| II. Review Paper: OBSERVATIONAL METHODS FOR THE STUDY OF OPTICAL TURBULENCE Remy Avila |
The study of optical turbulence in the atmosphere spreads over a wide range of fields, from fundamental science, like astronomy, to commercial and military applications. Different observational methods have been developed in different fields. In this talk I will review the existing methods, focusing attention in those emploid for optical astronomy. Two kinds of methods can be distinguished. The techniques that study the properties of the wavefront reaching the ground like its spatial and temporal correlations, and those that measure the vertical distribution of the turbulence properties like its strength, the wind velocity and the local outer scale.
The advantages and disadvantages of each known method will be outlined.
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| II.1 COMPARATIVE STUDY OF MODELS LEADING TO ATMOSPHERIC TURBULENCE PROFILE FROM OPTICAL OBSERVATIONS FOR DAY-TIME OBSERVATIONS Abd-Errezak Bouzid, Abdanour Irbah, Julien Borgnino & Henri Lanter |
Atmospheric turbulence is responsible of spatio-temporal fluctuations of astronomical object image
obtained by means of ground-based telescopes. They are globally due to the Angle-of-Arrival (AOA)
fluctuations of the incoming wave-fronts integrated all over the telescope pupil. Fried (1977) established
an expression that links the angular structure function d(theta) of the object image fluctuations observed
in the telescope focal plane to the atmospheric turbulence profile Cn2(h) (structure constant of the
air-refraction-index fluctuations) along the light of sight. In a previous work (Bouzid et al., 1999)
this model allowed us to estimate the optical turbulence profile Cn2(h) from fluctuations observed on
the solar limb. A new formulation of the angular structure function d(theta) also linked to Cn2(h) has
been developed considering furthermore the spatial coherence outer scale L0. We present first in this
work a brief recall of the model formulations and then develop a comparative study of the Cn2(h) profiles
obtained from the two models. Solar data recorded at the Calern Observatory astrolabe (France) are used for
this purpose. The obtained results are presented and discussed. Ref :
Bouzid A., Irbah A., Borgnino J. and Lanteri H., Atmospheric Turbulence Profile Estimation from Fluctuation Analysis of Extended Object Images, 14th ESA Symposium, Potsdam, 29/05/99 –03/06/99, ESA SP-437, Sep. 1999, pp. 453-458. Fried D. L., 1977, The Nature of Atmospheric Turbulence Effects on Imaging and Pseudo-Imaging Systems and its Quantification, in : High Angular Resolution Stellar Interferometry, Colloquium n0 50 (U.A.I.), 4.1-4.44. |
| II.2 TURBULENCE PROFILE ESTIMATION AT CALERN OBSERVATORY USING SOLAR DATA Mohamed Chibani |
Atmospheric turbulence is responsible of spatio-temporal fluctuations of astronomical object images obtained by means of ground-based telescopes. They appear in particular as angle-of-arrival fluctuations of the incoming wavefronts integrated all over the telescope pupil.
A new model linking the angular structure function of the solar limb fluctuations to the atmospheric turbulence profile through a Fredholm integral equation of the first kind has been proposed. The inversion of this new expression allows also to estimate atmospheric turbulence profile from the observed angular structure function. In this work, we use this model to estimate the atmospheric turbulence profile from the analysis of data obtained with the solar astrolabe of Calern Observatory (France). The results are presented and discussed relatively to those obtained from an other model.
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| II.3 EXPERIMENTAL TESTS OF TURBULENCE PROFILING USING SCINTILLATION PROFILING Mark Chun |
We present experimental results of determining the optical turbulence profile by measuring the scintillation variance at several conjugated altitudes of the detector. We used a Generalized SCIDAR instrument, scanning through 10 different altitudes. Agreement is found between the profiles derived in this way and from the Generalized SCIDAR method. We discuss the potential use of this technique for site monitoring of the vertical distribution optical turbulence.
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| II.4 SUN RADIUS MEASUREMENT AND ATMOSPHERIC EFFECTS Djelloul Djafer |
The experiment of measuring the solar diameter with a solar astrolabe has begun in 1975 at Calern
Observatory. Since this date and during more than two solar cycles, solar diameter measurements were made
regularly. These observations have shown oscillations in the estimation of the solar diameter. To make these
oscillations of data in evidence, a solar astrolabe will be developped and installed at Tamanrasset
Observatory situated at Algerian's sahara. This astrolabe will enlarge the solar diameter network survey
and permits to have a continuous data recording. In order to improve the quality of the image and together
with the measure of seeing parameters, some modification will be brought to the ancientt instrument and new
techniques using the wavelet transform and the FFTD (Finite Fourier Transform Definition) for measuring the
solar diameter will be introduced. This paper will recall the principle of the measurement and the used
instrumentation. In addition, using the data recorded by the solar astrolabe at Calern Observatory (France),
a comparison between the different methods used to estimate the solar diameter will be given, and a study
of atmospheric effects will be developped around the relation of the diameter measurement error with the
atmospheric seeing.
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| II.5 THE DESIGN OF A CORRELATION SCIDAR Edward Kibblewhite |
The design of a Correlation SCIDAR instrument will we described that consists of a number of small area photosensors connected to a real time correlator. This Instrument, is inexpensive, does not need a telescope to operate and can be used to obtain SCIDAR information on aroutine basis at Observatory site. Its use with single a double stars will be discussed.
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| II.6 A NEW METHOD TO ESTIMATE CHARACTERISTIC TIME OF ATMOSPHERIC TURBULENCE FOR DAYTIME OBSERVATIONS Lakhal Lyes |
In a previous work, we developed a numerical simulation to study the atmospheric effects on solar
diameter measurements performed with an astrolabe (Lakhal et al., 1999). The error behavior laws with
the observation conditions (Fried's parameter r0, wave front outer coherence scale, characteristic times)
were established. Using the simulation results, we propose here a new method to estimate the atmospheric
characteristic time for daytime observations. It is based on the measure of the contact time of the direct
and reflected solar images in the astrolabe focal plane. The contact time fluctuations with the observation
conditions modelized by the numerical simulation lead to estimate the characteristic time of the atmospheric
turbulence. After giving an overview of the numerical simulation and the most important results, we present
the method leading to atmospheric characteristic time estimations. We then apply it to estimate
characteristic times at Calern observatory (France) using data recorded at the solar astrolabe during the
period 1996 - 1997. We finally present the obtained results and discuss them. Ref.: Lakhal et al., Astron. Astrophys. Suppl. Ser. 138, 155-162 (1999). |
| P.II.1 (Poster) OPTICAL PARAMETERS FOR HIGH ANGULAR RESOLUTION ASTRONOMY ESTIMATED FROM THE GSM EXPERIMENT AT SEVERAL MAJOR ASTRONOMICAL SITES IN THE WORLD F. Martin, A. Ziad, R. Conan , J. Borgnino & A. Tokovinin |
Performances of ground based astronomical observations are severely limited by atmospheric turbulence. The Generalized Seeing Monitor (GSM) is dedicated to the measurement of the optical parameters which characterize the corrugated wavefront at the ground. These parameters are Fried's parameter, the spatial coherence outerscale of the wavefront, the isoplanatism angle and time(s) constant of the turbulence . The knowledge of these parameters is necessary for definition of adaptive optics systems and long baseline interferometers and for optimization of their implementation.
The GSM instrument consists of four 10-cm telescopes with image analysis devices to sense the angle of arrival fluctuations in one direction.
So, simultaneous angle of arrival estimation at four points of the wavefront are obtained with 5ms time resolution and for sequence of 2mn long data .
A second order statistical analysis (variance, covariance,correlations) of these data leads to the values of the parameters and to their variation during the night (continuous monitoring).
Since September 1997, GSM experiment has been successfully operated at five different important astronomical sites through the world:
La Silla (CHILE-1997), Oukaimeden (MOROCCO-1998), Maydanak (OUZBEKISTAN-1998), Cerro Pachon ( CHILE-1998) and Paranal (CHILE-1998).
The results obtained at these sites , the characteristics of which are somewhat different, will be presented.
This unique GSM data bank concerning the optical parameters , permits comparison of the values of the optical parameters
and show their variability. |
| II.8 DATA REDUCTION SOFTWARE ALGORITHMS (DRSA) FOR THE VLT INTERFEROMETR MID-INFRARED INSTRUMENT Mohammed Nafati |
This paper describes the algorithms and design of the DRS of MIDI which is based on the current knowledge of the different observing modes of the instrument, on the current data format and the general principle of the reduction algorithms that will be used for MIDI. It is qualified as a preliminary version because part of it still on research activities. The Data Reduction System is an off-line process with respect to the global observation process.
The purpose of the DRS is to provide the visibility (modulus and phase if available) of the observed science sources. MIDI being a single-mode instrument will have up to four outputs: two interferometric signals and two
photometric signals to monitor the fluctuations of phase due to turbulence and which are traded against intensity fluctuations when the spatial filter is used.
The data have four dimensions: optical path difference (or time), channel, space, wavelength.
The Data Reduction Software of MIDI is designed so that it is as simple as possible to use for any astronomer who will use MIDI. It has been thought so that it is not necessary for the user to know MIDI in great details. In particular the number of interactions between the user and the DRS have been reduced to a minimum.
But, whenever required, the DRS can also provide facilities to reduce and analyze the data acquired with MIDI at an expert level. This capacity is mandatory for observation programs which will require the best performances of MIDI and also in case of trouble to understand the origins of the problems. The first mode is called the scientific level in the following and the modules associated to this level constitute the Scientific_Workbench. The second mode is called the expert level with its associated
Expert_Workbench. When run, the Scientific_Workbench will submit to the observer's approval a list of calibrators (i.e. the list of objects among the set of observed sources which will be treated as calibrators) with their parameters (i.e. the diameter for simple sources). The observer is
free to change this list and to select different calibrators or correct for their parameters. After this interaction, the Scientific_Workbench will basically compute the visibilities for the non-calibrator targets or scientific sources. The Expert_Workbench will allow the observer to access his data frame by frame, scan by
scan, to run the functions of the Near-real time software, to perform statistics and to have access to technical parameters.
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| II.9 APPLICATION OF WAVEFRONT TURBULENCE MONITORS TO THE ANALYSIS AND PREDICTION OF IMAGE QUALITY PERFORMANCE OF A 4m TELESCOPE Neil O'Mahony |
Simultaneous seeing measurements, using the JOSE fast
Shack-Hartmann camera on the 4.2m WHT and a DIMM monitor
in the open air, have shown that "dome seeing" is probably insignificant in the case of WHT. The method derives its power from its insensitivity to telescope optical aberrations, its proven ability to isolate Non-Kolmogorov turbulence, and the comparison with intrinsic seeing from DIMM.
The results from the ING DIMM campaign are consistent with
those previously published by Munoz-Tunon et al (1997, 1998), quantifying the high quality of the ORM as an
optical observing site. An additional and unique contribution of JOSE has been to characterise for ORM the outer scale of the turbulence, a parameter that reduces image FWHM from a large telescope to less than that predicted by DIMM. Such an effect has been observed in WHT images and allows us to expect an uncorrected yearly median image FWHM better than 0.5 arcsec at 700nm from a telescope
of 10m aperture, such as the future GranTeCan.
In addition we suggest that deviations from log-normality commonly observed in DIMM FWHM, may imply practical limits to predictive AO scheduling. However, impressive results are still to be expected from AO for long periods on certain nights at ORM.
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| II.10 SURFACE LAYER SEEING CONTRIBUTION AT SAN PEDRO MARTIR: SIMULTANEOUS MICROTHERMAL AND DIMM MEASUREMENTS Leonardo Sanchez |
Results are presented on the optical image degradation due to sur
face layer and whole atmosphere turbulence, respectively. The measurements were
obtained during the 16 nights of the San Pedro Mártir Site Testing Campaign carr
ied out in May 2000, using microthermal sensors placed at 7 levels on a 17m-high
mast and a Differential Image Motion Monitor (DIMM) located close to the mast.
We evaluate the contribution of the surface layer to the image degradation for d
ifferent slabs in the 2-16.8m altitude range. Further analysis of the data shoul
d provide information about the optical turbulence behavior in the surface layer
and the influence of the vegetation. |
| II.11 MEASURING TURBULENCE PROFILES FROM SCINTILLATION OF SINGLE STARS A. Tokovinin, V. Kornilov |
Several new methods to extract the information on vertical turbulence
profile from the spatial structure of single-star scintillations were
recently suggested. Two of them have already been tested at Maidanak
in 1998-99. The first method involving quasi-simultaneous
measurements of scintillation indices with several apertures of
different size permitted to derive isoplanatic angle and to obtain
approximate estimates of free atmosphere seeing and effective
turbulence altitude by adjusting a single-layer model to the data.
The second method consists in measuring the differential scintillation
index in a pair of small concentric apertures. It gives the integrated
free-atmosphere turbulence directly. During the rare periods when
high atmosphere dominates the overall seeing, the results obtained by
both methods match simultaneous seeing data from DIMM, otherwise the
free atmosphere seeing is systematically better than the overall
seeing. The median contribution of free atmosphere to the total
turbulence integral at Maidanak is estimated to be 0.38.
Based on this experience, we plan to build the Multi-Aperture
Scintillation Sensor (MASS) - a turbulence profiler with 4 concentric
apertures. It will use single stars up to 2 mg. About 5 independent
points on turbulence profile will be measured, so that seeing,
isoplanatic and isokinetic angles will be derived with a good
precision. Additionally, the data will be fitted to a 3-layer model
to study the time evolution of altitudes and strengths of dominating
layers. MASS is conceived as a permanent automatic profile monitor to
provide real-time data in support of Adaptive Optics and
Interferometry operations. |
| II.12 GSM: WAVEFRONT OPTICAL PARAMETERS MONITOR FOR SITE EVALUATION A. Ziad, F. Martin, R. Conan , J. Borgnino & A. Tokovinin |
The main optical parameters (OP) relevant for high angular resolution (HAR) are provided with the new version of the Generalized Seeing Monitor (GSM) instrument. Indeed, The Fried parameter r0, the wavefront coherence outer scale L0, the isoplanatic angle q0 and the coherence time t0 have been monitored continuously with the GSM during several nights at different sites over the world (La Silla, Paranal, Cerro Pachon, Maydanak and Oukaimeden). The temporal variability of these measured OP have been analyzed and their effects on the optimization of the HAR techniques (adaptive optics and interferometry) have been studied.
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| II.13 GSM CAMPAIGN "L'OUKAIMDEN" II- SPATIO-TEMPORAL ANALYSIS OF THE WAVEFRONT Abdelhadi Jabiri |
The atmospheric turbulence study needs a deep knowledge of turbulent medium such as characterization of the incident wave front. In attemp to characterize the temporal properties of the perturbed wave front, we analysed angle of arrival temporal cross-correlation functions at all GSM baselines. The statistical analysis of the angles of arrival allows us to detect the more turbulent layers of the atmosphere and to measure the wind direction and velocity associated with predominant turbulent layers.
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