The use of larger, lighter, and more precise space optics requires not only a means of manufacture, but also a means of spacecraft integration and performance verification. Engineers at NASA's Goddard Space Flight Center (GSFC) have demonstrated a process capable of producing a high-precision, mounted, lightweight mirror, and have validated its on-orbit figure. This effort included the design of a mount capable of surviving the launch environment of a sounding rocket, as well as a mounting process that did not introduce performance-degrading figure distortion. Additionally, analysis techniques were developed and adapted to address the challenges in measuring an optic that exceeds its figure specification under the strain of its own weight.
The mirror mount and associated mirror mounting process enables ultra-lightweight high-precision mirrors to be mounted without distortions exceeding length scales of several nanometers (root-mean-squared, over the optical aperture); provides an on-orbit, or zero-gravity, mirror surface figure verification capability in the presence of much larger self-weight gravity distortions; and is proven to both mechanically survive a launch environment and optically maintain mirror surface figure.
This hardware design and mounting method are particularly innovative because they enable the in situ measurement of nanometer-sized, zero-g mirror figure distortions in the presence of large gravity-induced mirror distortions. Furthermore, the hardware and test results allow for the iterative analysis, isolation, and correction of any induced mirror distortions due to the mounting process before the mount is irreversibly locked. The core of this innovation is the means of integrating mount design, support hardware capabilities, modeling and analysis, and in situ optical testing.
A high-precision, ultra-lightweight 0.5-m mirror with ultraviolet grade tolerances on surface figure quality has been measured through the coating and mounting process, and shown to survive component vibration testing. This 4.5-kg, 0.5-m 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, the ability to isolate surface figure effects comparable to UV diffraction limited tolerances from much larger gravity and mount distortions was demonstrated. Being able 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. Nanometer-scale measurement accuracy was achieved, and the final mounted surface figure was 12.5 nm RMS, maintaining UV diffraction-limited performance with an aggressively lightweight mirror.
The in situ test approach, mount concept, and methodology enable the verifiable distortion-free mounting of lightweight optics in a manner compatible with spaceflight. This is particularly applicable to the production of any system that employs precision lightweight optics that must withstand a harsh launch environment, then operate in a zero-gravity environment. This includes, but is not limited to, earth-observing systems, optics used in space exploration, and spaceborne astronomical observatories.