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Figure verification of a precision ultra-lightweight mirror: techniques and results from the SHARPI/PICTURE mirror at NASA/GSFC

A high-precision ultra-lightweight 0.5m mirror with ultraviolet grade tolerances on surface figure quality has been measured from its delivery to the Goddard Space Flight Center, through the coating and mounting process, and shown to survive component vibration testing. This 4.5kg, 0.5m paraboloid mirror is the prime optic of two sounding-rocket telescopes: SHARPI (solar high angular resolution photometric imager) and PICTURE (planet imaging concept testbed using a rocket experiment). By integrating the analysis of interferometer data with finite element models, we demonstrate the ability to isolate surface figure effects comparable to UV diffraction limited tolerances from much larger gravity and mount distortions. The ability to measure such features paired with in situ monitoring of mirror figure through the mirror mounting process has allowed for a diagnosis of perturbations and the remediation of process errors. In this paper, we describe the technical approach used to achieve nanometer scale measurement accuracy, we report and decompose the final mounted surface figure of 12.5 nm RMS, and we describe the techniques that were developed and employed in the pursuit of maintaining UV diffraction-limited performance with this aggressively lightweighted mirror.