less.. Since it was repaired in 1993, NASA's Hubble Space Telescope has amazed scientists and citizens alike with its views of the universe, including glimpses of the farthest known galaxies. The mirror in Hubble, however, is relatively small at 94.5 inches (almost 8 feet) across, a limitation that has encouraged NASA to think bigger. The James Webb Space Telescope, slated for a 2013 launch, will boast a 20-foot mirror capable of providing seven times the light-collecting area of Hubble.

But NASA is also considering a more intriguing solution -- a special type of reflecting telescope that uses a liquid, not glass, as the primary mirror. Known as a liquid mirror telescope (LMT), it wouldn't view space from Earth's orbit, as Hubble does. Instead, it would peer into the universe from the moon's surface. The telescope would be anywhere from 66 feet to 328 feet wide, making it the largest telescope known to man. It would collect 1,736 times more light than Hubble and penetrate the depths of the universe to see objects nearly as old as the Big Bang.

This article will explain how a liquid mirror telescope works. It will look at the structure and function of an LMT, but it will do so in light of a moon-based deployment. How in the world does one build a telescope on the moon? How difficult is it going to be to build an LMT on the moon? And m­ost importantly, what opportunities can a lunar telescope provide?

What is a Liquid Mirror Telescope?

In principle, an LMT is no different from a normal reflecting telescope. Check How Telescopes Work for a thorough explanation of telescopes. Here's a quick recap.

A reflecting telescope uses mirrors to view distant objects. A primary mirror gathers light from the object, while a secondary mirror focuses the image to the eyepiece. In a conventional reflector, the primary mirror is made by painstakingly grinding and polishing glass to its desired shape, usually a parabola. Once the glass is prepared, a process known as aluminizing makes it reflective. Aluminizing involves vaporizing aluminum in a vacuum, causing a film of metal about 100 nanometers thick to be deposited on the glass. Flaws in the mirror production can affect how the telescope performs. This was the issue with Hubble: The curve in its primary mirror was off by just a fraction of a hair's width, which caused light to reflect away from the center of the mirror, leading to blurry images.

A liquid mirror telescope, as its name suggests, uses a liquid, not aluminized glass, as its primary mirror. The liquid, usually mercury, is poured into a rotating dish. The rotation creates two fundamental forces that act on the mercury -- gravity and inertia. Gravity pulls down on the liquid surface, while inertia pulls the liquid sideways at the edge of the dish. As a result, the liquid forms a uniform and perfect parabola, the ideal reflecting surface for a telescope. Best of all, the liquid mirror surface remains smooth and flawless with little or no maintenance. If the liquid is disturbed, gravity and inertia will act on the liquid to return it to its original state.

Ernesto Capocci, an Italian astronomer, was the first person to describe how an LMT might work in 1850. He conceived of the idea after reading about experiments, conducted by Isaac Newton and others, involving spinning liquids. In the early 20th century, the American physicist R.W. Wood actually built what Capocci had described 50 years earlier. Wood's LMT featured a one-centimeter layer of mercury placed in a rotating dish. He was able to observe the moon but noted that the image was distorted. Modern astronomers learned that the image quality of an LMT was greatly improved if a thinner layer of mercury was used, so today's LMTs use a one-millimeter layer of mercury.

The Advantages of Liquid Mirror Telescopes
The biggest advantage of an LMT is its relative low cost. Liquid telescopes cost much less to build than polished aluminum mirrors of similar size. For example, the Large Zenith Telescope carried a price tag of $1 million. A comparable glass mirror telescope would cost 100 times that much to build. And LMTs cost less to maintain, mainly because the liquid mirror doesn’t need to be cleaned, adjusted or aluminized.

Of course, there are some drawbacks. Mercury is extremely toxic, so working with it poses some long-term health risks. Not only that, the dish holding the mercury can only be tilted so far before the liquid spills out. This limits the view of an LMT, which can only look straight up.

What is a Lunar Liquid Mirror Telescope?

A liquid mirror telescope built on the surface of the moon is a lunar liquid mirror telescope (LLMT). It's really no different from the Large Zenith Telescope described in the last section, except that the liquid chosen must have just the right properties if it is to remain liquid in the moon's harsh climate. Mercury won't work because its freezing point is -101.966° F (-74.43° C). The low temperature on the moon can reach -243° F (-153° C), so mercury would solidify, making it an unacceptable choice for the primary mirror.

­Recently, scientists have discovered a class of liquids that might make an LLMT possible. They are known as ionic fluids, and they have these important properties:

• They are liquid at temperatures below -212° F (-136° C).
• They are composed entirely of ions.
• They possess no vapor pressure at room temperature or below, which means they won't evaporate.
• They are highly viscous.

Most importantly, ionic liquids can be coated with materials that give them high reflectivity. One ionic fluid showing promise is 1-ethyl-3-methyli-
midazolium ethylsulphate, commercially known as ECOENG 212. ECOENG 212 can be coated in silver, making it highly reflective. Its reflectivity can be improved even more by depositing a film of chromium first, followed by silver. ECOENG 212 has a freezing point of -144° F (-98° C), however, so it still could solidify in the moon's bitter-cold temperatures. Given that there are millions of ionic liquids, scientists feel confident that they will find another candidate with a better freezing-point profile.

They will also have to find another way to support the primary mirror. The air bearing used in the Large Zenith Telescope won't work on the moon because there's no air to feed the system. One solution would be a superconductor magnetic bearing. Such a bearing is based on the same technology used in maglev vehicles, which use a magnetic field to levitate a vehicle above a guideway. In this case, the magnetic field creates a zero-friction cushion between the spindle and its housing.

Of course, all of these materials will have to be shipped by rocket to the moon and assembled there. Even taking that into consideration, a liquid mirror telescope poses far fewer logistical problems than a conventional reflecting telescope made of glass. The mirror, because it's liquid, will simply be carried in a jug and stored until the telescope infrastructure is ready. Then an astronaut will pour the liquid into the dish to form the primary mirror. The truss system used to support the dish and mirror could be prebuilt and deployed robotically, its framework unfolding like an umbrella being opened. But using a robot to build an LMT on the moon would require that the instrument remain fairly small. As we'll see in the next section, the LMT envisioned by astronomers and NASA engineers is anything but small.