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.

But due to its location downwind of numerous large mountain peaks, astronomical seeing is rarely good.

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.

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.

Crews took to blocking off portions of the mirror using large pieces of black cloth to cover over the roughest areas.

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.

[5] Weather is another significant factor; on average, observing takes place on fewer than half of the nights throughout the year.

"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.

[6] 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.

[7] 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.

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.