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

Diameter measurements performed with the solar astrolabe are affected by in strumental and atmospheric effects. The problem is to know how these effect s contribute to the error on diameter measurements. Thus, a numerical simul ation is developed to bring some responses to this problem. For this purpos e, synthetic images of the Sun similar to the ones obtained by the instrume nt through the Earth atmosphere are simulated. A fractal model is used to g enerate randomly perturbed wavefronts and therefore optical response of the whole system, atmosphere and instrument. Many sequences of solar images ar e then simulated for various observation conditions (Fried's parameter ro, spatial coherence outer scale Lo and atmospheric characteristic times). The y are used to define the time of contact of the direct and reflected solar images which is fundamental in the sc,lar astrolabe experiment. It is then studied as a function of the observation conditions relatively to the exper imental characteristics (exposure time).