NLTE determination of the calcium abundance and 3D corrections in extremely metal-poor stars M. Spite, S. M. Andrievsky, F. Spite, E. Caffau, S. A. Korotin, P. Bonifacio, H.-G. Ludwig, P. François and R. Cayrel A&A, 541 (2012) A143 Published online: 16 May 2012 DOI: 10.1051/0004-6361/201218773