Liquid mirror

Liquid mirrors are mirrors made with reflective liquids. The most common liquid used is mercury, but other liquids will work as well (for example, low melting alloys of gallium). Liquid mirrors can be a low cost alternative to conventional large telescopes.

Isaac Newton noted that the free surface of a rotating liquid forms a circular paraboloid and can therefore be used as a telescope, but he could not actually build one because he had no way to stabilize the speed of rotation (the electric motor did not exist yet). The concept was further developed by Ernesto Capocci of the Naples Observatory (1850), but it was not until 1872 that Henry Skey of the Dunedin Observatory in New Zealand constructed the first working laboratory LMT.

Conventional land-based liquid mirror telescopes

These are made of liquid stored in a cylindrical container made of a composite material, such as Kevlar. The cylinder is spun until it reaches a few revolutions per minute. The liquid gradually forms a paraboloid, the shape of a conventional telescopic mirror. The mirror's surface is very precise and small imperfections in the cylinder's shape do not affect it. The amount of mercury used is small, less than a millimeter in thickness.

Moon-based liquid mirror telescopes

Low temperature ionic liquids (below 130 kelvins) have been proposed [cite journal|url=http://www.nature.com/nature/journal/v447/n7147/abs/nature05909.html|journal=Nature|issue=447|pages=979-981|date=21 June 2007|doi=10.1038/nature05909|title=Deposition of metal films on an ionic liquid as a basis for a lunar telescope|first=Ermanno F.|last=Borra|coauthors=et.al.|accessdate=2008-10-11] as the fluid base for an extremely large diameter Liquid mirror spinning liquid mirror telescope to be based on the earth's moon. Low temperature is advantageous in imaging long wave infrared light which is the form of light (extremely red-shifted) that arrives from the most distant parts of the visible universe. Such a liquid base would be covered by a thin metallic film that forms the reflective surface.

Space-based ring liquid mirror telescopes

The Rice liquid mirror telescope design is similar to conventional liquid mirror telescopes. However, it will only work in space, where gravity will not distort the mirror's shape into a parabola. The design features a liquid stored in a flat-bottomed ring-shaped container with raised interior edges. The central focal area would be rectangular, but a secondary rectangular-parabolic mirror would gather the light to a focal point. Otherwise the optics are similar to other optical telescopes. The light gathering power of a Rice telescope is equivalent to approximately the width times the diameter of the ring, minus a percentage based on optics, superstructure design, etc.

Advantages and disadvantages

The greatest advantage of a liquid mirror telescopic is its small cost, about 1% of a conventional mirror. This cuts down the cost of the entire telescope at least 95%. The University of British Columbia's 6 meter Large Zenith Telescope cost about a fiftieth as much as a conventional telescope with a glass mirror. [cite web|archiveurl=http://web.archive.org/web/20030818233315/http://www.govertschilling.nl/artikelen/science/030314_sc.htm|url=http://www.govertschilling.nl/artikelen/science/030314_sc.htm|title=Liquid-mirror telescope set to give stargazing a new spin|date=2003-03-14|archivedate=2003-08-18|publisher=Govert Schilling|accessdate=2008-10-11] Unfortunately, the mirror can only be pointed straight up. If it tilts from the zenith, it loses its shape. The mirror's view changes as the Earth rotates and objects cannot be physically tracked. An object can be briefly electronically tracked while in the field of view by applying a voltage to the CCD to shift electrons across it at the same speed as the image moves; this tactic is called "drift scanning." However, some types of astronomical research are unaffected by these limitations, such as long-term sky surveys and supernova searches. Since the Universe is believed to be isotropic and homogeneous (this is called the Cosmological Principle), the investigation of its structure by cosmologists can also use liquid mirror telescopes. However, research is underway to develop telescopes that can be tilted.

References

* [http://www.economist.com/displayStory.cfm?Story_ID=432060 The Economist - Mirror, Mirror]
* [http://vela.astro.ulg.ac.be/themes/telins/lmt/didac_e.html The 4m International Liquid Mirror Telescope Project]
* [http://www.astro.ubc.ca/LMT/lzt/ The Large Zenith Telescope]
* [http://wood.phy.ulaval.ca/liquid_mirrors/gallium/gallium.php Gallium Liquid Mirror]
* cite journal | author=Gibson, B. K. | year=1991 | title=PDF| [http://citeseer.ist.psu.edu/cache/papers/cs/2683/http:zSzzSzwww-astro.physics.ox.ac.ukzSz~bkgzSzLMT_history.pdf/liquid-mirror-telescopes-history.pdf Liquid mirror telescopes: history] |138 KiB | journal=Journal of the Royal Astronomical Society of Canada | volume=85(4) | pages=158–171
*cite web|url=http://www.americanscientist.org/issues/feature/2007/3/liquid-mirror-telescopes/1 |title= An old idea for astronomical imaging is undergoing a technology-driven renaissance |accessdate=2007-04-23 |last=Hickson |first=Paul |date=May-June 2007 |format=PDF |work=American Scientist