Ресейдің Оңтүстігіндегі БТА-6 телескопы

Optical telescope at the Special Astrophysical Observatory in southern Russia

The BTA 6 (Большой Телескоп Альт азимутальный) is a aperture optical telescope at the Special Astrophysical Observatory located in the Zelenchuksky District of Karachay Cherkessia on the north side of the Caucasus Mountains in southern Russia. The BTA 6 achieved first light in late 1975, making it the largest telescope in the world until 1990, when it was surpassed by the partially constructed Keck 1. It pioneered the technique, now standard in large astronomical telescopes, of using an altazimuth mount with a computer controlled derotator. For a variety of reasons, BTA 6 has never been able to operate near its theoretical limits. Early problems with poorly fabricated mirror glass were addressed in 1978, improving but not eliminating the most serious issue. But due to its location downwind of numerous large mountain peaks, astronomical seeing is rarely good. The telescope also suffers from serious thermal expansion problems due to the large thermal mass of the mirror, and the dome as a whole, which is much larger than necessary. Upgrades have taken place throughout the system's history and are ongoing to this day.

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.

The first attempt to fabricate the primary mirror was made by the Lytkarino Optical Glass Plant, near Moscow. They annealed the glass too quickly, causing cracks and bubbles to form, making the mirror useless. A second attempt fared better and was installed in 1975. 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 discussed in James Oberg's 1988 book Uncovering Soviet Disasters. A third mirror, with an improved figure and no cracks, was installed in 1978. Weather is another significant factor; on average, observing takes place on fewer than half of the nights throughout the year.

SAO astronomers planned to address one of the main problems with a new mirror made of the ultra low expansion glass ceramic Sitall, but this upgrade is not recorded as having taken place. With a Sitall primary mirror it would be possible to reduce the thickness from 65 to 40 cm, reducing thermal inertia. By 2007 the operational mirror, the third to be produced, had become heavily corroded by the use of nitric acid to neutralise the alkali based solvents used to clean the glass before applying a new layer of reflective aluminium. A major overhaul in order to re grind the mirror was needed, but this would have cut into the packed observing schedule. Instead, the second mirror, abandoned due to imperfections but sitting in storage throughout, was returned to Lytkarino for refurbishment. In 2012 a milling machine removed 8 mm of glass from the upper surface, taking with that all of the optical imperfections. Work was supposed to be finished in 2013, but was delayed due to funding shortage. The mirror was finally completed in November 2017, and mirror replacement took place in May 2018. However, the refurbished mirror was found to be still inadequate, and after a few months of testing, it was decided to replace it with the previous mirror.