High Resolution Adjustable Mirror Control for X-ray Astronomy

The goal of the proposed program is to enable increased angular resolution and collection areas for future major X-ray observatories by incorporating control of the mirror surfaces after fab-rication and mounting. We propose to develop and implement a method for preparing adjustable optics with integrated control elements on curved mirror segments for future X-ray space telescopes. Development of such mirror elements will provide a major advance to the field of X-ray astronomy, by enabling mirrors with half an arcsecond angular resolution using thin, lightweight glass to significantly increase the collection area. This is an enabling technology for mission concepts such as the X-ray Surveyor.

The heart of the proposed program is the integration of PbZr0.52Ti0.48O3 (PZT) piezoelectric cells to serve as adjusters for thin glass mirrors. The voltage on each cell is optimized to correct for figure errors that may be present due to mirror fabrication, mounting, gravitational or thermal effects. Previous work demonstrates that the piezoelectric thin films enable correction of the mirror surfaces using control voltages under 10 V. Control of a large array of piezoelectric cells requires an approach to reduce the complexity of the wiring array. We propose to address this challenge by integrating ZnO thin film transistors for row-column addressing, along with ancil-lary electrostatic discharge protection directly on the back of the mirrors. This will radically reduce the number of required electrical connections, while relying on a robust technology widely in used for display applications. The PSU team has demonstrated via previous NASA funding that ZnO transistors can be integrated on PZT piezoelectrics on flat mirrors while retaining the full functionality of both.

The extension we will make through this program is demonstration of an integration scheme appropriate for ZnO transistors on conical adjustable optics. We plan to achieve this through preparation of ZnO electronics on thin polyimide films (~5 micron in thickness) which can be lam-inated to the adjustable optics mirrors. This will allow conventional high-resolution lithography processes to be utilized for the transistors.

A second key component will develop PZT films with excellent uniformity of thickness, breakdown strength, phase purity, and piezoelectric response over the curved mirror segments. A process for insuring improved stability of the piezoelectric response will be identified by carefully controlling the alignment of the domain structure.

A third engineering challenge is associated with making the electrical connections between the adjustable optics and the control system. We will investigate anisotropic conductive film bonding, gold diffusion bonding, and conductive epoxy bonding for this step with particular at-tention to minimizing any effects on mirror figure from the bonding process. An electronics box will be built to enable control of a 22 by 22 cell actuator array.

We propose, by the end of a 3 – year program to fabricate a high yield conical mirror segment of an adjustable optic with ZnO transistor-based row-column addressing. The mirror will be mounted in a “flight-like” configuration, electrically connected, and functionally tested with optical metrology. This test will duplicate the optical pre- and post-X-ray testing planned for Dec. ’15 as part of our current adjustable X-ray optics APRA program. The part will be available for subsequent X-ray testing to demonstrate functionality. Close collaboration between teams at Penn State and the Smithsonian Astrophysical Observatory will enable quantification of the influence functions resulting from PZT actuation and a measurement of the angular resolution of the resulting curved mirror segments.

This proposal supports NASA's goals of technical advancement of technologies suitable for future missions, and training of graduate students.