Méthode nouvelle pour la mesure CCD du diamètre solaire avec un astrolabe

Abstract

Abstract. 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)(1990)(1991)(1992)(1993)(1994)(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 Send offprint requests to: F. Chollet 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 959. 39 ± 0. 03 with a scatter of 0. 29.It is interesting to remark that the values of the error bar and the scatter ob...

On solar radius measurements with Danjon astrolabes

“…It has been claimed that the first visual results that have been obtained at the other astrolabe stations were always different to those obtained simultanously by Laclare at CERGA. Nevertheless, during the course of time there is an evolution of the results that makes them finally converge to the results of CERGA (Chollet 1995;Sinceac et al 1998). However, such evolution has never been observed in the results of Santiago.…”

Solar diameter observations with the Astrolabe at Observatório Nacional - Rio de Janeiro

“…Following the Observatoire de Paris digital acquisition system software (Sinceac et al 1997), the solar limb edge, at each CCD line, is represented by the inflection point of the luminosity curve along the line. The observed solar limb can be adequately represented by a parabola that fits the set of inflection points.…”

Processing Method Effect on Sun Diameter Measurement With CCD Solar Astrolabe

“…This difference is mainly due to the effect of the value of the parameter a. A mean difference of 138 mas is obtained between the radius value of the Sun calculated with the compact wavelet and the smoothing methods used in Calern Sinceac et al 1998). Our method uses a procedure to automatically estimate the noise present in each acquired image of the Sun.…”

“…The method for CCD measurements of the solar diameter with the astrolabe has been described in detail by Sinceac et al (1998). The diameter value of the Sun depends on the difference between the two instants of transition of the Sun through the height circle defined by the astrolabe.…”

“…To show this, we compare three filtering methods using the CCD solar astrolabe data, i.e., the proposed method (see Djafer et al 2008b) based on the compact wavelet that automatically determines the amplitude of the noise present in the image and eliminate it without changing the inflection point position, the FFTD method (see Sofia et al 1983Sofia et al , 1994Egidi et al 2006), and the method actually used with the CCD solar astrolabe (see Sinceac et al 1998;Jilinski et al 1999;Kilic et al 2005). …”