Processing Method Effects on Solar Diameter Measurements: Use of Data Gathered by the Solar Disk Sextant

Abstract

To determine the apparent diameter of the Sun, it is first necessary to measure the shape of the intensity profile of the solar limb with an imaging optical system (hereafter denoted as a solar-limb profile). The inflection point of the limb profile is usually used as a reference for calculating the diameter. Because this point may be difficult to determine in the presence of noise, it is necessary to define an appropriate filtering process that eliminates noise while preserving the position of the inflection point. In this paper we study two filtering techniques, one based on the compact wavelet transform and the other on the finite Fourier transform definition, that meet these requirements. The application of these two techniques to data gathered by the Solar Disk Sextant experiment shows that the solar radius increased from 1992 to 1996 by about 197 mas. However, a previous analysis of the same data and our present analysis provide a difference in the measured radii of about 92 mas. We show that this difference is entirely traced to the filtering process

Processing Method Effect on Sun Diameter Measurement With CCD Solar Astrolabe

“…Its one-dimensional version is applied to SDS data; more detail is given in Djafer et al (2008b). In the present work, we use the two-dimensional version of this method and apply it to the solar images acquired by the CCD astrolabe.…”

“…For the FFTD method, the position of the inflection point is highly dependent on the value of the parameter a. The optimal value of a is 4 pixels (Djafer et al 2008b). For the solar astrolabe method, a window of 5 pixels is used.…”

“…All the methods necessitate a reliable noise filtering. Djafer et al (2008aDjafer et al ( , 2008b have presented in detail the filtering methods and their effects on the measurement of the diameter of the Sun using the solar disk sextant (SDS) data. With a CCD solar astrolabe, the filtering method currently being used is based on a smoothing process of 1, 3, 5, and 7 pixel windows.…”

“…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). …”

Latitude dependence of the solar granulation during the minimum of activity in 2009

“…From recent accurate measurements taken with space telescopes, the oblateness amounts to about 8 mas (Emilio et al 2007(Emilio et al , 2009Fivian et al 2008;Kuhn et al 2012). It is found to be constant over the activity cycle by some authors (Kuhn et al 2012), but found to increase with decreasing activity by some others (Egidi et al 2006;Djafer et al 2008). …”