Single Star Scidar Research Articles

Single star SCIDAR: Atmospheric parameters profiling using the power spectrum of scintillation

Optical parameters of atmospheric turbulence have a significant impact on high angular resolution, adaptive optics, and site testing. The single star SCIDAR technique provides vertical profiles of these parameters, including the refractive index structure constant Cn2(h). Its is based on the analysis of single stars scintillation. This study introduces a new approach for real-time measurement of atmospheric parameters using the Single Star SCIDAR. The key element of this approach is the utilization of a modified power spectrum of atmospheric speckles, that have more significant variation with altitude. By using that modified power spectrum, an objective function is computed, and the minimization process is performed using the Active-Set algorithm. With this approach, we successfully obtained real-time vertical profiles of Cn2(h) with good accuracy. The processing takes approximately 3 s per profile, and the recovery rate (i.e. sum of Cn2(h)) is about 95%. The reliability of this approach is validated through simulation results and a comparison with data obtained from sounding balloons. These validations confirm the accuracy and credibility of this method, which can be useful in many practical applications.

Optical turbulence characterization at LAMOST site: observations and models

Atmospheric optical turbulence seriously limits the performance of high angular resolution instruments. An 8-night campaign of measurements was carried out at the LAMOST site in 2011, to characterize the optical turbulence. Two instruments were set up during the campaign: a Differential Image Motion Monitor (DIMM) used to measure the total atmospheric seeing, and a Single Star Scidar (SSS) to measure the vertical profiles of the turbulence C_n^2(h) and the horizontal wind velocity V(h). The optical turbulence parameters are also calculated with the Weather Research and Forecasting (WRF) model coupled with the Trinquet-Vernin model, which describes optical effects of atmospheric turbulence by using the local meteorological parameters. This paper presents assessment of the optical parameters involved in high angular resolution astronomy. Its includes seeing, isoplanatic angle, coherence time, coherence etendue, vertical profiles of optical turbulence intensity _n^2(h)$ and horizontal wind speed V(h). The median seeing is respectively 1.01 arcsec, 1.17 arcsec and 1.07arcsec as measured with the DIMM, the SSS and predicted with WRF model. The history of seeing measurements at the LAMOST site are reviewed, and the turbulence measurements in this campaign are compared with other astronomical observatories in the world.

Multi-instrument characterization of optical turbulence at the Ali observatory

In order to characterize the atmospheric optical turbulence at Ali observatory, we have deployed multi-instruments, which are able to continuously monitor the optical turbulence for site evaluation. These instruments include the DIMM, MASS, Single Star SCIDAR and Polaris seeing monitor, and we also plan to install SNODAR and Micro-thermal sensors for the turbulence on surface layer by the end of this year. This configuration allows us to collect a substantial database and make cross-comparison of the results. We have successfully obtained the profiles of optical turbulence and wind speed with Single Star SCIDAR, as well as the key parameters for adaptive optics application, such as seeing, coherence time, and isoplanatic angle. The DIMM seeing measurements are also carried out simultaneously. The median seeing measured by the DIMM and SSS in 2013 is 0.69 and 0.79 arcsec, respectively.

Optical turbulence profiling with single star SCIDAR technique

In order to characterize optical turbulence, we have developed a single star SCIDAR (SSS) for measurement of the distribution of C n 2 with height. The SSS consists of a 40 cm telescope and a CCD camera for fast sampling of stellar scintillation pattern. Spatiotemporal auto and cross-correlation functions of the single star images are computed, providing vertical profiles of optical turbulence intensity C n 2 (h) and wind speed V(h). Using this new SSS experiment, profiles of turbulence can be obtained from the ground to the top of atmosphere, allowing the determination of seeing, isoplanatic angle and coherence time. Detailed characteristics of atmospheric optical turbulence are important for active and passive imaging, astronomical site testing, adaptive optics, laser communications, target tracking and designation, and laser beam control. We plan to improve the robotization of the SSS to be able to use it routinely even under harsh weather and altitude conditions that we expect to encounter on the high Tibetan plateau or at Dome A in Antarctica. SSS will also be applied for the site testing campaign of the future Chinese extremely large telescope.

SCIDAR: an optical turbulence profiler for Dome A

AbstractThis paper introduces a plan to detect turbulence profiles at Dome A with a Single Star Scidar (SSS), to enhance our understanding of the characteristics of the site. The development of a portable monitor for profiling vertical atmospheric optical turbulence and wind speed is presented. By analyzing the spatial auto and cross-correlation functions of very short exposure images of single star scintillation patterns, the SSS can provide the vertical profiles of turbulence intensity C2n(h) and wind speed V(h). A SSS prototype is already operational at Ali in Tibet which will be improved in order to become fully robotic and adapted to extreme weather conditions that prevail at Dome A in Antarctica.

Dome C Site Characterization in 2006 with Single-Star SCIDAR

ABSTRACTWe present observations made in 2006 with the single-star SCIDAR (SSS) at Dome C in Antarctica, allowing us to determine optical turbulence and velocity V(h) profiles from ice levels up to about 25 km above sea level (a.s.l.). SSS is a 16 inch telescope placed on an equatorial mount that continuously tracks the Canopus star. About 90,000 individual profiles are analyzed from March to September, where surface-layer contribution to seeing can be separated from the rest of the atmosphere. Medians of high angular resolution parameters relevant to astronomy are statistically studied, such as seeing (1.0′′), isoplanatic angle (6.9′′), and wavefront coherence time (3.4 ms). For a telescope placed above the turbulent surface layer, superb conditions are encountered (medians of seeing better than 0.3′′, isoplanatic angle better than 6.9′′, and coherence time larger than 10 ms). Astronomical conditions are twice as good at the beginning of the night, with ε0 ≈ 0.5′′, θ0 ≈ 11.5′′, and τ0 ≈ 15 ms. SSS wind-velocity profiles are consistent with National Oceanic and Atmospheric Administration analysis up to 17 km (a.s.l.), within a error bar. Coherence étendue (which is a combination of ε0, θ0, and τ0), well adapted to adaptive optics performances, is likely 4 times better at Dome C than at the already-known observatories such as Mauna Kea or ORM.

First single star scidar measurements at Dome C, Antarctica

Aims. First operational running of the Single Star Scidar (SSS) under harsh weather conditions at Dome C in Antarctica. Continuous monitoring of the optical turbulence and wind speed profiles throughout the whole atmosphere. Methods. SSS is mainly composed of “off the shelf” light-weight components, a 16 inches telescope installed on an equatorial mount. Scintillation patterns are computed (auto and cross-corre lations) in real time and analyzed off line to retrieve continuously vertical profiles of optical turbulence C 2 (h) and wind speed V(h), from ground up to 20 km . Results. Using a simulated annealing method, we have analyzed about 6.5 hours of observations, putting into evidence the strong s urface layer contribution to seeing degradation. SSS results show a good agreement with simultaneous measurements with a Differential Image Motion Monitor as well as weather archive from NOAA. Conclusions. SSS has shown its usefullness for site characterization sin ce it measures simultaneously C 2 N and V profiles, from which most of adaptative optic parameters are deduced, such as isoplanatic angle and coherence time of the wavefront. Due to its small size, it is well adapted to site characterization, even when low infrastructure is available.

Using meteorological forecasts to predict astronomical ‘seeing’

The main limitations affecting visible-light and IR astronomy are related to visibility, atmospheric humidity, and spatial resolution. High-resolution imaging is fundamentally limited by optical turbulence (OT) in the atmosphere. OT is induced by dynamical turbulence developing within a stratified temperature field and depends on weather conditions. The resulting degraded telescope resolution is called ‘seeing.’ At the best ground-based observatories, characterized by a seeing of order 0.5 arcseconds, the resolution of a telescope with an aperture diameter of 10m is no better than 20cm (at ground level) at optical wavelengths. At the present time, when large and expensive telescope projects are increasingly becoming the norm, observatory managers rely on a proper assessment and understanding of weather conditions for optimal scheduling of scientific operations. Flexible rotas (i.e., planning optimal use of the available instrumentation) are now frequently used at the largest astronomical observatories. Site testing in preparation for new observatory developments is another important challenge. For site characterization for the European 42m (diameter) Extremely Large Telescope (E-ELT)1 or the Antarctic ‘Dome C’ project,2 we designed the DIMM (differential image-motion monitor) and SSS (single-star scidar)3 optical monitors. Since monitoring requires the availability of specific instruments and logistics, and it is limited in place and time, we are investigating other ways to retrieve atmospheric OT conditions frommeteorological measurements or forecasts. Atmospheric conditions are defined by the speed, temperature, and composition of the constituent elements. The corresponding state of the atmosphere is governed by the Navier-Stokes equations prevalent above and around the site. For astronomical seeing prediction, one must obtain the local (microscopic) atmospheric conditions on the metric OT scale. Several models can be used, but most are computationally time Figure 1. Seeing forecasting scheme. OT: Optical turbulence. Cn2 (Cn): OT index. P.T.U.: Pressure, temperature, humidity.

Single Star Scidar at Dome C

Here we present the first light of the Single Star Scidar (SSS) at Dome C, during January 2005 and the first results obtained during the winter 2006. We recall the SSS principle, how it is designed to resist to very low temperatures and to be adjusted in a very simple way. No exemple of long period of observation can be given since the huge amount of data cannot be sent back to our laboratory to be offline processed, due to low data transfer available from Dome C. But, a short file is shown, which demonstrates the major contribution of the surface layer to the whole atmosphere optical turbulence. Almost 300 hours of observations have been taken successfully during winter 2006.

Single Star Scidar first light from Dôme C

In the recent years, a lot of instruments have been put into operation during the polar summer at DômeC., Then, during the first polar night when the Astro-Concordia sation was open for the first time during winter, about 40 balloons (Azouit & Vernin) instrumented to measure optical turbulence profiles and 2 Differencial Image Motion Monitors (DIMM) were setup. The main results from this first important campaign are found in Agabi et al. (2006). It appears from this first night time observations that almost all the optical turbulence was concentrated in the first 30 m above the ice. At an elevation of 8.5 m above the ice the seeing is about 1″.4, while above an elevation of 30 m the seeing drops down to 0″.36. This last figure is coherent with the estimation from Lawrence et al. (2004) if one takes into account that they were not sensitive to the first 30 m., which corresponds to the turbulent surface layer.