As a consequence of an astrometry program, conducted since 1975 on a solar astrolabe at the Calern Observatory (Observatoire de la C6te d'Azur), we have obtained a data set of apparent solar diameters which encompasses periods greater than one solar cycle. From a set of more than 5000 visual observations, made by the same observer between 1975 and 1994, the mean value of the semidiameter was measured at 959.42" -4-0.01". Also, a set of CCD measurements made with the same instrument between 1989 and 1994 yields the mean value 959.40" -4-0.01". Both results obtained by raw measurements are consistent but significantly differ from values obtained by other methods and on other instruments. We discuss some systematic effects that can affect our visual measurements and their precision. Taking account of a zenith distance effect provides for the semi-diameter a mean value closer to the value of the ephemeris. Our observations also reveal deviations around the mean diameter in the royal zones and for high heliographic latitudes; their amplitudes reaching as much as 0.08". Finally, semi-diameter variations appear in our series; their origin is unknown but they may possibly be related to observed variations of magnetic activity or other solar parameters.
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...
Abstract. Solar diameter measurements performed with the Calern Observatory astrolabe (O.C.A -France) during more than two solar cycles show temporal variations. Due to the weather, seasonal effects and instrumental characteristics, recorded solar data are non uniformly sampled and present temporal gaps. Thus, to analyze these data, diameter measurements averaged over one or more months have generally been considered. This limits the accessible harmonic terms to a low frequency range. To determine short-term periodicities from the observed variations, all daily solar data need to be considered and also corrected from the zenithal distance. In the present work, we use two methods to analyze solar diameter measurements recorded at Calern Observatory astrolabe during the observation period 1975-1996. They are based on least square fits and deconvolution of the observation window function. Results deduced from the analysis confirm harmonic terms already found by other authors but also reveal new higher frequencies.
Abstract. Observations of the solar diameter are reported from the astrolabe station at Observatório Nacional. All the observations presented here were taken with the 1986-CERGA prototype of Variable Prism, which enables many diameter measurements per day and to observe all year round. The 1996 DANOF software for digital CCD image acquisition and data processing has been used. We adopted the recording of 46 CCD frames with direct and reflected images of the limbs along the Sun's transit through the almucantar. The independent observable is the accurate UT datation of each frame.Here the first campaign results, from January to July/1997, are presented. An asymmetry between the east and west transit is noticeable. The west transit leads to a larger value for the mean semi-diameter and exhibits a larger standard deviation. No clear dependence is found upon the daily results, either on time or on observational parameters. The main outcome of this campaign remains the average solar semi-diameter, from 1149 transits (863 East and 286 West), obtained as 959. 21 ± 0. 03.
Abstract.Results are presented of measured variations of the photospheric solar diameter, as concurrently observed at three sites of the R2S3 (Réseau de Suivi au Sol du Rayon Solaire) consortium in 2001. Important solar flux variations appeared in that year, just after the maximum of solar activity cycle 23, make that time stretch particularly promising for a comparison of the multi-site results. The sites are those in Turkey, France and Brasil. All observations are made with similar CCD solar astrolabes, and at nearby effective wavelengths. The data reductions share algorithms, that are alike, the outcomes of which are here treated after applying a normalization correction using the Fried parameter. Since the sites are geographically quite far, atmospheric conditions are dismissed as possible causes of the large common trend found. Owing to particularities of each site, the common continuous observational period extends from April to September. The standard deviation for the daily averages is close to 0. 47 for the three sites. Accordingly, the three series are smoothed by a low-pass-band Fourier filter of 150 observations (typically one month). The main common features found are a declining linear trend, of the order of 0.7 mas/day, and a relative maximum, around MJD 2120, of the order of 100 mas. Standard statistical tests endorse the correlation of the three series.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.