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Multilayer Active Shell Mirrors


Steeves, John Bradley (2015) Multilayer Active Shell Mirrors. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z99W0CFB.


This thesis presents a novel active mirror technology based on carbon fiber composites and replication manufacturing processes. Multiple additional layers are implemented into the structure in order to provide the reflective layer, actuation capabilities and electrode routing. The mirror is thin, lightweight, and has large actuation capabilities. These features, along with the associated manufacturing processes, represent a significant change in design compared to traditional optics. Structural redundancy in the form of added material or support structures is replaced by thin, unsupported lightweight substrates with large actuation capabilities.

Several studies motivated by the desire to improve as-manufactured figure quality are performed. Firstly, imperfections in thin CFRP laminates and their effect on post-cure shape errors are studied. Numerical models are developed and compared to experimental measurements on flat laminates. Techniques to mitigate figure errors for thicker laminates are also identified. A method of properly integrating the reflective facesheet onto the front surface of the CFRP substrate is also presented. Finally, the effect of bonding multiple initially flat active plates to the backside of a curved CFRP substrate is studied. Figure deformations along with local surface defects are predicted and characterized experimentally. By understanding the mechanics behind these processes, significant improvements to the overall figure quality have been made.

Studies related to the actuation response of the mirror are also performed. The active properties of two materials are characterized and compared. Optimal active layer thicknesses for thin surface-parallel schemes are determined. Finite element simulations are used to make predictions on shape correction capabilities, demonstrating high correctabiliity and stroke over low-order modes. The effect of actuator saturation is studied and shown to significantly degrade shape correction performance.

The initial figure as well as actuation capabilities of a fully-integrated active mirror prototype are characterized experimentally using a Projected Hartmann test. A description of the test apparatus is presented along with two verification measurements. The apparatus is shown to accurately capture both high-amplitude low spatial-frequency figure errors as well as those at lower amplitudes but higher spatial frequencies. A closed-loop figure correction is performed, reducing figure errors by 94%.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Active Mirrors, Space Telescopes, CFRP Laminates, Nanolaminates, Replication Techniques
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Space Engineering
Awards:Charles D. Babcock Award, 2013; Ernest E. Sechler Memorial Award In Aeronautics, 2014; The Donald Coles Prize In Aeronautics, 2015
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Pellegrino, Sergio
Thesis Committee:
  • Ravichandran, Guruswami (chair)
  • Kochmann, Dennis M.
  • Burdick, Joel Wakeman
  • Pellegrino, Sergio
Defense Date:8 May 2015
Funding AgencyGrant Number
Natural Sciences and Engineering Research Council of Canada (NSERC)UNSPECIFIED
Keck Institute for Space Studies (KISS)UNSPECIFIED
Record Number:CaltechTHESIS:05282015-145339959
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:8902
Deposited By: John Bradley Steeves
Deposited On:02 Jun 2015 15:19
Last Modified:02 Nov 2021 19:52

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