M Sarazin

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Jorge Lima

Andrea Baruffolo

Carmelo Arcidiacono

Istituto Nazionale di Astrofisica

Nicholas Devaney

National University of Ireland, Galway

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Papers by M Sarazin

New challenges for adaptive optics: the OWL 100m telescope

Adaptive optics will be a key element of future Extremely Large Telescopes (ELTs) which need shar... more Adaptive optics will be a key element of future Extremely Large Telescopes (ELTs) which need sharp images both to avoid object confusion and to boost sensitivity to point sources against sky background. A challenging task of correcting turbulence in the visible part of the spectrum is addressed. With an R -10 natural guide star a Strehl ratio (SR) of 0.6 can be achieved at 500 nm, at the cost of an extremely low sky coverage. Thus, use of Multi-Conjugate Adaptive Optics techniques will be essential to widen the field of view (FOV). We summarize our recent theoretical results and show that with 3 deformable mirrors it is realistic to obtain a FOV diameter of 30"-60" in the visible, or 3'-6' in the K band. Contrary to the common belief, the layered structure of the turbulence vertical profile is not needed to achieve this limit, neither do we need to know the profile exactly.

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Testing turbulence model at metric scales with mid-infrared VISIR images at the VLT

Monthly Notices of The Royal Astronomical Society, 2007

We probe turbulence structure from centimetric to metric scales by simultaneous imagery at mid-in... more We probe turbulence structure from centimetric to metric scales by simultaneous imagery at mid-infrared and visible wavelengths at the VLT telescope and show that it departs significantly from the commonly used Kolmogorov model. The data can be fitted by the von Karman model with an outer scale of the order of 30 m and we see clear signs of the phase structure function saturation across the 8-m VLT aperture. The image quality improves in the infrared faster than the standard lambda^{-1/5} scaling and may be diffraction-limited at 30-m apertures even without adaptive optics at wavelengths longer than 8 micron.

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New challenges for adaptive optics: extremely large telescopes

Monthly Notices of The Royal Astronomical Society, 2000

The performance of an adaptive optics (AO) system on a 100-m diameter ground-based telescope work... more The performance of an adaptive optics (AO) system on a 100-m diameter ground-based telescope working in the visible range of the spectrum is computed using an analytical approach. The target Strehl ratio of 60 per cent is achieved at 0.5 mm with a limiting magnitude of the AO guide source near R magnitude , 10Y at the cost of an extremely low sky coverage. To alleviate this problem, the concept of tomographic wavefront sensing in a wider field of view using either natural guide stars (NGS) or laser guide stars (LGS) is investigated. These methods use three or four reference sources and up to three deformable mirrors, which increase up to 8-fold the corrected field size (up to 60 arcsec at 0.5 mm). Operation with multiple NGS is limited to the infrared (in the J band this approach yields a sky coverage of 50 per cent with a Strehl ratio of 0.2). The option of open-loop wavefront correction in the visible using several bright NGS is discussed. The LGS approach involves the use of a faint R , 22 NGS for low-order correction, which results in a sky coverage of 40 per cent at the Galactic poles in the visible.

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Optical parameters relevant for High Angular Resolution at Paranal from GSM instrument and surface layer contribution

Astronomy & Astrophysics Supplement Series, 2000

Main atmospheric parameters characterizing the turbulence were measured at the Paranal ESO Observ... more Main atmospheric parameters characterizing the turbulence were measured at the Paranal ESO Observatory (Chile) with the Generalized Seeing Monitor (GSM) during 19 nights in November-December 1998. The median seeing for this period was 0.88 , a little worse than average, the spatial coherence outer scale of the wavefront L 0 = 22.0 m and the median isoplanatic angle θ 0 = 1.90 . A very good agreement with seeing values given by ESO DIMM was found. Effects of ground layer turbulence were estimated from measurements of temperature micro-fluctuations at several altitudes. These measurements revealed that the first 21 m above ground account for 12.5% of the total turbulent energy. Comparison between GSM and ESO DIMM values have permitted to highlight a local phenomenon which occurs sometimes at the site in the middle of the night. Statistics of the refractive index structure constant C 2 n in the surface layer is also studied.

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The astroclimate of Maidanak Observatory in Uzbekistan

Astronomy & Astrophysics Supplement Series, 2000

Abstract. The atmospheric turbulence and meteorology of the Maidanak Observatory in Uzbekistan ar... more

First statistical data on wavefront outer scale at La Silla observatory from the GSM instrument

Astronomy & Astrophysics, 1998

Results of the first monitoring of the wavefront outer scale L 0 during 16 nights at La Silla, Ch... more Results of the first monitoring of the wavefront outer scale L 0 during 16 nights at La Silla, Chile, are presented. This parameter is of importance for high angular resolution observing techniques like adaptive optics and optical interferometry. It was measured with the Grating Scale Monitor (GSM) instrument which is based on the analysis of the spatial covariance of angle of arrival fluctuations simultaneously detected at several points on the wavefront. The outer scale distribution is found to be log-normal, with median value of 24 m and rms scatter of logL 0 of ±0.22. Short-duration bursts of L 0 are noted. During 1 hour the L 0 and seeing were simultaneously measured with the ADONIS adaptive optics system at the 3.6 m telescope, and a good agreement with our data was noted.

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New challenges for adaptive optics: the OWL 100m telescope

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Testing turbulence model at metric scales with mid-infrared VISIR images at the VLT

Monthly Notices of The Royal Astronomical Society, 2007

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New challenges for adaptive optics: extremely large telescopes

Monthly Notices of The Royal Astronomical Society, 2000

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Optical parameters relevant for High Angular Resolution at Paranal from GSM instrument and surface layer contribution

Astronomy & Astrophysics Supplement Series, 2000

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