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Plastic Film Makes for Light Travel

Photonics Spectra
Aug 2000
Steve Miller

Moving mass into space is an expensive proposition. The Next Generation Space Telescope, tentatively scheduled for launch in 2008, will weigh 15 kg/m2 of mirror surface, lean compared with Hubble's 250 kg/m2, but still not cheap. Future telescopes based on research at Sandia National Laboratories and the University of Kentucky in Lexington might weigh as little as 1 kg/m2.

In addition, the mirror might be folded into a compact package and expanded to its full size in orbit. Instead of a glass mirror, the telescope's reflective surface will be a polymer film, held in position by a piezoelectric reaction to a stream of electrons. This technology may enable the production of telescope mirrors as large as 30 m in diameter.

An electron gun controls the curvature of the reflective piezoelectric film, which expands or contracts in response to the beam of electrons, explained Tammy Henson, an optical engineer at Sandia. "We have determined that we can make very small changes -- in the order of 150 nm -- to the surface with this method," she said. "This is close to the tolerance needed for optical applications."

Tammy Henson displays the piezoelectric film that could enable large, lightweight telescope mirrors. Courtesy of Sandia National Laboratories.

Among the challenges that the research team faces is development of an optical sensor to provide feedback to the computer that controls the electron gun. Mirror shape corrections span many orders of magnitude, from an initial range of millimeters, as the mirror is unfolded, to the fine adjustments of tens of nanometers necessary for optical imaging applications. Although traditional interferometry provides the sensitivity and resolution required for this control, its range is insufficient.

The research is still at the lab scale, according to John Main, a mechanical engineer at the University of Kentucky who is developing the electron gun for the project. Although the largest mirror controlled to date is a 5 x 10-cm rectangle, results have indicated that sufficiently fine control is possible. Because the electron gun must operate in a vacuum, the technology is not likely to be applied to ground-based telescopes.

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