Aziz Ziad

Theoretical spatiotemporal analysis of angle of arrival induced by atmospheric turbulence as observed with the grating scale monitor experiment

Theoretical investigations of the statistical properties of the wave frontnperturbed by atmospheric turbulence are presented. They are deduced from thencalculation of the two-dimensional spatial covariance and the temporal crossnspectrum of the angle-of-arrival fluctuations with a finite outer scale overna pair of circular pupils as in the case of the grating scale monitor or anynother Shack–Hartmann-type sensor. Both calculations lead to integralnexpressions that are numerically evaluated and hold for any baseline vectornin the mean wave-front plane. It is proposed to retrieve the wave-front outernscale L0 from estimations ofnthis two-dimensional spatial covariance, normalized by the angle-of-arrivalnstructure function. To eliminate instrument vibration errors, the covariancenand the structure function are estimated from measurements obtained by mechanicallynindependent and mechanically coupled devices, respectively. The angle-of-arrivalntemporal cross spectrum is calculated for any mean wind velocity vector. Itnis shown that the baseline component in the mean wind direction affects thenphase of the angle-of-arrival temporal cross spectrum, whereas the componentnin the perpendicular direction affects the modulus. From simultaneous measurementsnof the phase of the angle-of-arrival temporal cross spectrum obtained withntwo nonparallel baselines, one can calculate the mean wind speed and direction,nwhich allows estimation of the coherence time for techniques of optical observationnat high angular resolution through the atmosphere.

EFFECTS OF ATMOSPHERIC SPECTRAL DECORRELATION ON VISIBILITY MEASUREMENTS IN MICHELSON INTERFEROMETRY

We analyze the degrading effects of spectral decorrelation introduced by atmospheric turbulence on astronomical images obtained from a long-baseline Michelson interferometer. Our approach consists of a theoretical computation of the polychromatic spectrum, based on a numerical model of the atmospheric turbulence. The model uses the von Karman definitions parameterized by the Fried parameter r0 and the outer scale L0. Through the theoretical analysis of fringe visibility estimated from the polychromatic spectrum, we show how the turbulence model can reproduce visibility behavior with respect to the spectral bandwidth and the separation of two spectral bands. This validation is realized by comparing theoretical to measured visibilities of α Cephei. The future use of this work is to calibrate these losses to improve the accuracy of the GI2T interferometers observations.

Optimized spectral bandwidth in high angular resolution imaging effect of a finite spatial-coherence outer scale

In the case of high angular resolution techniques (speckle interferometry, long baseline Michelson interferometry), one has studied how varies the signal-to-noise ratio (SNR) in terms of the spectral bandwidth Δλ. For values of Δλ varying from 0 up to 100 nanometers, it is shown that the SNR, contrary to the predictions, has no maximum value. In addition, in the case of the high frequency approximation, the effects of a finite spatial-coherence outer scale, the influence of the turbulence model used and of the “optical energy” of the turbulence on the optimal spectral bandwidth have been analyzed and discussed.

Experimental characterization of the turbulence inside the dome and in the surface layer

We present the concept of a new instrument dedicated to modeling turbulence inside the dome and in the surface layer. It consists of using parallel laser beams separated by non redundant baselines between 0.1 and 2-3m and measuring Angle-of-Arrival (AA) fluctuations from spots displacements on a CCD. We use weighted least-square method to fit the measured AA longitudinal and transverse covariances with theoretical forms deduced from the usual models of turbulence. Then, the whole parameters characterizing this turbulence are provided from a complete spatio-temporal analysis of AA fluctuations. Thus, the surface layer turbulence energy in terms of C2N constant is provided from the AA structure function as in the DIMM instrument.

Estimation of the spatial-coherence inner scale of the wavefronts perturbed by the atmospheric turbulence from first order angle-of-arrival statistics

The variance of the angle-of-arrival fluctuations is theoretically calculated taking into account the effects of limit scales for the spatial coherence of the wavefronts perturbed by an atmospheric turbulence described by the Von Karman model. A technique is proposed allowing the estimation of the spatial-coherence inner scale from measurements of the angle-of-arrival fluctuations (variance) with and without spatio-angular filterings. In order to support the theoretical study, a numerical simulation is performed. In addition the possibility to use in situ measurements has been evaluated introducing the vertical turbulence profiles, in the theory. A method is presented allowing, by means of spatial filters with linear amplitude transmittance, the observation of the angle-of-arrival fluctuations without spatial filtering, and afterwards an experimental concept is derived. Finally, the influence of the model used to describe atmospheric turbulence is discussed.

Optical turbulence in confined media. Part II:first results using the INTENSE instrument

Optical system performances can be affected by local optical turbulence created by its surrounding environment (telescope dome, clean room, or atmospheric layer). This paper follows a previous one introducing the INdoor TurbulENce SEnsor (INTENSE) instrument for optical turbulence characterization in a local area by exploitation of laser beam angle-of-arrival fluctuations. After a brief summary of the theoretical background, we present in this part results obtained using the INTENSE instrument in various optical integration testing clean rooms and telescope domes, each with specific air behavior conditions.

Review of the outer scale of the atmospheric turbulence

Outer scale is a relevant parameter for the experimental performance evaluation of large telescopes. Different techniques have been used for the outer scale estimation. In situ measurements with radiosounding balloons have given very small values of outer scale. This latter has also been estimated directly at the ground level from the wavefront analysis with High Angular Resolution (HAR) techniques using interferometric or Shack-Hartmann or more generally AO systems data. Dedicated instruments have been also developed for the outer scale monitoring such as the Generalized Seeing Monitor (GSM) and the Monitor of Outer Scale Profile (MOSP). The measured values of outer scale from HAR techniques, GSM and MOSP are somewhat coherent and are larger than the in situ results. The main explanation of this difference comes from the definition of the outer scale itself. This paper aims to give a review in a non-exhaustive way of different techniques and instruments for the measurement of the outer scale. Comparisons of outer scale measurements will be discussed in the light of the different definitions of this parameter, the associated observable quantities and the atmospheric turbulence model as well.

The Calern atmospheric turbulence station

From its long expertise in Atmospheric Optics, the Observatoire de la Côte dAzur and the J.L. Lagrange Laboratory have equipped the Calern Observatory with a station of atmospheric turbulence measurement (CATS: Calern Atmospheric Turbulence Station). The CATS station is equipped with a set of complementary instruments for monitoring atmospheric turbulence parameters. These new-generation instruments are autonomous within original techniques for measuring optical turbulence since the first meters above the ground to the borders of the atmosphere. The CATS station is also a support for our training activities as part of our Masters MAUCA and OPTICS, through the organization of on-sky practical works.

The INdoor turbulENce SEnsor (INTENSE) instrument

Optical system performances can be affected by local optical turbulence created by its surrounding environment (telescope dome, clean room, atmospheric surface layer). We present our new instrument INTENSE (INdoor TurbulENce SEnsor) dedicated to this local optical turbulence characterization. INTENSE consists of using several parallel laser beams separated by non-redundant baselines between 0.05 and 2.5m and measuring Angle-of-Arrival fluctuations from spots displacements on a CCD. We present detailed characterization of instrumental noise and first results for the characterization of the turbulence inside clean rooms for optical testing and integration.

LOTUCE2: a dome-seeing instrument for the E-ELT

Free-atmosphere, and surface-layer optical-turbulence have been extensively monitored over the years. The optical-turbulence inside a telescope enclosure en the other hand has yet to be as fully characterized. For this latest purpose, an experimental concept, LOTUCE (LOcal TUrbulenCe Experiment) has been developed in order to measure and characterise the so-called dome-seeing. LOTUCE2 is an upgraded prototype whose main aim is to measure optical turbulence characteristics more precisely by minimising cross-contamination of signals. This characterisation is both quantitative (optical turbulence strength) and qualitative (assessing the optical turbulence statistical model). We present the new opto-mechanical design, with the theoretical capabilities and limitations to the actual models.