|Location||near Mount Pastukhov in the Caucasus Mountains, Russia|
|Wavelength||0.3 to 10 μM|
|Built||First light 1975|
|Collecting area||26 m²|
|Focal length||f/4 (26 m)|
|Mounting||alt-azimuth fully steerable primary|
|Website||Special Astrophysical Observatory|
The BTA-6 (Большой Телескоп Альт-азимутальный, Bolshoi Teleskop Alt-azimutalnyi, Eng:Large Altazimuth Telescope) is a 6 m aperture optical telescope at the Special Astrophysical Observatory located in the Zelenchuksky District on the north side of the Caucasus Mountains in southern Russia. The BTA-6 achieved first light in late 1975, using a computer-controlled alt-az configuration, supporting a single piece glass mirror.
For many years the primary world-class observatory in the Soviet Union was the Pulkovo Observatory outside Saint Petersburg, originally built in 1839. Like many observatories of its era, it was primarily dedicated to timekeeping, weather, navigation and similar practical tasks, with a secondary role for scientific research. Around its 50th anniversary a new 76 cm telescope, then the world's largest, was installed for deep space observation. Further upgrades were limited due to a variety of factors, while a number of much larger instruments were built around the world over the next few decades.
In the 1950s the Soviet Academy of Sciences decided to build a new telescope that would allow first-rate deep space observation. Design work started at Pulkovo in 1959 under the leadership of future Lenin Prize winner Bagrat K. Ioannisiani. With the goal of building the largest telescope in the world, a title long held by the 200 inch (5 m) Hale telescope at the Palomar Observatory, the team settled on a new design of 6 m (236 inches). This is about the maximum size a solid mirror can have without suffering from major distortion when tilted.
A telescope's theoretical angular resolution is defined by its aperture, which in the case of the BTA's 6 m leads to a resolution of about 0.021 arcseconds. Atmospheric effects overwhelm this, so it becomes important to locate high-resolution instruments at high altitudes in order to avoid as much of the atmosphere as possible. The Pulkovo site, at 75 m above sea level, was simply not suitable for a high-quality instrument. While BTA was being designed another instrument, the RATAN-600 radio telescope, was also designed. It was decided that the two instruments should be co-located, allowing the construction of a single site to house the crews. To select the site, sixteen expeditions were dispatched to various regions of the USSR, and the final selection was in the North Caucasus Mountains near Zelenchukskaya at a height of 2,070 m. In 1966 the Special Astrophysical Observatory was formed to host the BTA-6 and RATAN-600.
BTA's first images were obtained on the night of 28/29 December 1975. After a break-in period, BTA was declared fully operational in January 1977. Almost immediately after it opened, rumors started in the West that something was seriously wrong with the telescope. It was not long before many dismissed it as a white elephant, so much so that it was even mentioned in James Oberg's 1988 book Uncovering Soviet Disasters.
The original mirror had significant imperfections, attributed to the Russians' inexperience with large optics. These included cracks on the surface, which were covered with black cloth to hide their effects. According to Ioannisiani, the primary directed 61% of the incoming light into a 0.5 arcsecond circle and 91% into one with twice the diameter.
A second mirror, with an improved figure and no cracks, was installed in 1978. Although this improved the major problems, a number of unrelated issues continued to seriously degrade the overall performance of the telescope. In particular, the site is downwind of a number of other peaks in the Caucasus, so the site's astronomical seeing is not nearly as good as the primary sites like Mauna Kea, La Palma or Chile; observations with a resolution better than an arcsecond are rare, and 2 arcseconds is considered good. Under favourable conditions (little temperature difference between the main mirror, the air inside the dome and also that outside of it), the seeing is limited by the atmospheric turbulence, the width of the seeing disc (FWHM) being ~1 arcsecond for 20% of observational nights. Weather is another significant factor; on average observing takes place on fewer than half of the nights throughout the year.
Perhaps the most annoying problem is the huge thermal mass of the primary, the telescope as a whole, and the enormous dome. Thermal effects are so significant in the primary that it can tolerate only a 2 degree Celsius change per day and still retain a usable figure. If the temperatures of the primary and the outside air differ by even 10 degrees, observations become impossible. SAO astronomers planned to address some of this problem with a new mirror made of ultra low expansion Sitall glass, but this upgrade is not recorded as having taken place. With a glass ceramic (Sitall) primary mirror, it would be possible to reduce the thickness from 65 to 40 cm, reducing also thermal inertia. The large size of the dome itself means there are thermal gradients within it that compound these problems. Refrigeration within the dome offsets some of these issues.
Despite these shortcomings, the BTA-6 remains a significant instrument, able to image objects as faint as the 26th magnitude. This makes it especially useful for tasks such as spectroscopy and speckle interferometry where light gathering performance is more important than resolution. BTA has made several contributions using these techniques.
Speckle Interferometry techniques allow today the diffraction-limited resolution of 0,02 arcseconds of 15th magnitude objects under good seeing conditions (EMCCD-based Speckle Interferometer - PhotonMAX-512B camera - in active use since 2007). "In contrast to the adaptive optics, which is effective today mainly in the infrared, speckle interferometry can be used for observations in visible and near UV bands. In addition, speckle interferometry is realizable under poor atmospheric conditions, while the adaptive optics always needs the best seeing".
In 2007 SAO RAS started to work on reconstruction and replenishment of the primary mirror.  There was no money to replace the primary mirror with one made of low-expansion ceramic glass, which would have lessened the thermal problems, so instead SAO RAS decided to refurbish the original mirror which had sat in storage for nearly 30 years. According to Katerina Kuchaeva, one of SAO's assistant directors "New optical technologies make it possible to reconstruct the surface of the 6-m mirror eliminating the existing defects and creating a practically ideal parabolic surface, thereby significantly increasing the angular resolution of the mirror."  In 2012 A milling machine has removed 8 mm of glass from the upper surface, taking with that all of the optical imperfections. All work should be completed by mid-2013. 
The BTA primary is a 605 cm f/4 mirror. This is a relatively slow primary compared to similar instruments; the Hale is a 5 m f/3.3. The telescope optics are a Ritchey-Chrétien telescope design, albeit without the traditional Cassegrain-style focus. Due to its large primary, the image scale at the prime focus is 8.6 arc seconds per millimeter, about the same as the Cassegrainian focus of a 4 m telescope. This eliminates the need for a secondary, and instead the observing instruments are placed at the prime focus. For secondary roles, two Nasmith foci can be used, with an effective f/30.
The long focal length and lack of a secondary placed in front of the prime focus makes for a long telescope overall; BTA's main tube is 26 m long. This would have required a massive equatorial mount, so BTA instead uses an altazimuth mount with computer controls to keep the motion of the sky still in the view. Since this also results in the rotation of the field of view as the telescope moves, the primary focus area containing the instruments is also rotated to offset this effect. With the widespread adoption of computer controls for almost all aspects of telescope operations, this style of mounting, pioneered on BTA, has since become common.
When working at the prime focus, a Ross coma corrector is used. The field of view, with coma and astigmatism corrected at a level of less than 0.5 arcseconds, is about 14 arcminutes. It takes about three to four minutes to switch from one focus to another, making it possible to use several different instrument sets in a short period of time.
BTA-6 is enclosed in a massive dome, 53 m tall at the peak, and 48 m tall from the cylindrical base it sits on. The dome is much larger than required, and there is a gap of 12 m between the telescope and dome.
Contemporaries on debut
Largest telescopes in early 1976:
Special Astrophysical Obs
|26 m²||2070 m
|20 m²||1713 m
|1949||George Ellery Hale|
Kitt Peak National Obs.
|12 m²||2120 m
|1973||Nicholas U. Mayall|
Siding Spring Obs.
|12 m²||1742 m
|1959||Nicholas U. Mayall|
The BTA-6 was the largest optical telescope in the world between its first light in late 1975, when it exceeded the famous 5 m Hale telescope, and 1993, when the first 10 m Keck Telescope opened. However, a variety of problems meant that BTA-6 was never able to perform anywhere near its theoretical limits, so it may not have been the largest effective telescope throughout that period. It uses a computer-controlled altazimuth mount, which offers advantages and disadvantages compared to the equatorial mountings used many on previous designs.
- World's Largest Astronomical Telescope, Cherkessk 1978
- Uncovering Soviet Disasters: Exploring the Limits of Glasnost, James Oberg, ISBN 0-7090-3725-2
- Galaxies Through a Red Giant, William C. Keel, Sky and Telescope, 1992
- "6 meter telescope". Russian Academy of Sciences Institution, Special Astrophysical Observatory. 28 October 2010.
- Snezhko LI. "Проект БТА: исследование, состояние и перспективы" [BTA project: research, status and prospects] (in Russian). Russian Academy of Sciences Institution, Special Astrophysical Observatory. Retrieved 14 December 2010.
- Maksimov AF, Balega YuYu, Dyachenko VV, Malogolovets EV, Rastegaev DA, and Semernikov ЕА (2009). "The EMCCD-Based Speckle Interferometer of the BTA 6-m Telescope: Description and First Results". Astrophysical Bulletin 64 (3): 296–307. arXiv:0909.1119. Bibcode:2009AstBu..64..296M. doi:10.1134/S1990341309030092. ISSN 1990-3413. / Astrofizicheskij Byulleten (in Russian) 64 (3): 308–321. 2009.
- The 200-inch Hale Telescope
- Ioannisiani BK, Neplokhov EM, Kopylov IM, Rylov VS, Snezhko LI. (1982). "The Zelenchuk 6M telescope (BTA) of the USSR Academy of Sciences". ASSL Vol. 92: IAU Colloq. 67: 3–10. Bibcode:1982ialo.coll....3I.
- Special Astrophysical Observatory (SAO).
- BTA control info and webcams. (Webcams, telescope aiming, outdoor/indome/main mirror temperature, etc.). Retrieved 13 December 2010.
- BTA dome from Zeiss-1000 webcam. Retrieved 14 December 2010.
- 6m telescope and other SAO telescopes. (Instrumentation, observation schedules, request submission, observation reports, etc.). Retrieved 14 December 2010.
- BTA local web site (in Russian). (Staff, equipment, climate parameters, etc.). Retrieved 14 December 2010.
- BTA "indome" main server computer. (Manuals, software documentation). Retrieved 13 December 2010.
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