Thin, Active Mirrors
In order to facilitate the development of future large space telescopes, there is a need for new mirror technologies that will allow for the construction or deployment of larger primary apertures. The primary mirror of the Hubble Space Telescope is 2.4 meters, and the primary of the James Webb Space Telescope, currently under development, is 6.5 meters. Larger sizes for future telescopes will require innovative mirrors that will reduce the overall mass and cost of the mission.
Use of thin shell mirrors are one possible avenue for drastic reduction of mass per unit area. Many thin mirrors could be replicated from a single accurate mold in order to populate a segmented aperture. However, such mirrors suffer from a lack of structural stability, stiffness, and shape accuracy. Active materials and actuators can be used to alleviate this deficiency. This research is an exploration of thin mirror design concepts and their effectiveness at providing accurate shape control. Small-scale prototype mirrors are currently under development for the AAReST mission concept (more information on AAReST can be found here).
Two active mirror concepts are currently under development. The first makes use of micro-manufacturing techniques to produce active mirrors comprised of an optical-quality glass substrate with layers of piezoelectric polymer bonded to its backside. The second builds upon the methodology of the first but looks towards the construction of larger diameter (0.5m-1m) mirrors. In this method, the mirror substrate is fabricated from carbon fiber composites using relatively simple manufacturing processes.
Work has also been performed in order to create optimal actuator patterns for the mirror concepts. Two approaches are used in order to do so: a "top-down" method where patterns are created using a-priori knowledge of the expected mirror deformation, and a more general "bottom-up" approach using an evolutionary algorithm to produce the actuation pattern.
Dedicated metrology systems have been developed in order to characterize mirror deformations and define associated control methods. Two systems have been developed: The first uses a Shack-Hartmann wave-front sensor to measure and control the wave-front error upon reflection off the mirror. The second, allowing the measurement of larger amplitudes, uses a reverse Hartmann technique by projecting a regular grid of light onto the mirror.
Reverse Hartmann measurements have been used to characterize the most recent Carbon Shell DM. We looked at initial resting shape, which shows that the dominant manufacturing error is astigmatism.
We then implemented closed loop control to make an absolute measurement of the achievable flatness, and found that an RMS surface error of 200nm can be reached. An inspection of the voltages required to achieve this show that several edge actuators are saturated, suggesting further reduction in manufacturing errors are still required.
We also measured the controllability of the mirror by examining its ability to form specific Zernike modes. This was executed using relative modal control, where the initial unactuated shape is measured and then the mirror is driven to a differential modal shape; using this technique, we can use the full dynamic range of each actuator without allocating any stroke to correcting initial shape errors. Results show that we are approaching the theoretical limit of mirror controllability.
Collaboration with JPL
This link to page 38 of the JPL Microdevices Laboratory 2011 Annual Report shows Keith Patterson and Dr. Risaku Toda testing a deformable mirror.
- Steeves, J., Laslandes, M., Pellegrino, S., Redding, D., Bradford, S. C., Wallace, J. K., Barbee, T. (2014) Design, fabrication and testing of active carbon shell mirrors for space telescope applications. (pdf)
- Laslandes, M., Pellegrino, S., Steeves, J., Patterson, K. (2014) Optimization of electrode configuration in surface-parallel actuated deformable mirrors. (pdf)
- Steeves, J., Pellegrino, S. (2013) Ultra-thin highly deformable composite mirrors. (pdf)
- Patterson, K., Pellegrino, S. (2013). Ultra-lightweight deformable mirrors. (pdf)
- Patterson, K., Yamamoto, N., Pellegrino, S. (2012) Thin deformable mirrors for a reconfigurable space telescope. (pdf)
- Patterson, K. Pellegrino, S. (2011) Shape correction of thin mirrors. (pdf)
- Patterson, K., Pellegrino, S., Breckinridge, J., (2010) Shape correction of thin mirrors in a reconfigurable modular space telescope. (pdf)