Environmental Testing of MEMS Deformable Mirrors for Exoplanet Detection
Michael Helmbrecht/Iris AO, Inc.
Many of the coronagraphs being studied and developed for space-based exoplanet detection rely on microelectromechanical system (MEMS) deformable mirrors (DM) to control optical wavefronts at picometer levels. Failure of even a single DM actuator could render the coronagraph useless because of the stringent contrast requirements. Despite this, relatively little effort has been directed towards environmental testing of MEMS mirrors for space qualification. Attending to MEMS DM flight qualification is essential to further advancing the TRL of coronagraphs in general.
A simple-minded "shake and bake" approach is wholly inadequate to achieve a space qualified design that is traceable to flight hardware. Instead, environmental testing must be accompanied with FEM modeling to understand the testing results. Because of tolerances associated with MEMS fabrication techniques, the FEM models must be validated with appropriate sampling to actual hardware prior to environmental testing. The best approach, based on experience garnered by flight qualifying the NIRSPEC microshutter arrays for JWST, is to have an FEM model that can be used to predict performance for various launch environments the DMs will be subjected to. Further, the model should be able to predict performance for any design modifications required to increase performance. Combining the FEM modeling with environmental testing yields a DM technology that can meet space qualification requirements and one that can be transitioned to a fabrication line that is capable of supporting flight builds.
The first milestone for this research is to develop FEM models of existing Iris AO DM designs and validate them with measured data from an appropriate sample of DMs. Early in the research, a set of DMs will be destructively tested and the failures will be correlated with the validated FEM model. Also, the mirrors will be tested for shock and acoustic loads over a flight envelope of typical launch vehicles to assess any potential weaknesses in the designs that could result in failures during launch.
The validated FEM models and results for launch load testing will be used to modify the MEMS DM design to make it robust to launch loads. The modified designs will be fabricated and then used to validate the FEM models that were used for the design. This will demonstrate the validity of using the FEM models in a predictive fashion for design modifications.
The robust DMs will be tested with a more comprehensive set of tests to validate launch robustness and radiation hardness. Prior to and after environmental testing, the DMs will be operated in vacuum in the Visible Nulling Coronagraph (VNC) at NASA/Goddard Space Flight Center. Error budgets for the DM will be determined and compared to simulations. These tests will be used to determine successful completion of the second milestone of this research in order to mature the technology to TRL-5: acoustic, vibration, shock, and radiation testing coupled with performance validation in the VNC before and after qualification.
The end result will be a TRL-5 DM technology that will be traceable to flight hardware. Validated predictive FEM models will exist for two independent modeling tools, thus enabling design modifications that can be simulated for launch loads and cross checked prior to fabrication of any future design modifications required for flight hardware.
The FEM studies and testing here will be focused on the Iris AO DM technology and its use in the VNC. However, the methods developed here will be invaluable to the entire coronagraph community baselining MEMS devices.
Strategic Astrophysics Technology