A liquid mirror telescope (LMT) is a telescope that consists of a spinning horizontal disk containing a reflective liquid, typically mercury. This may seem like a needlessly complex method of building a telescope, but it has certain distinct advantages.

Traditional telescopes use a system of lenses and/or mirrors to magnify objects visible in the sky. Light enters the telescope as parallel rays that are focused onto one single spot. It is obvious that larger diameter telescopes are able to capture more light, and are thus able to detect fainter features in the sky.

Lenses can be used to converge the light to the focal point. However, glass is never fully transparent; a part of the light gets absorbed by the lens, and is lost in the form of heat. Furthermore, the lens system becomes heavy, expensive, and very difficult to manufacture for larger telescope sizes.

A reflecting telescope uses a parabolic mirror to focus the light. The parabolic shape is important, because it ensures that incoming light will always be reflected towards the focus of the parabola, independently of where the light hits the mirror.

Reflecting telescopes have been used since its invention by Isaac Newton. The size of the reflecting mirror ranges from 15 cm to 1 m for amateur astronomers. Professional telescopes use 4 to 8 m parabolic mirrors. However, at such large sizes these mirrors become very expensive to manufacture: an 8 m reflecting telescope costs tens of millions of dollars to build for the mirror alone.

There are several problems with building very large parabolic mirrors. First of all, a perfect parabolic shape must be attained to focus the light. This is difficult to achieve, because the mirror will be deformed by its own weight. Also, small changes in temperature can alter the shape during operation. Next, surface roughness becomes an issue; the reflecting surface needs to be as smooth as possible, otherwise part of the light is not reflected towards the focus. Finally, over time the mirror's reflective surface becomes obscured by dust or oxidation. Aluminized glass mirrors are difficult to clean, because it ultimately affects their parabolic shape.

The difficulties with making parabolic mirrors are known since their first application in reflecting telescopes by Isaac Newton. However in 1982, Professor Ermanno Borra of the Laval University published a paper that would overcome some of the limitations of conventional parabolic mirrors. His idea was to use a spinning liquid to create an almost perfect parabolic mirror for use in astronomy.

Take a large bowl, filled with a liquid. Spin the bowl around its center point, and due to centrifugal forces the surface of the liquid will form a perfect parabola. By using a reflective liquid such as mercury, very large parabolic mirrors can be generated this way.

The following diagram is a typical setup for a Liquid Mirror Telescope. At the heart of the LMT is a large pan that contains mercury. Mercury is a liquid metal at room temperature. The inside of the bowl is parabolic, to minimize the amount of mercury that is required. Approximately 28 L of mercury is needed to ensure a proper parabolic mirror of 6 m. The mirror is supported on an air bearing, to minimize vibrations. A motor drives spins the mirror at around 0.12 revolutions per second (one full revolution every 8.5 seconds). The reflected signal goes through a set of optical correction systems and enters a CCD Detector.


                   ------  CCD Detector
                   |    |
                   |    | Alignment system
                    -  -  Focusing System
                     ||
                    /  \
                   /    \
                  /      \
                 /        \
                /          \
               /            \
              /              \
             /                \
            /                  \ Tripod
           /                    \
          /                      \
         /                        \
        /                          \
       /       Mercury Mirror       \
      /  |---___            ___---|  \
     /   |      ---______---      |   \
    /     \______________________/     \
   |               |    | Air Bearing  |
   |              /      \             |
   |             |        | Motor      |
   |             |        |            |
---------------------------------------------
/////////////////////////////////////////////

The parabolic shape of the mirror is excellent. The surface roughness for these liquid mercury mirrors is approximately 1/20,000,000,000 (one in twenty billion). To put that in perspective: if the mirror was the size of the earth, the largest bumps on the surface would be 0.3 mm (0.01") high.

As mentioned before, the price is a major consideration for these large telescopes. A typical 6 m telescope would cost on the order of 100 million dollar. A LMT of the same size would cost only 1 million dollar.

The major drawback of LMTs is that the mirror can only point straight up. However, using overlaps of electronic images, and the rotation of the earth, this telescope can be used to map a large portion of the sky. It is often used to monitor space debris, but also stars and galaxies.

Older types of LMTs used to have a mylar cover to prevent escape of toxic mercury fumes. However, researchers of Laval University have discovered that the mercury forms a small mercury oxide layer on the surface that prevents further evaporation. Around the mirror, the mercury concentrations are too low to form a hazard. Researchers are also studying gallium and gallium alloys for use as mirrors, because this element does not have such severe toxicity issues as mercury.


sources:

http://wood.phy.ulaval.ca/home.html
http://www.sciam.com/specialissues/1299engineering/1299musserbox6.html
http://www.abc.net.au/science/k2/moments/s64751.htm
http://www.space.com/science
astronomy/astronomy/liquid_mirror_000924.html
http://www.astro.ubc.ca/LMT/

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