Perspectives on primary mirror figure control for a space-based optical telescope

Large space based telescopes such as the Next Generation Space Telescope (NGST) have motivated the study of large deployable primary mirror concepts. This paper will explore the rationale used to develop the trade space between rigid body adjustment of segmented mirrors and full primary mirror active figure control. This discussion covers the relative merits of the two fundamental approaches with regard to complexity and system performance, including performance at cryogenic operating temperatures such as envisioned for NGST. Some of the areas covered will be mirror segment size as it relates to complexity of control, ability to address radius of curvature adjustment, impact of different mirror substrate materials choices, and other system implications such as launch loads. An area of trade considered is the amount of control achieved at the primary mirror compared to augmented control using a deformable mirror at a subsequent pupil plane.