Remy Avila

Whole atmospheric-turbulence profiling with generalized scidar

Statistical analysis of stellar scintillation on the pupil of a telescope, known as the scidar (scintillation, detection, and ranging) technique, is sensitive only to atmospheric turbulence at altitudes higher than a few kilometers. With the generalized scidar technique, recently proposed and tested under laboratory conditions, one can overcome this limitation by analyzing the scintillation on a plane away from the pupil. We report the first experimental implementation of this technique, to our knowledge, under real atmospheric conditions as a vertical profiler of the refractive-index structure constant C (N)(2) (h). The instrument was adapted to the Nordic Optical Telescope and the William Hershel Telescope at La Palma, Canary Islands. We measure the spatial autocorrelation function of double-star scintillation for different positions of the analysis plane, finding good agreement with theoretical expectations.

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.

San Pedro Mártir: astronomical site evaluation

The Observatorio Astronomico Nacional at San Pedro Martir is situated on the summit of the San Pedro Martir Sierra in the Baja California peninsula of Mexico, at 2800m above sea level. For as long as three decades, a number of groups and individuals have gathered extremely valuable data leading to the site characterization for astronomical observations. Here we present a summary of the most important results obtained so far. The aspects covered are: weather, cloud coverage, local meteorology, atmospheric optical extinction, millimetric opacity, geotechnical studies, seeing, optical turbulence profiles, wind profiles and 3D simulations of atmospheric turbulence. The results place San Pedro Martir among the most favorable sites in the world for astronomical observations. It seems to be particularly well-suited for extremely large telescopes because of the excellent turbulence and local wind conditions, to mention but two characteristics. Long-term monitoring of some parameters still have to be undertaken. The National University of Mexico (UNAM) and other international institutions are putting a considerable effort in that sense.

A New Method to Estimate the Temporal Fraction of Cloud Cover

High altitude astronomical sites are a scarce commodity with increasing demand. A thin atmosphere can make a substantial difference in the performance of scientific research instruments like millimeter-wave telescopes or water Cerenkov observatories. In our planet reaching above an altitude of 4000 m involves confronting highly adverse meteorological conditions. Sierra Negra, the site of the Large Millimeter Telescope (LMT) is exceptional in being one of the highest astronomical sites available with endurable weather conditions.