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The Most Rigid Telescope

June 17, 2002

The scientists from NPO Astrofizika, have designed a terrestrial telescope, which has no match all over the world. Fundamentally new technical solutions ensure that a unique telescope with the mirror of 25 meters in diameter is able to investigate previously invisible celestial objects of up to the 29-th magnitude.
        
What makes astronomers design the telescopes with the larger and larger mirrors? Certainly, astronomers are driven by the capacity of a telescope to provide more information about the Universe. The larger the mirror is, the larger amount of light from one source the telescope can catch, thus enabling the scientists to descry and investigate remoter or smaller objects. At present there are telescopes available with the main mirror of 8 and even 10 meters in diameter. For instance, in Russia the largest is the Zelenchuk telescope with a mirror of 6 meters in diameter. The Americans have installed the telescope in Hawaii and the Europeans - in Chile, the mirrors being 10 meters in diameter, but the astronomers have almost exhausted capacities of these telescopes. Now the astronomers are eager to use a telescope with a larger mirror - as big as of 100 meters in diameter.
        
However, large mirrors entail significant, sometimes insoluble problems. Such enormous mirrors are difficult to manufacture, install and maintain. Even minor deviations from the standard lead to tremendous distortions and consequently errors. That is why, before starting the development of the super-telescope, the Moscow astrophysicists have analysed the sources of possible errors (they have calculated the budget of errors, as they put it) and have come to the conclusion that it is unreasonable to manufacture a terrestrial telescope with the mirror of more than 25 meters in diameter, as the inevitable distortions will not allow astronomers to obtain more information.
        
However, a huge mirror is difficult to manufacture. It should not be too heavy, it should be strongly fastened and reliably protected from vibrations - a gust or an earthquake tremor, or even the auxiliary devices operation can loosen the dish which is as big as a playground. And finally, the mirror should be easy to operate, i.e. to turn it in the required direction.
        
To solve these and multiple other problems, the scientists applied several technical ideas at a time, having had previously patented them. First, they have decided to make a composite mirror, instead of a monolithic one - it will consist of individual controllable mirrors of 1-meter in diameter, the shape of mirrors being that of regular hexagons. The 10-meter mirror of the existing Keck-telescope (USA) was designed in the same way. On top of that, at each moment the mirror will automatically take the shape of the surface, which ensures the high image quality (i.e., adaptive mirror). The Russian scientists have thought out to make even these small mirrors non-monolithic. To enlighten the entire structure and to facilitate the manufacturing, the designers have suggested that these mirrors should be similar to a sandwich consisting of two thin layers, rigidly fastened between themselves by a open-work metal construction. According to the scientists, the remaining free space may be filled in by a light heat-insulating material.
        
The major thing the scientists have suggested is to reject the traditional form of a telescope, the so-called tube, in which the mirror is normally fixed on the support, an individual dome protecting the tube from the environmental impact. The Russian astronomers` design has made the body fulfil all these functions (support, placement of optical elements and protection). The body is a hollow sphere of 50 meters in diameter with an aperture slightly exceeding the mirror size. The body presents a two-layer truss shell. Its immovable lower part carries the mobile upper part with the main mirror fastened in it. Between them the so-called lodgement is placed, which easily, precisely and accurately turns the upper part with the fastened mirror.
        
As a result, the designers have managed to ensure unprecedented solidity, reliability and rigidity of the entire structure. In addition, such a design allows the scientists to protect the telescope from vibrations - the whole construction is non-resonant. And finally, the entire telescope turns out to be light (the weight of the mobile part being 800 tons) and inexpensive (for its class, of course), its cost making only USD 99 million.
        "In principle, our AST-25 telescope is currently the most rigid, simple, inexpensive and reliable telescope in the world among similar large telescopes already being operated or under development", says Professor Sychev, one of the project designers.

Informnauka (Informscience) Agency