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Lunar Telescope to Have Liquid Mirror

Photonics Spectra
Aug 2007
Device at the moon’s pole will be 1000 times as powerful as the James Webb Space Telescope.

Hank Hogan

Thanks to out-of-this-world thinking and down-to-earth materials research, scientists — from Université Laval in Quebec City, from the University of Arizona in Tucson, from the University of British Columbia in Vancouver, Canada, and from Queen’s University in Belfast, UK, and from NASA Ames Research Center in Moffett Field, Calif. — have taken the first steps needed to create a telescope with a liquid mirror that will operate on the moon. The result could be a 100-m-aperture infrared telescope at one of the moon’s poles. The instrument would observe objects 1000 times fainter than is possible with the yet-to-be-launched James Webb Space Telescope (see “Building a Telescope to Supplant Hubble,” page 46).

TWTelescope_Fig1_IMG_0052.jpg

A liquid mirror composed of an ionic liquid coated with colloidal silver could form the basis of a lunar telescope. Shown is a 2-in.-diameter flat liquid mirror sample reflecting a resolution chart. Courtesy of Omar Seddiki, Université Laval.


The lunar liquid-mirror telescope would have only a small field of view of the sky at the zenith. However, that would not be a drawback for astronomers seeking deep-space images of distant objects.

In a liquid-mirror telescope, a reflective liquid sits in a spinning container. Under the prodding of gravity, inertia and friction, the liquid assumes an ideal light-collecting shape. Large terrestrial liquid-mirror telescopes have been built using mercury.

TWTelescope_Fig2_Sky6-m.jpg
This image of stars and galaxies was acquired with a 6-m liquid-mirror telescope at the University of British Columbia in Vancouver. Courtesy of Paul Hickson.


Building one on the moon, however, offers some advantages to astronomers. The moon rotates slowly, and locating the telescope at one of the lunar poles would reduce object-blurring movement to almost zero. The moon also has no air currents to disturb the mirror’s surface.

There is a challenge, however, because the liquid must be reflective and viscous and must maintain its qualities at low vapor pressure and at low temperature. After studying various possibilities, the researchers concluded that an ionic liquid had the best chance of meeting these requirements.

TWTelescope_Fig3_DifferentS.jpg
Shown here is an artist’s rendering of a possible 20-m lunar liquid-mirror telescope. The instrument is shielded from the sun, with scientific instruments below the bearing pier. Drawn by Tom Connors,University of Arizona.


Such a liquid, as the name suggests, is composed of discrete ions rather than molecules. Ionic liquids already exist with the proper vapor-pressure profile, but they lack a reflective surface. With some materials research and engineering, the investigators demonstrated that they could coat the liquid with chromium and then with silver to achieve infrared reflectivities of 80 percent or more. That figure must be higher, and the group is working on that.

The liquid used — 1-ethyl-3-methylimidazolium ethylsulfate — solidifies at 175 K, about 45 K above the temperature needed for infrared astronomy. However, researcher Ermanno F. Borra of the Université Laval’s Center for Optics, Photonics and Lasers believes that a liquid exists that will be up to the task.

“It should be possible to find an ionic liquid that will do the job, because there are on the order of a trillion possible ionic liquids we can synthesize.”

Nature, June 21, 2007, pp. 979-981.


GLOSSARY
infrared telescope
A telescope that transmits, enlarges and converts infrared images.
liquid mirror
A mirror composed of liquid, taking advantage of the parabolic shape of a spinning liquid and the fact that the mirror's focal length can be adjusted by altering the velocity at which the liquid's container spins.
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