How Lunar Liquid Mirror Telescopes Work

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 largest LMT on Earth is the Large Zenith Telescope in British Columbia. Its spinning liquid mirror is almost 20 feet across and weighs three tons, making it the third-largest telescope in North America. The dish that holds the mercury is fabricated from hexagonal segments glued together to form a shell. Each piece has a high-density foam core covered with fiberglass. To give the shell a concave shape, it is heated in a large oven. A wall at the rim of the mirror prevents mercury from spilling.

A steel truss and 19 adjustable pads support the dish. The truss, in turn, is supported by a stainless-steel air bearing designed just for the Large Zenith Telescope. An air bearing is a special type of bearing that uses a thin film of pressurized air as the lubricant around the shaft that turns the mirror. Normal bearings that use oil lubricants are less effective, because they produce vibrations and unstable rotations that degrade image quality. As a zero-friction solution, an air bearing eliminates these problems, leading to a perfectly smooth, vibration-free rotation. A built-in brushless DC motor turns the air bearing spindle and can rotate a load up to 10 tons at approximately 10 revolutions per minute.

­Six support legs attach the primary mirror to a ring at the top of the telescope. The ring supports the detector and a smaller refracting lens that helps focus the image. The detector includes a charge-coupled device (CCD), which gathers photons of light and converts them into picture elements, or pixels. These pixels are transferred to a computer screen and pieced together to form an image that can be manipulated and enhanced to improve the image detail. The computer is not housed in the telescope's observatory structure, but in a nearby building.

The one problem with the Large Zenith Telescope -- a problem it shares with all earthbound telescopes -- is its location. Even at an altitude of 1,295 feet, the atmosphere still shields its view of the heavens. If a liquid mirror telescope could be placed on the moon, where there is no atmosphere to block ultraviolet, infrared and other forms of energy, it could provide even more spectacular results. But, as we'll see in the next section, building an LMT on the moon presents its own challenges.­

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:

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.­

A liquid mirror telescope placed on the moon instantly has a major advantage over an earthbound telescope: It's free from atmospheric distortion, which affects celestial images. For the same reason, it's also able to detect more forms of electromagnetic energy. Most types of electromagnetic radiation, except for visible light and radio waves, are absorbed by the Earth's atmosphere. On the moon, which has no atmosphere at all, a telescope would be exposed to the full spectrum of electromagnetic radiation -- gamma rays, X-rays, ultraviolet light, visible light, infrared radiation, microwaves and radio waves.

A telescope using an ionic liquid as its primary mirror would be particularly sensitive to visible light and infrared radiation. This would be important for observing the universe's most distant objects, which are moving rapidly away from the Earth. The Doppler effect causes them to create radiation in the longer-wavelength, infrared portion of the spectrum.

Size is also a key factor. In the low-gravity environment of the moon, it's much easier to build big structures. The team designing the LLMT believes it can build a primary liquid mirror that is 66 feet to 328 feet wide. Such a mirror would be able to observe objects 100 to 1,000 times fainter than the next generation of telescopes -- including the James Webb Space Telescope -- are able to. That means astronomers could use the instrument to peer deeper into space and time than ever before. For the first time, we could be able to detect the very early phases of the universe right after the Big Bang, expanding our understanding of how the newly formed universe behaved.

When Might a Lunar Liquid Mirror Telescope Become a Reality?

Right now, the LLMT is still a concept. The project has received funding from the NASA Institute for Advanced Concepts for a study to show how a telescope on the moon might support astronomy. This is important because the moon is the first target in the Vision for Space Exploration, an initiative seeking how to go out beyond Earth's orbit for purposes of human exploration and scientific discovery. If NASA can demonstrate that lunar outposts would be practical, with both economic and scientific value, then the public -- and ultimately Congress --­ might be willing to show appropriate financial support.

A lunar liquid mirror telescope is among several projects that will help NASA prove the feasibility of space exploration. Even still, the earliest it could be deployed is 2020. Until then, astronomers will have to be satisfied with liquid mirror telescopes, such as the Large Zenith Telescope, that view the heavens from Earth.

To find out more about lunar liquid mirror telescopes, check out the links on the next page.

­ ­­