Notwithstanding the experimental character of the 1986 CERGA prototype Variable Prism, used to define the observational zenith distance, the measurements dispersion is comparable to what was obtained with zerodur fixed angle prisms (Laclare 1983). The lower quality measurements in the afternoon, as compared with the morning ones, is of no surprise. It may arises from the temperature differences that can affect the lower layers of the atmosphere and immediate vicinity of the instrument. The stability of the results is a good indication of the variable angle prism performance. A perfectioned prism of this type is currently being developed at OCA/CERGA and will soon replace the prototype used so far.
The digital CCD acquisition methods display a number of advantages
over visual observations. It enables an accurate definition of the
solar limb, though further modeling of it is in progress.
Special care ought to be given to define the wavelength and
bandwidth of observation with precision. The method frees the
measurement from any personal bias and it helps evaluating the
atmospheric turbulence, hence the images quality (Irbah et al. 1994)
The method keeps in memory the whole raw data for an eventually
better processing. In the case of Observatório Nacional, the
acquired images are stored in CD-ROM and DAT tapes.
The site of the Observatório Nacional in Rio de Janeiro comes out as specially fitted for long term surveys of the solar diameter. It characterizes by the great number of possible observations as much as the prospective quality that can be expected, combined with the observation facilities. The seeing of the site is larger than that of Calern, as shown by the averages of similar CCD measurements from this campaign and the 1996-97 Calern's one. The linear dependence between the spreading of the border function and a smaller diameter is confirmed, in line with prior results (Rösch et al. 1983; Neckel 1995).
Since much care is required to define the limb of the solar disk, in order to distinguish between observed diameter variations due to physical shape changes and due to variability in other solar properties affecting the limb definition (Hill & Stebbins 1975; Hill et al. 1981), further observations are highly desirable of the largest density on time.
This work was partially supported by the CNPq/CNRS, Brazil/ France binational agreement, under the number 910172/95-1. The authors wish also to express their thanks to Francis Laclare (CERGA/OCA), Roberto Vieira Martins, Alberto G.F. dos Santos, the ON Time Service Department, Nelson V. Leister (IAG/USP) and Clemence Gneiding (CNPq/LNA) for the contribution to the present research. E.J. and S.P. thank CNPq for the financial support under the contracts 300016/93-0 and 300017/93-6.
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