Five centimetre diameter flat liquid mirror. It is made of an ionic liquid coated with a thin layer of silver. A reflection of a resolution chart is visible in the liquid.
Credit: Omar Seddiki (U. Laval)
PARIS: Scientists have successfully coated an ionic liquid with silver, an achievement that may one day find use as a liquid mirror in an immensely powerful lunar telescope.
Desolate, airless and with no people around for hundreds of thousands of kilometres, the Moon is a great place – for astronomers. Skywatchers have an enduring hope of one day building a lunar observatory, where gleamings from the earliest stars can be snared without the curse of man-made light pollution and Earth's atmospheric distortion.
But making telescopic mirrors – dozens are needed in a giant complex – is eye-wateringly expensive, for it requires grinding and polishing glass to an accuracy of a few tens of billionths of a metre. And, after making a mirror, there's the risk of breaking it when you haul it to the Moon.
Enter an idea that has been kicked around for more than a century and a half: the liquid mirror telescope.
Gentle spin
Under this, mercury is gently spun on a round table, so that centrifugal force combined with gravity forces the quicksilver to spread out, with its edges thicker than its centre.
The parabolic shape is exactly what is needed to focus the reflected light on a detector, and supporters say the optical qualities are as good as with glass.
So far, several pilot schemes using liquid mirror telescopes (LMTs) up to six metres across have been launched on Earth, and backers say their cost is just one or two per cent of large conventional scopes.
According to their calculations, an LMT on the Moon, with an aperture of 20 to 100 m, would be able to observe objects 100 to 1,000 times fainter than the US James Webb Space Telescope, a 4.1-billion-A$ next-generation orbiting telescope scheduled for launch in 2013.
Now a team of U.S. pioneers believe they have made strides towards resolving the big challenge facing a lunar LMT – finding a substitute for mercury, which is no good as it would freeze on an unlit surface of the Moon.
"Excellent optical quality"
The researchers – led by Ermanno Borra, a physicist at Laval University in Quebec, Canada – have used a process called vacuum vaporisation to apply liquid chromium to a water-repelling, commercially available solvent called ECOENG 212.
They then added liquid silver on top of the chromium, delivering a result with "excellent" optical quality and which remained stable throughout months of study, they report today in the British journal Nature.
The outcome is not the Holy Grail, because the reflectivity is still not up to scratch and the solvent freezes at -98°C, which is still too high for lunar temperatures as low as -130°C.
The good news, though, is that ECOENG 212 is part of a vast category of chemicals called ionic compounds, which are environmentally-friendly, crystalline solids that melt into liquids at low temperatures.
"Ionic liquids seem to defy common sense," said chemist Robin Rogers of the University of Alabama in Tuscaloosa in a commentary published also in today's Nature. "Most ionic compounds are crystalline solids with high melting points, but these fascinating salts melt at temperatures below 100°C; indeed, many are liquids at room temperature," he said.
Mobility issues
"As there are at least a million simple ionic liquids and a trillion [more complex] ionic liquid systems, there is a phenomenally wide choice for optimising the properties of the liquid substrate, to minimise [freezing] point and volatility, while maintaining optimal infrared reflectivity," wrote Borra's team.
Critics of lunar LMTs, however, point out that these telescopes have a disadvantage, in that they cannot be shifted from their horizontal axis, which thus blinkers them to a view of just a few degrees.
That is frustrating for astronomers who like to track an object across the heavens. For cosmologists, who peer into the distant, still Universe, this is not a problem, though.
A small lunar LMT could be deployed automatically, unfolding on the Moon's surface like an umbrella.
But everyone agrees a large-scale version would require money – lots of it – as well as human hands to assemble it.
That means waiting for Man to return to the Moon, a prospect that lies at the end of the next decade, if not longer.
with Agençe France-Presse
