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Published online by Cambridge University Press: 14 August 2015
At the 1973 eclipse, S. Koutchmy (Institut d'Astrophysique, Paris) obtained several pictures of the white-light corona, using polarizers and a radially-compensated filter. These pictures provide a very good opportunity for studying the large coronal hole at the north polar cap; this hole has been extensively studied during the Skylab period. On the plates of Koutchmy, we could record reliable intensities between 1.3 and 3.2 R⊙. The absolute calibration was made using the stars observed in the field at the same time. This method allows a direct comparison of well-exposed objects on a same plate and must therefore be highly reliable (see Koutchmy et al., 1978). It is well-known that the absolute calibration of eclipse plates is a difficult problem.