Progress on creating thin curved glass facesheets for deformable mirrors via free form slumping

Adaptive optics systems that use adaptive secondary mirrors can potentially broaden the use of AO in astronomy. However, unlike post-focal plane AO systems, ASM-based AO systems require large (200-1500 mm diameter), curved, aspheric membranes to replicate the standard telescope secondary mirrors that they replace. A key limitation of current devices is the cost (and associated risk) of manufacturing thin (2-3 mm) curved facesheets. We have developed a new approach for the fabrication of these facesheets that initially uses flat, polished sheets of glass that are formed into curves via heating. This technique traditionally relies on an accurate negative form to slump the glass into. We take a different approach of letting the glass slump freely within a support ring that is outside the final optical diameter of the shell. Such an approach allows for the creation of shells over a variety of radii of curvature, simply by adjusting the peak temperature. However, to create an approximately parabolic shape, an additional force is required, which can be applied by weights added on to of the shell. We have tested this over a variety of size ranges from 300-600 mm, and for a range of radii of curvature from 1000-5000 mm. We report on the fabrication of shells for the IRTF ASM, as well as initial experiments for a shell of 620 mm diameter.