SOLAR DIAMETER MEASUREMENTS WITH CALERN OBSERVATORY ASTROLABE AND ATMOSPHERIC-TURBULENCE EFFECTS

In order to deduce significant astrophysical results from solar diamet er measurements it is necessary to take an accurate account of instrum ental and atmospheric effects. This paper presents a comparison betwee n visual and CCD camera measurements performed by means of the Calern Observatory solar astrolabe during the last 4 years; this allows us to evaluate visual measurements done previously (from 1975 to 1989). The n, a study of atmospheric effects is developed. From CCD measurements, the image quality, expressed by Fried's seeing parameter, r0, is esti mated and related to the errors occurring in solar diameter measuremen ts. A statistical analysis gives about 0.26 arc sec (or 0.13 arc sec f or the semi-diameter) as the lowest value that this error may reach at Calem Observatory. One conclusion of this work is that it is importan t in the future to have image quality observations, obtained using a d edicated monitor, in order to evaluate and classify the measurements. A survey of the seeing might so lead to improve the precision of the r esults by weighting each diameter estimation and eventually to schedul e the observations.