Thermally formed inflatable reflectors for space telescopes

Imaging distant objects with increasing spatial resolution is instrumental towards furthering space exploration abilities. Telescopic imaging of exoplanets and other objects requires mirrors with large surfaces which when used at terahertz frequencies can further capture object chemistry, mass structure and dynamics. Membrane mirrors could lead to a dramatic scale up in size of telescope mirrors deployed on orbit. Large membrane reflectors built in the past have had a primary challenge of uncontrolled inflation dynamics and surface shape. A major contribution to such inaccuracies has been attributed to manufacturing techniques employed. The surface shape attained by a tensioned membrane has been described as an oblate spheroid or Hencky surface. Traditionally built out of smaller gore units, membrane mirrors tend to attain faceted final shapes that deviate from the intended. They have unreliable and unrepeatable final surface shapes. This makes the design of corrective optics difficult. Further, complex assembly jigs are required for the precise manufacture of such membrane units. A repeatable and scalable manufacturing method is required to harness the advantages offered by membrane reflectors. Our present work is focused on thermally formed membrane reflectors. This involves heating a whole flat membrane close to its glass-transition region followed by pressurization at a constant fixed temperature. The intent is to induce plastic deformation of the membrane causing a retention of induced curvature when cooled down. This method eliminates breaking down the membrane structure into smaller gore units and can be scaled over to vast membrane sizes. An experimental set-up has been designed and built to thermally form a 1-meter diameter Mylar membrane reflector and conduct shape measurement on its surface. We present development efforts in the design, manufacture and surface shape measurement of thermally formed reflectors. The results are being used to validate thermo-structural simulations conducted on the expected membrane surface behavior. Further analysis is underway to understand optimal circumferential stress distributions to improve the reliability of obtained membrane shapes. Our work contributes towards an understanding of key design variables in the development of tensioned thermally formed membrane reflectors that can provide a potential pathway towards dramatic scale up in size of such mirrors.