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The Determination of Seeing, Isoplanatic Patch Size and Coherence Time by Solar Shadow Band Ranging
Abstract
Using an array of solar scintillometers (Seykora 1993; Beckers and Mason
1998), I characterize the shadow band patterns as seen from the
un-eclipsed sun. The spatial and temporal structure of these patterns
provide a measure of the optical turbulence in the earth' atmosphere
from which the r_0 (Fried Parameter), Θ_0 (isoplanatic patch
diameter) and τ_0 (coherence time) can be estimated.
... 14 In 1993, Seykora 15 showed that scintillation measurements can be used to estimate the C 2 N (h) profile. It was further extended by Beckers [16][17][18] and is known as SHABAR (SHAdow BAnd Ranging). It was extensively used in the DKIST (Daniel K. Inouye Solar Telescope, formerly ...
The Indian Institute of Astrophysics (IIA) is developing a Multi-Conjugate Adaptive Optics (MCAO) system for the Kodaikanal Tower Telescope (KTT). In this context, we have measured the daytime turbulence strength profile at the Kodaikanal Observatory. The first method based on wavefront sensor (WFS) images, called S-DIMM+ (Solar-Differential Image Motion Monitor+), was used to estimate the higher altitude turbulence up to a height of 5 - 6 km. The second method used balloon-borne temperature sensors to measure the near-Earth turbulence up to 350 m. We also carried out simulations to validate the performance of our system. We report the first-ever daytime turbulence strength profile measurements at the observatory. We have identified the presence of a strong turbulence layer about 3 km above the observatory. The measured near-Earth turbulence matches the trend that is expected from the model for daytime component of turbulence and gives an integrated of about 4 cm at 500 nm. This is consistent with earlier seeing measurements. This shows that a low-cost setup with a small telescope and a simple array of temperature sensors can be used for estimating the turbulence strength profile at the site.
... However, the probed turbulence volume is limited to distances below ≈ /Θ, where represents the pupil diameter. A third strategy, which provides an alternative to triangulation, is to perform single source scintillation measurements and use the dependence with of the scintillation signature (iii) to perform the profiling, a feature exploited by MASS [30,31], by SHABAR [32], and by Single Star SCIDAR [33]. However, relying on this sole information leads to a limited spatial resolution. ...
CO-SLIDAR is a very promising technique for the metrology of near ground profiles. It exploits both phase and scintillation measurements obtained with a dedicated wavefront sensor and allows profiling on the full line of sight between pupil and sources. This technique is applied to an associated instrument based on a mid-IR Shack-Hartmann wavefront sensor, coupled to a 0.35 m telescope, which observes two cooperative sources. This paper presents the first comprehensive description of the CO-SLIDAR method in the context of near ground optical turbulence metrology. It includes the presentation of the physics principles underlying the measurements, of our unsupervised profile reconstruction strategy together with the error bar estimation on the reconstructed values. The application to data acquired in a heterogeneous rural landscape during an experimental campaign in Lannemezan (France) demonstrates the ability to obtain profiles with a sampling pitch of about 220 m over a 2.7 km line of sight. The retrieved profiles are presented and their variability in space and time is discussed.
... However, the probed turbulence volume is limited to distances below ≈ /Θ, where represents the pupil diameter. A third strategy, which provides an alternative to triangulation, is to perform single source scintillation measurements and use the dependence with of the scintillation signature (iii) to perform the profiling, a feature exploited by MASS [30,31], by SHABAR [32], and by Single Star SCIDAR [33]. However, relying on this sole information leads to a limited spatial resolution. ...
Coupled slope and scintillation detection and ranging (CO-SLIDAR) is a very promising technique for the metrology of near ground C n 2 profiles. It exploits both phase and scintillation measurements obtained with a dedicated wavefront sensor and allows profiling on the full line of sight between pupil and sources. This technique is applied to an associated instrument based on a mid-IR Shack–Hartmann wavefront sensor coupled to a 0.35 m telescope, which observes two cooperative sources. This paper presents what we believe is the first comprehensive description of the CO-SLIDAR method in the context of near-ground optical turbulence metrology. It includes the presentation of the physics principles underlying the measurements of our unsupervised C n 2 profile reconstruction strategy together with the error bar estimation on the reconstructed values. The application to data acquired in a heterogeneous rural landscape during an experimental campaign in Lannemezan, France, demonstrates the ability to obtain profiles with a sampling pitch of about 220 m over a 2.7 km line of sight. The retrieved C n 2 profiles are presented and their variability in space and time is discussed.
... [19][20][21] Note that triangulation brings a strong and simple geometrical constrain (see (iv)) that helps profiling, however the probed turbulence volume is limited to distances below Z max ≈ D/Θ which often implies meter class telescopes to probe a few tens of kilometers, not to mention the issue of the availability of double stars with appropriate magnitude and angular separation. A third strategy, which gives an alternative to triangulation, is to perform single source scintillation measurements and use the dependence with z of the scintillation signature (iii)) to perform the profiling (MASS, 22,23 SHABAR,24 Single Star SCIDAR 25 ), relying on this sole information however leads to limited spatial resolution. We have on our side developed a fourth strategy called CO-SLIDAR 26, 27 that has the advantage of using all physical properties (i) to (iv) thanks to the association of slope and scintillation measurements on a double source. ...
Cn² est la constante de structure de l'indice de réfraction provoquée par les variations de température, d'humidité et de pression dans l'atmosphère. Le profil de Cn² caractérise localement la force de la turbulence. La méthode CO-SLIDAR, développée par l’ONERA, permet de réaliser des profils de Cn² le long de la ligne de visée d'un télescope à partir des pentes et des scintillations mesurées par un Analyseur de Shack-Hartmann sur source double. Les deux expériences à Lannemezan et à Châtillon-Meudon ont vu la mise en place d'un nouveau profilomètre de Cn² doté d'un Shack-Hartmann Infrarouge : le SCINDAR. Elles ont été réalisées sur des surfaces respectivement hétérogène et homogène par morceaux, et elles participent à la validation de la méthode pour des applications agronomiques et écologiques.Mon étude consiste à améliorer le traitement du signal du profilomètre SCINDAR et à valider la méthode CO-SLIDAR pour des mesures de la turbulence atmosphérique proche du sol. Cette méthode a été adaptée en utilisant un formalisme de propagation en onde sphérique.L'étude a permis d'identifier et prendre en compte des sources d'erreur dans le traitement : à savoir la vibration de la machine à froid de l'analyseur de front d'onde cryogénique du SCINDAR et l'étendue des sources dans les fonctions de poids du modèle direct posé pour le traitement des données. La régularisation L1L2, qui est adaptée pour des mesures de Cn² proches du sol, a été choisie. La méthode de réglage des hyperparamètres de cette régularisation est non-supervisée. Mon étude se consacre ainsi aux améliorations du traitement des données du SCINDAR et à la validation expérimentale des profils de Cn² obtenus avec des mesures acquises par des scintillomètres. Ces améliorations permettent d’augmenter la fiabilité et la précision de l’estimation du profil de Cn² de façon pragmatique à l'aide des erreurs relatives sur les paramètres turbulents : paramètre de Fried et taux de scintillation. Leur application aux données SCINDAR acquises sur un paysage rural hétérogène à Lannemezan montre la possibilité d’obtenir une résolution de 170 m sur une ligne de visée de 2,7 km. Pour l'expérience de Meudon, le profil Cn² est estimé sur les quatre zones urbaines et forestières. Les différentes natures du sol et la topographie le long de la ligne de visée expliquent la variabilité spatiale du Cn². Le profilomètre SCINDAR avec la méthode CO-SLIDAR produit finalement des profils de Cn² d'excellente qualité avec de petites barres d'erreur statistique, qui sont comparés avec succès aux mesures des scintillomètres. La connaissance précise de la turbulence atmosphérique en visée horizontale permettra de mieux appréhender la physique des flux de chaleur à l’interface sol-atmosphère.
An array of photodetectors, responding to scintillation of light from
the Moon, provides a probe of the turbulence structure along the line of
sight through the atmosphere. The measured quantities are time series of
intensity fluctuations received by the individual detectors in the
array. From the covariance of these fluctuations, taken between all
possible detector pairs, one can determine the C2N
turbulence profile from the ground up to a maximum height determined by
the separation between the detectors. This paper outlines the
theoretical basis of the technique and discusses methods of data
analysis. We then describe the design and construction of a prototype
eight-element lunar scintillometer array and present first results of
observations using this system.
In this article I describe a site survey facility, which measures the signals of the solar equivalent of a Differential Image Motion Monitor (S-DIMM) and of a six element linear array of solar scintillometers. Combining the S-DIMM r
o andscintillometer I observations allows the determination of the fractions of the seeing in the free atmosphere and in the ground/lake layer. From the scintillometer array observations C
n
2
(h) is determined for heights corresponding to the first 500 m along the line-of-sight. With minor changes this seeing monitor can also be used for other extended objects like the Moon and planets.
To evaluate site quality and to develop multi-conjugative adaptive optics systems for future large solar telescopes, characterization of contributions to seeing from heights up to at least 12 km above the telescope is needed. We describe a method for evaluating contributions to seeing from different layers along the line-of-sight to the Sun. The method is based on Shack Hartmann wavefront sensor data recorded over a large field-of-view with solar granulation and uses only measurements of differential image displacements from individual exposures, such that the measurements are not degraded by residual tip-tilt errors. We conclude that the proposed method allows good measurements when Fried's parameter r_0 is larger than about 7.5 cm for the ground layer and that these measurements should provide valuable information for site selection and multi-conjugate development for the future European Solar Telescope. A major limitation is the large field of view presently used for wavefront sensing, leading to uncomfortably large uncertainties in r_0 at 30 km distance. Comment: Accepted by AA 22/01/2010 (12 pages, 11 figures)
In 1996 we started a seeing survey of a number of existing and potential
solar observing sites using solar scintillometry. This paper reports the
result of the first year of that survey. It confirms earlier reports
about the superior observing conditions of lake sites. I also describe
the first results of atmospheric structure constant (C(subscript
n)(superscript 2)) probing using a solar scintillometer array.
A non-telescopic method of determining the quality of atmospheric seeing is discussed for large angular diameter objects, such as the Sun. In this method the concept of thermodynamic fluctuations is used to relate the observed intensity fluctuations to the r.m.s. angular diameter of the atmospheric seeing cells and telescopic angle of arrival fluctuations. Comparison between the measured r.m.s. cell size and telescopic angle of arrival fluctuations are presented for various degrees of seeing. Cross-correlation coefficients of 0.95 have been measured during such comparisons.