Liquid Mirror Lunar Telescope

Liquid mirror made of mercury. A transparent layer of oxide on the surface prevents the mercury vapors from escaping. Credit: Guy Plante (Laval)

Thursday, June 21st, 2007--

Scientists may have figured out a way to make a new telescope that can out-observe the most powerful space telescopes by a factor of a thousand. The secret to the design? A liquid mirror.

Ermanno Borra, a physicist from the Universite Laval in Quebec, Canada and colleagues, took an ionic liquid (basically a salt that melts at low temperatures) and coated the surface of the ionic liquid with silver. The liquid remained stable for months, giving new life to the 150-year-old idea of making a liquid mirror telescope for the moon. They reported their findings in the journal Nature this week.

To turn liquid into a mirror, it must be spun. The spinning creates a centrifugal acceleration that pulls the liquid to the sides of the vat, while gravity pulls the liquid down. These forces act in concert to create a perfect parabola, which can serve as a telescope’s mirror. These natural forces make the mirror self-mending: if its surface is perturbed, the mirror will naturally right itself.

But the biggest advantage, Borra says, is the simplicity of its design: “They are extremely simple and really inexpensive." A glass mirror of equivalent optical quality would cost about one hundred times more than a liquid mirror, he says.

A liquid mirror would be ideal for a lunar telescope because there is no wind on the moon. “There is no weather on the moon because there is no atmosphere,” Borra says, adding, “this is the reason we would want it on the moon, because it would be outside the Earth’s atmosphere.”

Plus, liquid mirrors can be large. “Making a 100-meter or even 20-meter glass mirror on the moon is absolutely unthinkable,” says Borra. A glass mirror of that size would have to broken down and re- assembled on the moon. “It’s difficult enough to do on the Earth. But on the moon? Give me a break...it’s impossible,” he says.

A liquid mirror on the other hand, can be transported in a jug and poured into a container. While coating the liquid with silver on the moon could be tricky, Borra argues it is simpler (and cheaper) than reconstructing a glass mirror of equivalent size.

Borra and his colleagues expect that a 100-meter liquid mirror lunar telescope, because of its size, would let us see deeper into the universe than ever before. There is also an advantage to escaping Earth's atmosphere. Borra says, "We want to understand how stars form, and how galaxies form." These deep space occurrences are masked by the light in our own atmosphere. "That's why we have to get out of the atmosphere--the upper atmosphere glows in the infrared region of the spectrum, which is where all the light will be."

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--Flora Lichtman