Optical turbulence characterization at LAMOST site: observations and models (original) (raw)
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Ground-based and Airborne Telescopes II, 2008
Since November 2004 we measured the optical turbulence (C 2 N profiles) with a Generalized Scidar (GS) placed at the focus of the Vatican Advanced Technology Telescope at Mt.Graham, Arizona. The present statistic consists in measurements related to 43 nights covering different periods of the solar year. In this paper we calculate the statistics of the astroclimatic parameters (C 2 N , seeing ε , isoplanatic angle θ 0 , wavefront coherence time τ 0 ) and we compare these values with those measured above other top level astronomic sites. All profiles are reduced into a form suitable to be used as inputs for adaptive optics point spread function simulations for the conceptual design of the Laser Guide Star Facility supported by a GLAO system of the Large Binocular Telescope. With GS measurements done observing wide binaries (30-35 arcsec), the turbulence in the first kilometer above the ground is characterized with the vertical resolution (200-250 m) required for the optimization of a 4 arcmin field of view AO system. It is the first time that are published measurements of the optical turbulence vertical distribution above a mid-latitude site with such a high vertical resolution and such a high statistical reliability. On 8 of those nights, employing cross-correlation scintillation maps of wide binaries and the method described in Ref.
Atmospheric turbulence measurements at Ali Observatory, Tibet
Ground-based and Airborne Telescopes IV, 2012
The atmospheric turbulence characteristics are important to evaluate the quality of ground-based astronomical observatory. In order to characterize Ali observatory, Tibet. we have developed a single star Scidar (SSS) system, which is able to continuously monitor the vertical profiles of both optical turbulence and wind speed. The main SSS configuration includes a 40cm telescope and a CCD camera for fast sampling the star scintillation pattern. The SSS technique analyzes the scintillation patterns in real time, by computing the spatial auto-correlation and at least two cross-correlation images, and retrieves both C 2 n (h) and V (h) vertical profiles from the ground up to 30km. This paper presents the first turbulence measurements with SSS at Ali observatory in October, 2011. We have successfully obtained the profiles of optical turbulence and wind speed, as well as the key parameters for adaptive optics, such as seeing, coherence time, and isoplanatic angle. The favourable results indicate that Ali observatory can be an excellent astronomical observatory.
Solar seeing monitor MISOLFA: A new method for estimating atmospheric turbulence parameters
Astronomy & Astrophysics, 2016
Aims. Daily observation conditions are needed when observing the Sun at high angular resolution. MISOLFA is a daytime seeing monitor developed for this purpose that allows the estimation of the spatial and temporal parameters of atmospheric turbulence. This information is necessary, for instance, for astrometric measurements of the solar radius performed at Calern Observatory (France) with SODISM II, the ground-based version of the SODISM instrument of the PICARD mission. Methods. We present a new way to estimate the spatial parameters of atmospheric turbulence for daily observations. This method is less sensitive to vibrations and guiding defaults of the telescope since it uses short-exposure images. It is based on the comparison of the optical transfer function obtained from solar data and the theoretical values deduced from the Kolmogorov and Von Kàrmàn models. This method, previously tested on simulated solar images, is applied to real data recorded at Calern Observatory in July 2013 with the MISOLFA monitor. Results. First, we use data recorded in the pupil plane mode of MISOLFA and evaluate the turbulence characteristic times of angle-ofarrival fluctuations: between 5 and 16 ms. Second, we use the focal plane mode of MISOLFA to simultaneously record solar images to obtain isoplanatic angles: ranging from 1 to 5 arcsec (in agreement with previously published values). These images and our new method allow Fried's parameter to be measured; it ranges from 0.5 cm to 4.7 cm with a mean value of 1.5 cm when Kolmogorov's model is considered, and from less than 0.5 to 2.6 cm with a mean value of 1.3 cm for the Von Kàrmàn model. Measurements of the spatial coherence outer scale parameter are also obtained when using the Von Kàrmàn model; it ranges from 0.25 to 13 m with a mean value of 3.4 m for the four days of observation that we analyzed. We found that its value can undergo large variations in only a few hours and that more data analysis is needed to better define its statistics.
Two campaigns to compare three turbulence profiling techniques at Las Campanas Observatory
2008
In preparation to characterize the Giant Magellan Telescope site and guide the development of its adaptive optics system, two campaigns to systematically compare the turbulence profiles obtained independently with three different instruments were conducted at Las Campanas Observatory in September, 2007 and January 2008. Slope detection and ranging (SLODAR) was used on the 2.5-m duPont telescope. SLODAR measures the C2n profile as a function of altitude through observations of double stars. The separation of the observed double star sets the maximum altitude and height resolution. Ground layer (altitudes < 1 km) and free atmosphere turbulence profiles are compared with those obtained with a lunar scintillometer (LuSci) and a multi-aperture scintillation sensor (MASS), respectively. In addition, the total atmospheric seeing was measured by both SLODAR and a differential image motion monitor (DIMM).
Turbulence Profiles with Generalized Scidar at San Pedro Mártir Observatory and Isoplanatism Studies
Publications of the Astronomical Society of the Pacific, 1998
The results obtained from 3398 vertical pro les of atmospheric turbulence measured during 11 nights at the Observatorio Astron omico Nacional in San Pedro M artir (Baja California, M exico) are presented. The observations were carried out with the generalized scidar (GS) installed at the 1.5 m and the 2.1 m telescopes of that site, in March and April 1997. The open air seeing was measured with a di erential image motion monitor (DIMM). The GS can detect turbulence pro les along the whole optical path, unlike the classical scidar which is insensitive to low altitude turbulence. For the rst time, to our knowledge, pro les including turbulence near the ground are monitored and statistically analyzed. Isoplanatic angles for speckle interferometry and adaptive optics (AO) in either full or partial compensation are deduced, as well as the focus anisoplanatism parameter for sodium laser guide stars. The advantage of minimizing the distance between the turbulent layers and the conjugated plane of the deformable mirror of an AO system is studied. The comparison of GS pro les obtained at both telescopes, together with DIMM measurements, show that the turbulence near the ground is more strongly dominant at the 1.5 m telescope than at the 2.1 m telescope, where the median values of the seeing near the ground, in the free atmosphere and in the whole optical path are 0.56, 0.44 and 0.77 arcsec, respectively. These values are comparable to or better than those of the major astronomical observatories, although a larger data sample is needed for a de nitive comparison.
SITE CHARACTERISATION: ASTRONOMICAL SEEING FROM A TURBULENCE-RESOLVING MODEL
South African Journal of Geology, 2011
A Lunar Laser Ranging (LLR) system is to form part of geodetic instrumentation to be located at a new fundamental space geodetic observatory for South Africa. For optimal efficiency, LLR requires optical resolution or so-called astronomicai seeing conditions of -1 arc-second in order to deliver usable ranging data. Site characterisation should include a description of astronomical seeing for various locations on-site and overall atmospheric conditions. Atmospheric rurbulence degrades astronomical seeing. In-siru methods of determining astronomical seeing are difficult, time-consuming and costly. \7e propose the use of a turbulence-resolving model to determine and predict astronomical seeing at a site. Large Eddy Simulation NERSC (Nansen Environmental and Remote Sensing Centre) Improved Code (LESNIC) is a turbulence-resolving simulation code which models atmospheric turbulence. It has been used to compile a database of turbulence-resolving simulations, referred to as DAfABASE64. This database consists of a collection of LESNIC runs for a stably stratified planetary boundary layer (SBL) over a homogeneous aerodynamically rough surface. Results from DATABASE64 for the nocturnal boundary layer are employed to render profiles of the vertical distribution of optical turbulence (Cniprofiles). Seeing parameter values are aiso obtained by making use of DAIABASE64 results. The 6112 profiles and seeing parameter values obtained from DATABASE64 results are compared with general observational results that have been published in the literature. The values obtained are consistent with results from fieid campaigns as reported. Turbulence-resolving models, such as LESNIC, show potential for delivering and predicting profiles and parameters to characterise astronomical seeing, which are essential prerequisites for establishing an LLR system at the most suitable site and most suitable on-site location. A two-pronged approach is envisagedin addition to modelling, quantitative seeing measurements obtained with an on-site seeing monitor will be used to veri$' and calibrate results produced by the LESNIC model.
Optical turbulence characterization at the SAAO Sutherland site
Monthly Notices of the Royal Astronomical Society, 2013
We present results from the first year of a campaign to characterize and monitor the optical turbulence profile at the South African Astronomical Observatory's Sutherland observing station in South Africa. A Multi Aperture Scintillation Sensor Differential Image Motion Monitor (MASS-DIMM) was commissioned in 2010 March to provide continuous monitoring of the seeing conditions. Over the first month of the campaign, a Slope Detection And Ranging (SLODAR) from Durham University was also installed, allowing an independent verification of the performance of the MASS-DIMM device. After the first year of data collection, the overall median seeing value is found to be 1.32 arcsec as measured at ground level. The ground layer which includes all layers below 1 km accounts for 84 per cent of the turbulence, while the free atmosphere above 1 km accounts for 16 per cent with a median value of 0.41 arcsec. The median isoplanatic angle value is 1.92 arcsec, which is similar to other major astronomical sites. The median coherence time, calculated from corrected MASS measurements, is 2.85 ms. The seeing conditions at the site do show a strong correlation with wind direction, with bad seeing conditions being associated with winds from the southeast .
Astronomy & Astrophysics, 2014
Aims. An experiment was set up at the Concordia station in Antarctica during the winter-over period in 2012 to determine the behaviour of atmospheric optical turbulence in the lower part of the atmospheric boundary layer. The aim of the experiment was to study the influence of turbulence and weather conditions on the quality of astronomical observations. The Concordia station is characterised by the high quality of astronomical images thanks to very low seeing values. The surface layer in the interior of Antarctica during the winter is very stably stratified with the differences of temperature between the surface and the top of the inversion, which reach 20−35 • C. In spite of this strong static stability, considerable thermal optically active turbulence sometimes occurs and extends to several tens of metres above the surface, depending on weather conditions. It is important to know the meteorological characteristics that favour good astronomical observations. Methods. The optical measurements of the seeing made by differential image motion monitors installed at two levels of 8 and 20 m were accompanied by observations of turbulence in the lowest one hundred meters. Turbulence was detected and evaluated using a high-resolution sodar developed specially for this purpose. The statistics of some relevant meteorological variables including the long-wave downward radiation, which indicates cloudiness, were determined. Results. Typical patterns of the vertical and temporal structure of turbulence shown by sodar echograms were identified, analysed, and classified. The statistics of the depth of the surface-based turbulent layer and the turbulent optical factor for different height layers are presented together with the seeing statistics. We analysed the dependence of both seeing and integral turbulence intensity within the first 100 m on temperature and wind speed. Conclusions. Seeing and turbulence intensity in the atmospheric boundary layer appear to be correlated. The best values of the seeing (<1 arcsec) are observed when the sodar shows very low turbulence intensity. The main contribution to the image distortion is due to turbulence generated within the lowest 30−50 m near the surface. The presented statistics of the vertical distribution of the atmospheric optical turbulence can be used to determine the optimal location for astronomical instruments.
LOLAS-2: Redesign of an Optical Turbulence Profiler with High Altitude-resolution
Publications of the Astronomical Society of the Pacific, 2016
We present the development, tests and first results of the second generation Low Layer Scidar (LOLAS-2). This instrument constitutes a strongly improved version of the prototype Low Layer Scidar, which is aimed at the measurement of optical turbulence profiles close to the ground, with high altitude-resolution. The method is based on the Generalised Scidar principle which consists in taking double-star scintillation images on a defocused pupil plane and calculating in real time the autocovariance of the scintillation. The main components are an open-truss 40-cm Ritchey-Chrétien telescope, a german-type equatorial mount, an Electron Multiplying CCD camera and a dedicated acquisition and real-time data processing software. The new optical design