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Primary mirror BTA-6: history and modern times (restoration works of 6m giant is continued) Print

The history of Large Altazimuth Telescope (BTA-6, Karachai-Cherkess) began on the 25th of March, 1960, when at the suggestion of Soviet Academy of Sciences and Государственный комитет по оборонной технике, USSR council of ministers passed a decision to create a complex with reflecting telescope the primary mirror of which would be 6 meter in diameter. His function is “to examine the structure, physical nature and the evolution of extragalactic objects; to study physical features and chemical composition of non-stationary and magnetic stars in detail”. The main executors were Государственный оптико-механический завод (ГОМЗ) им. ОГПУ ( later, on its base, was organized LOMO) and the main constructor - Bagrat Konstantinovich Ioannisiani.

BTA-6 was the newest space machinery for its time. It was the machinery that contained many revolutionary decisions. Since that time the assembly of all large-size world telescopes are accomplished according to altazimuth scheme, which was used firstly in world practice by our scientists in BTA-6 and brilliantly proved its value. Professionals labored over its creation and that ensured the high quality of this gigantic telescope.

BTA–6 is watching the stars for more than 30 years already. This telescope is able to discern space objects of 27size. Imagine, that the Earth were flat: than, if someone would light a cigarette in Japan, it could be easily seen in this telescope. The telescope’s resolving power is in 2000 times more than the resolution ability of a human eye; its “eyesight radius” is in 1, 5 times broader than analogous index of the biggest telescope of that time ( the USA telescope in Mount Palomar ) – 8-9 billion of light years vs. 5-6 accordingly. BTA-6 is called “eye of the planet” not by chance. Its sizes are striking: its height is 42 meters and its weight is 850 tons. Due to the special construction of hydraulic mountings the telescope is kind of “swimming” on the thinnest oil “pillow” (thickness – 0,1 mm). Human being can turn it around without any machinery or additional instruments.

On the 25th of March, 1960 the USSR government confirmed Lytkarino optical glass factory as the main executor of technological process’s development for casting from glass the mirror’s 6m workpiece and of mirror’s workpiece manufacture. Specially for this project two new manufacturing blocks were built. We had to cast a 70 tons workpiece, to anneal it and to turn all the surfaces and to manufactory 60 firment blind bores on its back side and the central bore, etc. The pilot-plant workshop was created since 3 years from the moment of government’s decision. The workshop’s tasks were as follows: the equipment’s mounting and adjusting, industrial technical process development and mirror’s workpiece production. The researches of optimal processing conditions, made by LZOS specialists, allow to create and to realize the technology of the primary mirror’s workpiece production. The workpiece processing run for nearly 1,5 years. For making it, Коломенский завод тяжелого станкостроения made special turning mill КУ-158 in 1963. The big scientific work was spent on technology and control of this unique mirror. The mirror was ready to certification in June 1974. It passed all the tests. The mirror’s transportation began in June 1974. On the 30th of December, 1975, the State Inter-Department Commission signed a document on acceptance and exploitation of Large Altazimuth Telescope. During its 30-year exploitation period, the telescope’s mirror was re-covered several times, what led to the surface substantial damage, its corrosion. As the result, almost 70% of mirror’s reflectance was lost. But BTA-6 was and stay the unique instrument for astronomers, both Russian and foreign ones. To save its functionality and to increase its effectiveness, there is the necessity for reconstruction and replenishment of the primary mirror. Presently, the form-building technology and technology of mirror’s unloading, known by LZOS specialists, allow to improve its optical features, including its angular resolution, in 3 times.

Today the technology of form-building of space optical details surface is on the new level on LZOS. The quality of surface form deviation from theoretical has increased considerably due to the production automation and modernization and also computer controls. Both the mechanical base and the technology of mirror’s weight reduction and unloading with the use of computer equipment have substantially improved. Machines for 6m mirror’s milling, grinding and polishing are also modernized in accordance with modern requirements. The tools for optics control are also improved.

The primary mirror is delivered on LZOS. The milling stage is finished. The 8mm-upper layer is removed from mirror working area. The mirror is transported in thermostabilized block and is established on the machine for working area grinding and polishing. According to main enterprise’s engineer, Mr S.P.Belousov, it will be the most difficult and the most important stage of mirror’s processing. It’s necessary to get the surface form which will have smaller departure from the ideal paraboloid form than of those that was achieved in 1970s. After that the mirror with improved resolving power and improved transparent power will be able to serve both Russian and world science for no less than 30 years. Those who took part on mirror’s production – mechanic Zhikharev A.G., optician Kaverin M.S., locksmith Panov V.G., milling machine operator Pisarenko N.I.. They work and now, sharing their knowledge of large-size optics with the youngsters. Optician Bochmanov Yu.K. and milling machine operator Yegorov E.V. have recently retired. The letter made repeated milling of mirror last year and this year.

The same work can be done nowhere in Russia. It the world, except for LZOS, there are only two firms that make large-size mirrors. There are the Steward Observatory Mirror Laboratory (Arizona, USA) and SAGEM-REOSC (France). But the towers for mirrors control are shorter than needed even there, because BTA-6 mirror’s radius is 48 meters wide.

A.P. Semenov
The head of НПК-95 design-engineering office

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