Solar radius measurements with the DORAYSOL instrument (1999-2006) at the Calern site of the observatoire de la Côte d'Azur

International audienceThe series of measurements of the Solar radius at CALERN site of the observatoire de la Côte d'Azur (OCA) spans about 30 years. Measurements of the Sun were first achieved visually by the same single observer, then gradually grew more man-independent with the use of CCD acquisitions. The DORAYSOL instrument was designed and built up to keep improving the number and the quality of these semi-diameter measurements. The principle of this instrument remains the same as that of the Solar Astrolabe (timing the crossing of a parallel of altitude by the Sun) but a varying prism enables larger numbers of daily measurements. Digital CCD acquisition of images improves the credibility of the data and five computer-driven motors are giving a better control of the attitude of the instrument. The results for eight years and nearly 20,000 measurements are presented. Analysis of instrumental biases shows that differences between East and West measurements are systematic and then allow one to choose the mean values to calculate the Solar radius. This results display a good agreement with the Solar Astrolabe series, namely the mean value of the Sun radius (959.48″±0.01″), its time-dependence and the apparent dissymmetry of the photosphere. In the framework of the PICARD CNES Space mission, Solar radius ground measurements at CALERN simultaneously with the onboard SODISM telescope were projected. It appeared in 2007 that the maintenance of the DORAYSOL program was not possible alongside the development of the atmospheric turbulence monitor MISOLFA and the preparation for setting up the model of the SODISM 2 telescope, the staff at hand becoming insufficient. We do hope that it will be resumed, in a fully automated way. A continuation of the ground-based series would permit one to detect possible long time variations of the global geometry of the Sun