Liquid Telescope Progresses

An idea to build a highly reflective liquid-mirror telescope on the moon became a little more practical this week with an international team of researchers' announcement that they have achieved an optics first by successfully vacuum-coating a liquid.

Liquid-mirror telescopes differ from conventional telescopes by their primary mirrors -- the ones that gather and focus light -- which are made from a reflective liquid instead of polished glass. Poured into a spinning container, the liquid spreads out and forms a thin, perfectly smooth, and parabolic shape that can be used as a telescope mirror.

Researchers behind a 3.7-m-diameter liquid mirror at Laval University. The liquid is mercury, but there are no detectable mercury vapors in the air because a thin transparent layer of oxide covers the surface. (Photo courtesy Guy Plante, Laval University)

The international team included Ermanno Borra of Laval University's Center for Optics, Photonics, and Laser (COPL), who has long advocated the idea of a lunar liquid-mirror telescope (LLMT). In a 1991 paper published in the Astrophysical Journal, professor Borra first suggested it, demonstrating the practical and economic advantages of liquid mirror telescopes over their conventional counterparts and explaining how an observatory free from the Earth’s atmospheric disturbance could further our understanding of the early universe.

The project, which seemed almost like science-fiction at the time, gained renewed interest in 2004 when it received financial support from the NASA Institute for Advanced Concepts, which funds projects that can potentially push back the limits of science and space technology.

The project’s main challenge was finding a liquid capable of resisting the conditions on the moon’s surface and functioning in temperatures required for infrared observations, i.e. below -143 °C.

A 2-in.-diameter flat liquid mirror made of an ionic liquid coated with a thin layer of silver. The reflection of a resolution chart can be seen in the liquid. (Photo courtesy Omar Seddiki, Laval University)

In their new article, appearing in the June 21 edition of Nature, Borra and his colleagues explain how they may have solved the problem with a combination of materials. They successfully coated an ionic liquid with silver by vaporizing it in a vacuum. Ionic liquids are salts which are liquid at temperatures below 373 K, composed entirely of ions, and usually possess no significant vapor pressure at room temperature or below. The resulting silver layer is perfectly smooth, highly reflective, remains stable for months, and the ionic liquid on which it lies does not evaporate.

The liquid mirror envisioned for the lunar telescope would be 20 to 100 meters in diameter, making it up to 1000 times more sensitive than the proposed next generation of space telescopes, the researchers said.

Instead of having to transport a heavy, expensive piece of glass to the moon, the liquid mirror could be brought in bottles, and the whole telescope assembled on the lunar surface, Borra said.

Such a telescope will not be available to researchers in the near future, Borra said. "However, if we hadn't found the solution described in our article in Nature, it would have meant the end of the whole project."

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