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Mirror men stake their claim - New Scientist report by Marcus Chown - 12 Sept 1985

Hands up anyone who does not have a technique for making telescope mirrors from silvered plastic films. In the past week, New Scientist has been deluged with letters claiming that the idea, presented this month by Peter Wadell and Bill King of Strathclyde University (last issue, p19) is theirs. Claimants include:

Maurice Gavin, an amateur astronomer who published his idea in Sky & Telescope in 1979;

Richard Taylor, whose plans for a giant inflatable telescope will appear in Space magazine next month;

Robin Frost and John Beckman, who presented their ideas for an inflatable mirror at a symposium on space manufacture at Princeton University in 1980;

K. Mallen, an unemployed man from Boston, Lincs. He filed a patent application - "A process for the production of parabolic reflectors" in July this year.

Wadell says he received a provisional patent in Britain in February this year and his US patent came through in August. The British patent has yet to be published so would-be objectors will have to bide their time. Wadell has been reluctant to reveal the details of his technique, and he admits that his patent omits some important elements. So it is difficult to tell whether he has something the other claimants do not have.

Certainly, if he has overcome two major problems which plague the method, he may be justified in saying be bas made a breakthrough. The aim is to make a high quality reflecting mirror by stretching a plastic membrane over a hoop and sucking out the air from behind. It is important to have control over the stress at all points in the film. The shape that the film takes up, be it spherical or paraboloidal or some shape nore esoteric, depends on the stress pattern across the membrane. Wadell claims to lave solved this first problem. His membrane is not anchored to the hoop but is sprung, enabling it to move wben suction is applied, he says.

A second problem concerns the material of the membrane. Wadell uses a commercially available thin film and he admits it produces horrendous images if used in its raw state. The manufacturing process stretches the film, aligning the molecules and giving it directional properties. When fabricated into a mirror it therefore affects light differentially according to its polarization. Wadell subjects his film to an "unspecified" treatment to make it isotropic and iron out the unevenness.

"If Wadell can change the stress pattern in his membrane," says Gavin, an architect and amateur astronomer, "then he really does have something new." Gavin demonstrated a 52-cm reflector made from aluminized mylar in 1978 and published the idea in Sky & Telescope the following year. He was unable to do all that Wadell claims for his technique. He says the surface he produced was that of an oblate spheroid and unsuitable for focusing a point image. He was able to compensate for optical defects by combining with an inflatable secondary mirror.

The inflatable, or deflatable, certainly an idea whose time seems to have come. Richard Taylor of British Steel and the Extramural Department of London University has been thinking on similar lines to Wadell since 1978. He is to present his plans for a 200-metre inflatable spherical space mirror in the aerospace magazine Space later this month.

Wadell claims that a company in the US has offered him upwards of £10 million for his idea, and negotiations are under way which could net Strathclyde University a large reward as well. Wadell still has many problems to overcome if his idea is to end in a space mirror useful for star wars. Taylor has looked into the danger of micro meteoroid impacts for his own mirrors. he believes that leakage will be slow and gas can be added continually to compensate. Nevertheless he believes that an inflatable space mirror would suffer serious damage within five years of being placed in orbit.

The real problem, Taylor says, is oscillation in the mirror surface caused by variations in solar heating, gravity variations, any movement in of the mirror and interactions with the earth's magnetic field. Wadell believes the problem of interference from the magnetic field can be overcome by placing the mirror in a high orbit where the earth's field is weak. Taylor points to a final problem for a space mirror. He thinks the conducting surface will charge up, giving any vehicle which strays close by (a space shuttle for instance) an almighty electric shock!

Marcus Chown