A reflecting telescope, of the type invented by Isaac Newton on Earth.
Like every other material encountered in the tower, this telescope was manufactured without the use of high-temperature processes like fire. The reflector is a silver mirror electroplated over quartz.
It seems possible that the makers invented telescopes far earlier than humanity did. Perhaps they were inspired by light bending at the boundary between air and water.
The telescope was once sealed against water, so that it could produce a clear image. The seals corroded long ago.