Infobox Telescope
bgcolour =
name = BTA-6

caption = BTA-6 as seen from in front of the main entrance
organization = SAO
location = near Mt. Pastukhova, Russia
wavelength = 0.3 to 10 μM
built = First light 1975
website =
style = Ritchey-Chrétien
diameter = 605 cm
area = 26 m²
focal_length = f/4 (26 m)
mounting = alt-azimuth fully steerable primary mirror!primary
The BTA-6 (ольшой Телескоп Азимутальный", or "Large Altazimuth Telescope") is a large 6 m optical telescope at the Special Astrophysical Observatory located on the north side of the Caucasus Mountains in southern Russia. The BTA-6 was the largest telescope in the world between its first light in 1975, when it beat out the famous 5 m Hale telescope, and 1993, when the 10 m Keck Telescope opened. However, a variety of problems meant that BTA-6 was never able to perform anywhere near its theoretical limits, allowing Hale to claim it was the largest "effective" telescope throughout this period. [ [ The 200-inch Hale Telescope] ] One feature of the BTA design has been the pattern for every large telescope since, the use of a computer-controlled altazimuth mount, which is both simpler and more flexible than equatorial mountings used on previous designs.


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. Ioannisian. Setting 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, a new design of 6 m (236 inches) was settled on. 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 being 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 for its installation sixteen expeditions were dispatched to various regions of the USSR. As a result of extensive selection, the decision was made to locate BTA in the North Caucasus Mountains near Zelenchukskaya at a height of 2,070 m. [ World's Largest Astronomical Telescope] , Cherkessk 1978] 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". [ [ Uncovering Soviet Disasters: Exploring the Limits of Glasnost] , James Oberg, ISBN 0709037252]

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. [ Galaxies Through a Red Giant] , William C. Keel, "Sky and Telescope", 1992] For comparison, the Hale telescope was able to reach angular resolutions close to its theoretical limit of 0.025 arcseconds, well over twice the resolution of BTA-6. [ [ Angular Resolution Example: Hale Telescope] ]

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 nowhere near 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". 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 low-expansion Sitall glass, but this upgrade is not recorded as having taken place. 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 where light gathering performance is more important that resolution, which is true for spectroscopy and speckle interferometry. BTA has made several contributions using these techniques.


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 focal area at the prime focus was 8.6 arc seconds per millimeter, about the same as the Cassegrainian focus of a 4 m telescope. This allowed them to dispense with 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 a 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 <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. [ 6m telescope] , Optical scheme of the telescope]

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.

See also

* List of largest optical reflecting telescopes


Further reading

*cite journal | author=Ioannisiani BK, Neplokhov EM, Kopylov IM, Rylov VS, Snezhko LI.| title= "The Zelenchuk 6M telescope (BTA) of the USSR Academy of Sciences" | journal=ASSL Vol. 92: IAU Colloq. 67 | year=1982| volume= | issue= | pages= 3–10 | url=

External links

* [ 6m telescope] , SAO home page

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