Status of the production of the thin shells for the Large Binocular Telescope adaptive secondary mirrors

摘要

We report on the current status of production of the thin shells for the LBT adaptive secondary mirrors. These shells are made of Zerodur and have a front (optical) surface highly aspherical whereas the other (rear) surface is spherical. They have a 910mm diameter and an average thickness of 1.6mm. The manufacturing of these shells starts with a thick blank of Zerodur and follows the steps of: 1) optical surface figuring, 2) blank thinning, 3) rear surface grinding and polishing, 4) edges machining and 5) rear surface aluminizing. Of the three (two plus a spare) shells planned for LBT the first shell was completed and shipped to Italy for integration with magnets and the second is in advanced state of production. The third shell (spare) is planned to start production soon. In the paper we report details of production of these shells as well as the 'as built' characteristics. Details concerning the operations that follow the production, i.e. surface aluminum coating as well as handling and shipping fixtures are also reported.