T. Corbard, F. Morand, F. Laclare, R. Ikhlef, M. Meftah
Several efforts are currently made from space missions in order to get accurate solar astrometric measurements i.e. to probe the long term variations of solar radius or shape, their link with solar irradiance variations and their influence on earth climate. These space missions use full disk solar imagery. In order to test our ability to perform such measurements from ground on the long term, we need to use similar techniques and instruments simultaneously from ground and space. This should help us to model and understand how the atmosphere affect ground based metrologic measurements. However, using full imagery from ground instead of the traditional astrolabe technique immediatly raise the question of the effect of refraction and how well we can correct from it. The goal is to study in details the influence of pure astronomical refraction on solar metrologic measurements made from ground-based full disk imagery and to provide the tools for correcting the measurements and estimating the associated uncertainties. We use both analytical and numerical methods in order to confront commonly or historically used approximations and exact solutions. We provide the exact formulae for correcting solar radius measurements at any heliographic angle and for any zenith distance. We show that these corrections can be applyed up to 80 degree of zenith distance provided that full numerical integration of the refraction integral is used. We also provide estimates of the absolute uncertainties associated with the differential refraction corrections and shows that approximate formulae can be used up to 80 degree of zenith distance for computing these uncertainties. For a given instrumental setup and the knowledge of the uncertainties associated with local weather records, this can be used to fix the maximum zenith distance one can observe depending on the required astrometric accuracy.
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http://arxiv.org/abs/1304.0147
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