A&A Supplement series, Vol. 128, March II 1998, 605-615
Reçu le 5 juin; accepté le 22 juillet, 1997
V. Sinceac, F. Chollet, F. Laclare et C. Delmas
Send offprint request: F. Chollet
Observatoire de Paris, 61 avenue de l'Observatoire, 75014 Paris, France
Observatoire de la Côte d'Azur, France
Observing the Solar disk is a challenge and, as for the past visual observations, we have many results depending on observers and/or instruments. This was due to the differences in visual perceptions of the Sun's limb, instrumental errors and atmospheric disturbances. After a long series of visual observations at Calern Observatory, Francis Laclare felt the need for more impersonal and automatic measurements of the Solar diameter. After a series of analog CCD measurements (1989-1995), a digital data acquisition and processing was tested by the Paris Observatory group (F. Chollet and V. Sinceac) during the 1996 spring at Calern Observatory. Before starting a new continuous campaign of observations, to confirm eventual variations of the diameter and solar flatness, the aim is to find the best definition of the solar edge. The test campaign was spent comparing different solutions that were tried on two different astrolabes at Calern Observatory: The "classical'' one, outfitted with eleven zerodur ceramic prisms (S astrolabe), that has been used for twenty years in the Laclare series and on the other hand an instrument equipped with a varying angle prism (V astrolabe) enabling many measurements (385 in 1996) for perfecting the know how.
This article focusses on acquisition techniques and their feasibility. Two procedures were tried: The first one used alternately the direct and reflected images (separated using a revolving shutter in front of the objective) and the second one mathematically sorts out both components inside the computer (an image being a two-dimensionral array of numbers). According to the principle of the astrolabe, the measured quantity is the exact time crossing the parallel of altitude (defined by the prism angle) by the Sun's edge, i.e. the time of merging of the two images of the Sun in the focal plane of the telescope where the CCD matrix stands. Here comes the definition of the Solar edge for one frame as the collection of the inflect points on the luminosity function along each of the 256 useful lines (the matrix is 512 by 512 pixels). This means that a numerical derivation is performed on every other line of the CCD video camera which has to stand as vertical as possible. Then, for every frame, and through the 256 points, a parabola is fitted, using the least squares method. The top of this parabola materializes the prospective characteristic point. The sets of such points associated with the corresponding times of acquisition, are collected for both images and the exact time of contact of the two images may be obtained. This time is also the time when the solar edge crosses the almucantar.
The results for the semi-diameter obtained during 1996 campaign are derived from sixty measurements with the revolving mask and sixty seven without it, performed on the Solar Astrolabe. They give a mean value of 95939 0 03 with a scatter of 0 29.
It is interesting to remark that the values of the error bar and the scatter obtained do not depend on the definition of the Solar edge, whereas the mean value does depend on it.
It is noticed that going with the method is made a systematic error which slightly shrinks the diameter, but this value can be known statistically and the correction can easily be done. Choosing the best definition of the Solar edge will be the matter of a following article.
The main advantage of such a digital acquisition procedure has to be stressed, as it enables to store the full data for further reference and, if possible, better future processing.
keywords: astrometry -- methods: observational -- Sun: fundamental parameters