Tartu Ülikooli muuseumi kogudes olevate 19. sajandi teleskoobipeeglite keemiline analüüs. Analysis of the Chemical Composition of Two telescope Mirrors from the University of Tartu Museum Collections

Abstract

The purpose of this research was to try and determine the suitability of a portative X-ray fluorescence spectrometer (pXRF) for ascertaining the chemical composition of telescope mirrors in a non-invasive way. Both of the mirrors in question very probably date back to the early 19th century and were made by William Herschel and Johann Gottlieb Friedrich Schrader (although this is not 100% clear). One aim was to see whether the chemical composition of the objects was similar to what we know about the telescope mirrors of the time. The search for the perfect composition of a telescope mirror haunted telescope makers from the 17th century until the second part of the 19th century when silvered glass mirrors were developed by Léon Foucault and Marc Secretan. A mixture of copper and tin was
used to make speculum metal with the addition of a variety of metals and other compounds to add strength and lustre to the telescope mirror. The results of the pXRF tests show both mirrors are similar in composition. The chemical composition of the mirrors is also similar to contemporary recipes. However, the mirrors show less copper and more tin than the recipes would allow. It cannot be conclusively determined whether this is due to the restrictions of xPRF measuring (only from the surface of the mirror) or less copper was included while making smaller mirrors in the early 19th century.