I wonder which method the author will go with. Reflecting telescopes could work, but that’s not where the plot is headed towards. Plus, they’re not exactly known for their durability. For the material, resin or gems could be used, but the author seems intent on glassblowing. Given their tech, a multistep process where they try to grind a sphere and turn it into a lens by cutting or melting it would be a lot easier, albeit of of lower quality. Honestly, they already have a lathe, so they could just opt to grind it the entire way. But the author seems to want the glass to be blown, and the specific problem is the clarity of the glass. The only solution I can really think of for that is to toss in some soda and lime to make soda lime glass, or lead oxide to make lead glass. Both are soft and clear, so they could be used to make somewhat workable telescopes