Telescopes with liquid mirrors may revolutionize astronomy
Economist
A telescope with a liquid mirror? Sounds about as useful as a chocolate teapot. The solid giant mirrors, which capture and focus the light into large telescopes, weigh several tons, and require years of painstaking effort to cast, shape, and polish their surfaces to a precision of several billionths of a meter. So how could a pool of molten mercury do the same job? The answer is surprising. Pour any liquid into a cylindrical container, rotate the container at a constant speed, and the surface of the liquid will become a paraboloid (a surface with the cross section of a parabola), which just so happens to be the perfect shape for telescope mirrors.
The idea is not new. In fact, it may date back to the days of Newton, who was also one of the fathers of the reflecting telescope. But liquid mirror telescopes (LMTs) were for a long time considered a purely theoretical matter. Only in the last decade did the idea seem more and more attractive, for both economic and technical reasons. Several such telescopes have been built in the 90s, and the largest yet built, with a 6-meter diameter mirror, will soon capture its first rays of light in a forest east of Vancouver, Canada. Proponents of these telescopes believe that the technology has now reached maturity and that it could even spell the end of conventional telescopes.
Compared to making a solid mirror, making a liquid mirror is incredibly easy. The main ingredient is a small amount of mercury, which is placed in a strong but light container of composite material. Of all this it is a paraboloid, but it is not necessary to shape its shape with the degree of precision required for a solid appearance. As soon as the container rotates around its axis at the right speed (usually a few revolutions per minute) the mercury spreads and creates a layer that is less than a millimeter thick; The differences in the thickness of the mercury layer compensate for the imperfect shape of the container. The result is a liquid mirror whose shape is designed with the same level of precision as a conventional solid mirror, but the cost is about one hundredth the cost of a normal mirror.
A telescope is more than its appearance, of course. But when the other components are taken into account, still the cost of a telescope with a liquid mirror (Taman) only reaches about 5% of the price of a conventional telescope. The "Large Zenith Telescope", the telescope currently being built by Paul Hickson and his team at the University of British Columbia in Vancouver, was built for a pittance - it is expected to cost less than a million dollars. For comparison, the construction cost of the two "Gemini" telescopes in Hawaii is $184 million, and the "Astronomical Research Southern" telescope being built in Brazil will cost $28 million.
The ability to build large telescopes at such a low cost, relative to the normal price, will surely change the face of astronomy. This way, small research groups will be able to use their own telescopes, and they won't have to book viewing time at shared telescopes several months in advance, as they do now. Exclusive use of a single-purpose telescope will also make new types of research possible. At the moment it is impossible to spend all the viewing time of a telescope with a six or eight meter diameter looking for exploding stars, called supernovae, in other galaxies. This is a shame, because in an ideal world studies of this type could include repeated observations of the same galaxies every few days. With Teman, the use of a large telescope for this type of focused research will become possible.
But to her and a thorn in her side: you can aim Taman only directly up; Tilting the rotating container causes the mirror to lose its shape. Thus, such a telescope cannot be aimed at a specific point in the sky, and even when a celestial object of interest is directly overhead, it cannot be tracked by moving the telescope to compensate for the Earth's rotation.
However, this limitation is not as problematic as it sounds, at least when it comes to certain types of astronomy. Cosmologists, who study the structure of the universe, don't care where the telescope is pointed, because the universe is isotropic and homogeneous - a sophisticated way of saying that it looks the same in all directions. Similarly, long-term observing missions can be carried out with a zenith-oriented telescope. Thus, for a whole year the telescope can observe an entire strip of the sky.
One of the leading followers of the Taman is Dr. Armano Bora, a physicist at the University of Laval in Quebec. Bora's team was the first to create a mercury layer less than a millimeter thick. Bora also performed a series of key experiments in the 90s, showing that taste buds do work. His research motivated NASA scientists to build such a telescope, with a diameter of three meters. This telescope is now tracking "space junk" (remnants of satellites and the like): another application where the ability to point the telescope in a certain direction does not mean much.
Dr. Bora is particularly enthusiastic about the prospect of devoting a large space to the search for supernovae, because these are used by astronomers as rulers for measuring intergalactic distances. The discovery from last year, that the rate of expansion of the universe seems to be increasing, was based on observations of several dozen supernovae. A liquid mirror telescope should be able to find thousands of supernovae a year, and confirm or disprove this discovery.
In addition to spreading the news about the advantages of the taman, Bora is doing his best to overcome his great shortcoming: the inability to look in different directions. The problem today is that when the liquid mirror is tilted, it distorts. The solution is to increase the viscosity of the liquid. Bora calculated exactly how viscous the liquid should be, and found that it should have the texture of thick honey. The problem is that mercury is much less viscous than honey, and honey does not reflect light, so it cannot be used. Thus began the search for a liquid that is both viscous and reflects light effectively.
Dr. Bora came across a group of suitable materials. They are called liquid-like metallic membranes - tiny particles of silver, which are coated with organic molecules to coat them and then added to silicone oil. The result is a thin reflective layer floating on the surface of the oil. So far, Dr. Bora has achieved a reflection of 50% (that is, half of the light falling on the mirror is reflected), but he is confident that he will be able to match the 80% reflection of mercury. Bora believes that if he manages to build an observatory with a diameter of four meters that can observe at an angle of at least 30 degrees, it will be the end of the classic telescope.
Economist
{Appeared in Haaretz newspaper, 28/11/2000}
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