Liquid mirror telescope for the Moon
A powerful telescope built on the Moon is one step closer, say scientists, whose work could be used to develop a liquid mirror that would not break during the journey from Earth.
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 dozens of telescopic mirrors are needed in a giant complex and making them is eye-wateringly expensive, as 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.
The idea is that 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 percent of large conventional telescopes.
According to their calculations, an LMT on the Moon, with an aperture of 20 to 100 metres, would be able to observe objects 100 to 1,000 times fainter than the US James Webb Space Telescope, a 3.5-billion-dollar next-generation orbiting telescope scheduled for launch in 2013.
Finding a substitute for mercury
One of the biggest challenges facing a lunar LMT is finding a substitute for mercury, which would freeze on an unlit surface of the Moon.
And this is what Borra and team believe they have found.
They used vacuum vaporisation to apply liquid chromium to a water-repelling, commercially available solvent called ECOENG 212.
They then applied liquid silver on top of the chromium, delivering a result with "excellent" optical quality and which remained stable throughout months of study.
The outcome is not the Holy Grail, because the reflectivity is still not up to scratch and the solvent freezes at -98ºC, which is still too high for lunar temperatures as low as -130ºC.
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.
Borra and team suspect one of these will be useful in building an LMT.
Limitations of LMTs
Critics of lunar LMTs 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 lots of money 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.