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Astron. Astrophys. Suppl. Ser. 138, 155-162

Error due to atmospheric turbulence effects on solar diameter measurements performed with an astrolabe

L. Lakhal1 - A. Irbah1 - M. Bouzaria1 - J. Borgnino2 - F. Laclare3 - C. Delmas3

Send offprint request: J. Borgnino

1 - C.R.A.A.G, Observatoire d'Alger, BP. 63, Bouzaréeah Alger, Algeria e-mail:
2 - Université de Nice-Sophia Antipolis, UMR 6525 Astrophysique, Parc Valrose, F-06108 Nice Cedex 2, France
3 - Observatoire de la Côte d'Azur, Département C.E.R.G.A, Avenue Copernic, F-06130 Grasse, France

Received March 15; accepted May 26, 1999


Diameter measurements performed with the solar astrolabe are affected by instrumental and atmospheric effects. The problem is to know how these effects contribute to the error on diameter measurements. Thus, a numerical simulation is developed to bring some responses to this problem. For this purpose, synthetic images of the Sun similar to the ones obtained by the instrument through the Earth atmosphere are simulated. A fractal model is used to generate randomly perturbed wavefronts and therefore optical response of the whole system, atmosphere and instrument. Many sequences of solar images are then simulated for various observation conditions (Fried's parameter r0, spatial coherence outer scale $\mathit{L}_{0}$ and atmospheric characteristic times). They are used to define the time of contact of the direct and reflected solar images which is fundamental in the solar astrolabe experiment. It is then studied as a function of the observation conditions relatively to the experimental characteristics (exposure time).

Key words: Sun: fundamental parameters -- atmospheric effects -- methods: numerical

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