Are the great mirrors and lenses of historic telescopes preserved? Those of Herschel and Rosse and many others and back to the 18th and 17th centuries. The wooden structures of those telescopes I can imagine rottened and became firewood. But didn't they preserve the mirrors and lenses in some basement because they didn't know what to do with them?
What historic telescope mirrors are being conserved?
Many of these objects are collected in museums. The German "Deutsches Museum" for instance keeps famous original instruments of Fraunhofer http://www.deutsches-museum.de/sammlungen/meisterwerke/meisterwerke-ii/refraktor/ Such institutions are scattered all over the globe.
Further there are many museums devoted to individual astronomers like https://en.wikipedia.org/wiki/Herschel_Museum_of_Astronomyreplica keeping original relics and replica.
Most relics will have been preserved at their original places. The Göttingen Sternwarte keeps original instruments used by Gauss. A list of such institutions would get fairly long.
One of the most important mirrors historically is that of Sir Isaac Newton's telescope shown below. In the Periodic Video Amazing piece of metal (speculum), Professor Martyn Poliakoff (CBE, CChem, FRS, FRSC, FIChemE) describes the telescope, notes about building the telescope, and what is believed to be the speculum mirror used in the telescope, at the Royal Society in London.
