In a paper published in volume 50 of Annals of Science an astrolabe at the Istituto e Museo di Storia della Scienza, Florence, was attributed to the hand of Gerard Mercator, c. 1570, when his workshop was in Duisburg. This was the first scientific instrument by Mercator to be identified. Since then two further astrolabes by Mercator have been identified, one of them bearing his monogram: GMR. They belong to the Städtische Kunstsammlungen, Augsburg, and the Moravian Gallery, Brno. All three (...) instruments are described as a group, and reasons for believing that the Brno astrolabe was made earlier than 1550, and therefore in Louvain, are given. (shrink)