Generalized SCIDAR Measurements at Mount Graham

n/

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

Multi-Conjugate Adaptive Optics for LINC-NIRVANA : Laboratory Tests of a Ground-Layer Adaptive Optics System and Virtical Turbulence Measurements at Mt. Graham

Turbulence in Earth's atmosphere severely limits the image quality of ground-based telescopes. With the technique of Adaptive Optics, the induced distortions of the light can be measured and corrected in real-time, regaining nearly diffraction-limited performance. Unfortunately, when using a single guide star to measure the distortions, the correction is only useful within a small angular area centered on the guide star. The first part of this thesis presents a laboratory setup, which uses four guide stars to measure the turbulence-induced distortions and one deformable mirror to correct the most turbulent layer. With such a Layer-Oriented Ground-Layer Adaptive Optics (GLAO) system, the area of useful correction is significantly increased. The system is characterized in static and dynamic operation, and the influence of non-conjugated turbulent layers, the effect of brightness variations of the guide-stars and the impact of misalignments are studied. Furthermore, calibration strategies and the performance of the Kalman control algorithm are examined. The second part of this thesis focuses on SCIDAR measurements of the atmospheric turbulence above Mt. Graham. This dataset provides for the first time a statistical and thorough analysis of the vertical turbulence structure above the LBT site. Based on 16 nights of measurements, spread over one year, Mt. Graham appears to be an excellent site for an astronomical observatory. By extending an analytical model, describing the filtering of the turbulence-induced distortions by an AO system, we calculate performance expectations of the LINC-NIRVANA instrument. In particular, the optimal conjugation heights of the deformable mirrors are studied. Furthermore, we present a new method to measure the atmospheric turbulence near the ground with 40 times increased vertical resolution, compared to standard SCIDAR. First on-sky results demonstrate the power of this technique

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

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

Efficient, Dual-particle Directional Detection System Using A Rotating Scatter Mask

A directional radiation detection system and an omnidirectional radiation detector. The omnidirectional radiation detector detects radiation comprising at least one of: (i) gamma rays; and (ii) neutron particles. A radiation scatter mask (RSM) of the radiation detection system includes a rotating sleeve received over the omnidirectional radiation detector and rotating about a longitudinal axis. The RSM further includes: (i) a fin extending longitudinally from one side of the rotating sleeve; and (ii) a wall extending from the rotating sleeve and spaced apart from the fin having an upper end distally positioned on the rotating sleeve spaced apart or next to from a first lateral side of the fin and a lower end proximally positioned on the rotating sleeve and spaced apart from or next to a second lateral side of the fin

Prevalence of Two-Syllable Digits Affecting Forward Digit Span Test Score: A Potential Reliability Factor in Digit Span Tests and New Light to the Word Length Effect

The word length effect shows a connection between word length and working memory performance. Although the relationship between digit verbal length and digit span has been investigated between languages, it has not been investigated within a language. It was hypothesized that this effect can be shown as a connection between the prevalence of digits with two syllables and digit span score. The study examined the effect of amount of syllables on Norwegian digit span test scores by altering the prevalence of two-syllable digits using three conditions in a repeated measures design (N = 54). Results suggest that an elimination of two-syllable digits in a digit span test significantly reduced forward span test score (Cohen’s d = 0.36), but had no effect on backward span scores. These results suggest that a balanced distribution of two-syllable digits in a forward digit span tests should theoretically increase the test’s comparability and reduce language-related biases thus increasing the test’s parallel-form reliability. A peak-span model is proposed to integrate the findings into previous research on the word length effect