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
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
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
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
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.