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. Mirror, Mirror On The Moon -- The Most Powerful Telescope Of All

So far, several pilot schemes using liquid mirror telescopes (LMTs) up to six metres (20 feet) across have been launched on Earth, and backers say their cost is just one or two percent of large conventional scopes.
by Richard Ingham
Paris (AFP) June 20, 2007
Desolate, airless and with no people around for hundreds of thousands of kilometers (miles), the Moon is a great place -- for astronomers, that is. 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.

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 (20 feet) across have been launched on Earth, and backers say their cost is just one or two percent of large conventional scopes.

According to their calculations, an LMT on the Moon, with an aperture of 20 to 100 metres (65-325 feet), 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.

Now a team of US pioneers believe they have made strides towards resolving the big challenge facing a lunar LMT -- finding a substitute for mercury, which would freeze on an unlit surface of the Moon.

A team led by Ermanno Borra, a physicist at the Universite Laval in Quebec, Canada, 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 -98 C (-144 F), which is still too high for lunar temperatures as low as -130 C (-202 F).

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.

"As there are at least a million simple ionic liquids and a trillion ternary [triple-ingredient] ionic liquid systems, there is a phenomenally wide choice for optimising the properties of the liquid substate, to minimise [freezing] point and volatility, while maintaining optimal infrared reflectivity," Borra's group report on Thursday in Nature.

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 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.

earlier related report
Another Step Toward A Liquid Telescope On The Moon
Quebec City, June 20 - An international team including researcher Ermanno Borra, from Universite Laval's Center for Optics, Photonics, and Laser, has taken another step toward building a liquid telescope on the moon. The researchers have found a combination of materials that allows the creation of a highly reflective liquid mirror capable of functioning even under harsh lunar conditions. The details of the discovery made by Borra and his colleagues will be published in the June 21 edition of Nature.

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.

In a 1991 paper published in the Astrophysical Journal, Professor Borra had suggested the building of a liquid telescope on the moon. In that paper , Borra demonstrated the practical and economic advantages of liquid mirror telescopes over their conventional counterparts and explained 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, an organization which funds projects that can potentially push back the limits of science and space technology.

The project's main challenge consisted in 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 degrees Celsius.

In their Nature article, the researchers explain how they successfully coated an ionic liquid with silver by vaporizing it in a vacuum, something never achieved before in the field of optics. 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 1,000 times more sensitive than the proposed next generation of space telescopes.

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

Source: Agence France-Presse

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Related Links
Center for Optics, Photonics, and Laser
Universite Laval
Space Telescope News and Technology at Skynightly.com




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Laser Guide Star System On ESO VLT Starts Regular Science Operations
Garching, Germany (ESO) Jun 20, 2007
An artificial, laser-fed star now shines regularly over the sky of Paranal, home of ESO's Very Large Telescope, one of the world's most advanced large ground-based telescopes. This system provides assistance for the adaptive optics instruments on the VLT and so allows astronomers to obtain images free from the blurring effect of the atmosphere, regardless of the brightness and the location on the sky of the observed target.

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