Hot forming of a large deformable mirror facesheet

Conventionally, facesheets for deformable mirrors (DM) are manufactured through grinding, polishing and finishing of a piece of optical glass. Due to the amount of material that needs to be removed, the required surface form errors and the complex handling of the fragile product throughout all manufacturing steps, this is a lengthy, risky and expensive process. For the 620mm-diameter adaptive secondary mirror (ASM) of the University of Hawaii's 2.2-meter telescope, we develop an alternative process where the facesheet is manufactured by hot forming ('slumping') of a flat glass pre-form in a mold directly to the proper shape. Since low-spatial surface form errors can be corrected with the DM actuators to a large extent, only the mid/high-spatial errors beyond the period of the actuator spacing are considered critical. Through accurate modeling, and an iterative approach of hot-forming, metrology and adjustment of process parameters, the forming process is optimized to obtain the near-final shape. This approach limits the amount of required finishing operations, resulting in a facesheet meeting specifications at significantly reduced cost. We present results for the modeling & simulations, and the progress in manufacturing of this ASM facesheet.