Automatic Methods of Locating the Solar-Disk Center in Images Taken by a Solar-Halo Photometer

A modern Solar-Halo Photometer(SHP) is a precision instrument used to measure various atmospheric parameters for astronomical day-time observation sites. Such an instrument plays a key role in our solar-observation site survey program in the west China. The instrument takes the photometric measurements of multiple wavelength bands. The accuracy of locating the solar-disk center can decisively affect the photometric results. The solar-disk center in an SHP image varies with time because of imperfect observation conditions. With fast increasing SHP data, there is a strong need to automatically reduce the data. Here we develop two automatic methods for locating the solar-disk center coordinates: the total solar-disk intensity curve method and the Fourier-coefficients correlation method. The first method searches for the maximum of the variation curve of the total solar-disk intensity, which is taken as the solar-disk center in the image. The second method searches for the most significant correlation between the Fourier coefficients of a mask of the solar-disk and the recorded image. We have compared the solar-disk coordinates obtained by these two methods and find that the results are very close, showing the two methods are consistent and either one can be used. With the two methods we will be able to reduce SHP data in future analyses.