mid-latitude sites, Dome C shows worse thermodynamic instability conditions than those predicted above the South Pole and Dome A above 100 m. A rank of the Antarctic sites done with respect to the strength of the wind speed in the free atmosphere (ECMWF analyses) as well as the wind shear in the surface layer (radiosoundings) is presented.

Keywords
turbulence, atmospheric effects, site testing, ANTARCTIC PLATEAU, SITE, ASTRONOMERS, TURBULENCE
National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:uu:diva-87235 (URN)10.1111/j.1365-2966.2008.13361.x (DOI)000257706200013 ()
Available from: 2008-09-23 Created: 2008-09-23 Last updated: 2017-12-13Bibliographically approved
2. Optical turbulence vertical distribution with standard and high resolution at Mt Graham
Open this publication in new window or tab >>Optical turbulence vertical distribution with standard and high resolution at Mt Graham
2010 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 404, no 1, p. 144-158Article in journal (Refereed) Published
Abstract [en]

A characterization of the optical turbulence vertical distribution (C-N(2) profiles) and all the main integrated astroclimatic parameters derived from the C-N(2) and the wind speed profiles above the site of the Large Binocular Telescope (LBT) (Mt Graham, Arizona, USA) is presented. The statistics include measurements related to 43 nights done with a Generalized SCIDAR (GS) used in standard configuration with a vertical resolution Delta H similar to 1 km on the whole 20 km and with the new technique (High Vertical Resolution GS) in the first kilometre. The latter achieves a resolution Delta H similar to 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 C-N(2) . A discretized distribution of C-N(2) , 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 'grey zones' for J, H and K bands. Mt Graham is confirmed to be an excellent site with median values of the seeing without dome contribution epsilon = 0.72 arcsec, the isoplanatic angle theta(0) = 2.5 arcsec and the wavefront coherence time tau(0) = 4.8 ms. We find that the OT vertical distribution decreases in a much sharper way than what has been believed so far in the proximity of the ground above astronomical sites. We find that 50 per cent 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 demonstrated in the principle of the GS technique affects these measurements by an absolutely negligible quantity (0.04 arcsec).

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-132793 (URN)10.1111/j.1365-2966.2010.16313.x (DOI)000276794600036 ()
Available from: 2010-10-27 Created: 2010-10-27 Last updated: 2017-12-12Bibliographically approved
3. Wind speed vertical distribution at Mt Graham
Open this publication in new window or tab >>Wind speed vertical distribution at Mt Graham
2010 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 407, no 4, p. 2230-2240Article in journal (Refereed) Published
Abstract [en]

The characterization of the vertical distribution of wind speed, V (h), is fundamental for anastronomical site for many different reasons: (i) the wind speed shear contributes to triggeroptical turbulence in the whole troposphere; (ii) a few of the astroclimatic parameters, suchas the wavefront coherence time (τ0), depend directly on V (h); (iii) the equivalent velocityV0, controlling the frequency at which the adaptive optics systems have to run in order towork properly, depends on the vertical distribution of the wind speed and optical turbulence.Also, too strong a wind speed near the ground can introduce vibrations in the telescopestructures. The wind speed at a precise pressure (200 hPa) has frequently been used to retrieveindications concerning τ0 and the frequency limits imposed on all instrumentation based onadaptive optics systems. However, more recently, it has been proved that V200 (the wind speedat 200 hPa) alone is not sufficient to provide exhaustive elements concerning this topic, andthus the vertical distribution of the wind speed is necessary. In this paper, we report on acomplete characterization of the vertical distribution of wind speed strength, which has beencarried out above Mt Graham (Arizona, USA), the site of the Large Binocular Telescope.We provide a climatological study extended over 10 yr using the operational analyses fromthe European Centre for Medium-Range Weather Forecasts (ECMWF). We prove that this isrepresentative of the vertical distribution of the wind speed at Mt Graham, with the exceptionof the boundary layer. We also prove that a mesoscale model can provide reliable nightlyestimates of V (h) above this astronomical site from the ground up to the top of the atmosphere(∼20 km).

Keywords
turbulence, atmospheric effects, methods: data analysis, site testing
National Category
Earth and Related Environmental Sciences Physical Sciences
Identifiers
urn:nbn:se:uu:diva-132795 (URN)10.1111/j.1365-2966.2010.17102.x (DOI)000281948100015 ()
Available from: 2010-10-27 Created: 2010-10-27 Last updated: 2017-12-12Bibliographically approved
4. Optical turbulence simulations at Mt Graham using the Meso-NH model
Open this publication in new window or tab >>Optical turbulence simulations at Mt Graham using the Meso-NH model
2011 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 412, no 4, p. 2695-2706Article in journal (Refereed) Published
Abstract [en]

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 C2(N) profiles obtained with a generalized scidar from 41 nights are used to calibrate and quantify the model's ability to reconstruct the optical turbulence above the site. 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 C2(N) 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 C2(N)) 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 per cent. The median value of the relative error night by night is equal to 18.7 per cent, 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 evidence in the model mainly affects, at present, 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 per cent. 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.

Keywords
turbulence, atmospheric effects, methods, numerical, site testing
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-132797 (URN)10.1111/j.1365-2966.2010.18097.x (DOI)000289295800046 ()
Available from: 2010-10-27 Created: 2010-10-27 Last updated: 2017-12-12Bibliographically approved

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