New Glass Revealed!

This week at work (Steward Observatory Mirror Lab), the latest mirror casting was revealed as the oven was disassembled. While the 12-meter diameter oven is normally used to cast either 6.5 or 8.4 meter diameter substrates, this time the project was for a replacement telescope mirror for the 1.2 meter telescope at Fred Lawrence Whipple Observatory, operated by the Smithsonian Astrophysical Observatory at Mount Hopkins south of Tucson. While a 1.2 meter (48") diameter is a sizable telescope, it just looks tiny on an oven usually used for mirrors nearly 50 times larger (in area). In fact, the last big mirror we cast has a center hole as big as this mirror!

The original 1.2 meter mirror, originally cast at the Mirror Lab in the '80s, used true Pyrex glass from a U.S. supplier. However, the original rough edges of the glass pieces did not fuse together cleanly, and the substrate was full of bubble veils that not only weakened the substrate, but the multitude of bubbles on the mirror surface made cleaning for aluminizing nearly impossible, thus achieving a good reflecting surface was difficult. So with a gap in the casting schedule, a mold was machined and the new substrate was cast over the last 2 months. This substrate too has a few bubbles near the surface, most of which will be removed in generating, but it is very clean compared to the mirror in the telescope. The standard hexagonal lightweighting technique was used, and the white refractory material of the mold will be removed the next few weeks to leave a hollow glass structure. I'm not sure what the mirror weighs (I'll ask tomorrow), but I'm guessing about 500 pounds (220kg). Edit: I guessed correctly - as-cast weight is 575 pounds (260kg). Finished weight after processing will be 400 pounds (180kg).

I've got a few friends in the local astronomy club that would love to see this made into a telescope that we could haul around in the back of a pickup. Now that would be fun!