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We describe the magnetic design problems involved in the realization of the permanent magnet undulator ‘‘Nouvel Onduleur pour l’Etude du Laser (NOEL)’’currently being used for the free electron laser experiments on the storage ring ACO. The undulator has been compensated for the magnetic imperfections of the magnet elements used for its construction. Each pole piece was made from two elements so that we could cancel out the transverse field errors. Each undulator pole was selected from a two‐dimensional magnet distribution graph which was drawn with the results of a set of Hall‐probe measurements. Magnetic measurements of the assembled undulator as well as the first experiments on the ACO storage ring indicate that the method has been successful.