He curated the Hymenoptera collections of the Natural History Museum Vienna from 1955-1994.
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Later, the Museum became part of the Natural History Museum of Vienna, Austria (in German, "Naturhistorisches Museum Wien" or NHM-Wien).
According to Hahn, the focus is initially to be placed on projects from the big museums, such as the anthropological collection in the Vienna Natural History Museum, a study on copper corrosion in manuscripts by the Austrian National Library, or the expropriation of motor vehicles under the Nazi regime by the Vienna Technical Museum.