View source for BTA-6

{{Short description|Optical telescope at the Special Astrophysical Observatory in southern Russia}}
{{Infobox Telescope}}
The '''BTA-6''' ({{langx|ru|Большой Телескоп Альт-азимутальный|translit=Bolshoi Teleskop Alt-azimutalnyi|translation='''Large Altazimuth Telescope'''}}) is a {{convert|6|m|adj=on}} aperture [[optical telescope]] at the [[Special Astrophysical Observatory of the Russian Academy of Science|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 (astronomy)|first light]] in late 1975, making it the largest telescope in the world until 1990, when it was surpassed by the partially constructed [[W. M. Keck Observatory|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.
==History==
===Background===
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&nbsp;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 Ioannisiani|Bagrat K. Ioannisiani]]. With the goal of building the largest telescope in the world, a title long held by the 200&nbsp;inch (5&nbsp;m) [[Hale Telescope]] at the [[Palomar Observatory]], the team settled on a new design of 6&nbsp;m (236&nbsp;inches). This is about the maximum size a solid mirror can have without suffering from major distortion when tilted.
{{multiple image
|direction = vertical
|align= right
|width1= 225
|width3= 225
|width2= 225
|image1=Sao-6m-Telescope.jpg
|image3=SAO-6m-Telescope-main-mirror.jpg
|image2=Главная обсерватория.jpg
|caption1=The telescope's building, with a special crane on the right used for maintenance. A Zeiss 1-m telescope is housed in the building on the right. A 60-cm telescope was added in 1994.
|caption3=The telescope's 6 metre diameter main mirror is visible in the lower right part of the image.
|caption2=In front of the main entrance
}}
A telescope's theoretical [[angular resolution]] is defined by its aperture, which in the case of the BTA's 6&nbsp;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&nbsp;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&nbsp;m.<ref name=Cherkessk>[http://www.astr.ua.edu/keel/telescopes/bta.html World's Largest Astronomical Telescope], Cherkessk 1978</ref> In 1966 the Special Astrophysical Observatory was formed to host the BTA-6 and RATAN-600.
===Problems===
The first attempt to fabricate the [[primary mirror]] was made by the [[Lytkarino Optical Glass Plant]], near Moscow. They [[Annealing (glass)|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.<ref name=Cherkessk/>
However, it was clear the second mirror was only marginally better than the first, and contained major imperfections. Crews took to blocking off portions of the mirror using large pieces of black cloth to cover over the roughest areas.<ref name=Beatty/> According to Ioannisiani, the primary directed only 61% of the incoming light into a 0.5-[[arcsecond]] circle and 91% into one with twice the diameter.<ref name=sky>William Keel, [http://www.astr.ua.edu/keel/telescopes/bta.html "Galaxies Through a Red Giant"], ''Sky and Telescope'', 1992</ref>
[[File:Главная обсерватория внутри.jpg|thumb|upright=0.8|left|Inside the main observatory]]
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''.<ref>[https://books.google.com/books?id=Vnm-AAAACAAJ&dq=Uncovering+Soviet+Disasters ''Uncovering Soviet Disasters: Exploring the Limits of Glasnost''], James Oberg, {{ISBN|0-7090-3725-2}}</ref>
A third mirror, with an improved figure and no cracks, was installed in 1978.<ref name=Beatty/> 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 rarely better than one arcsecond resolution, and anything under 2 arcseconds is considered good.<ref name=sky/> In comparison, most major astronomical sites average seeing under one arcsecond.<ref name=Beatty/> Under favourable conditions the width of the seeing disc ([[FWHM]]) is ≈1&nbsp;arcsecond for 20% of observational nights.<ref name=six>{{cite web |url=http://www.sao.ru/Doc-en/Telescopes/bta/descrip.html |title=6 meter telescope |publisher=Russian Academy of Sciences Institution, Special Astrophysical Observatory |date=28 October 2010 }}</ref> Weather is another significant factor; on average, observing takes place on fewer than half of the nights throughout the year.<ref name=sky/>
Perhaps the most annoying problem is the huge thermal mass of the primary mirror, the telescope as a whole, and the enormous dome. Thermal effects are so significant in the primary that it can tolerate only a 2&nbsp;°C 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. 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.<ref name=sky/>
Speckle interferometry techniques today allow the diffraction-limited resolution of 0.02&nbsp;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".<ref>{{cite journal |journal=Astrophysical Bulletin |year=2009 |volume=64 |issue=3 |pages=296–307 |title=The EMCCD-Based Speckle Interferometer of the BTA 6-m Telescope: Description and First Results |author1=Maksimov AF |author2=Balega YuYu |author3=Dyachenko VV |author4=Malogolovets EV |author5=Rastegaev DA |author6=Semernikov EA |name-list-style=amp |issn=1990-3413 |doi=10.1134/S1990341309030092 |bibcode = 2009AstBu..64..296M |arxiv = 0909.1119 |s2cid=118435912 }} / {{cite journal |journal=Astrofizicheskij Byulleten |year=2009 |volume=64 |issue=3 |pages=308–321 |language=ru }}</ref>
===Improvements===
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&nbsp;cm, reducing thermal inertia.<ref>{{cite web |url=http://w0.sao.ru/hq/sekbta/40_SAO/Snezhko.htm |script-title=ru:Проект БТА: исследование, состояние и перспективы |author=Snezhko LI |publisher=Russian Academy of Sciences Institution, Special Astrophysical Observatory |language=ru |trans-title=BTA project: research, status and prospects |access-date=14 December 2010 }}</ref>
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.<ref name=Beatty>Kelly Beatty, [http://www.skyandtelescope.com/community/skyblog/newsblog/New-Eye-for-Giant-Russian-Telescope-148547035.html "New Eye for Giant Russian Telescope"], ''Sky and Telescope'', 23 April 2012</ref><ref>[http://www.sao.ru/Doc-k8/Science/Public/Reports/Y07-08/022-BTA-ktbt.pdf "Review Of The 6-M Telescope Operation"]</ref> In 2012 a milling machine removed 8&nbsp;mm of glass from the upper surface, taking with that all of the optical imperfections. Work was supposed to be finished in 2013,<ref>{{cite web |url=http://lzos.ru/en/index.php?option=com_content&task=view&id=209&Itemid=71 |title=LZOS - Primary mirror BTA-6: history and modern times (restoration works of 6m giant is continued) |website=lzos.ru |url-status=dead |archive-url=https://web.archive.org/web/20120510202813/http://lzos.ru/en/index.php?option=com_content&task=view&id=209&Itemid=71 |archive-date=2012-05-10}} </ref> but was delayed due to funding shortage. The mirror was finally completed in November 2017, and mirror replacement took place in May 2018.<ref>{{Cite web|url=https://www.sao.ru/Doc-en/Events/2018/MirrorChronicle/index.html|title = MirrorChronicle}}</ref> 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.
==Description==
The BTA primary is a 605&nbsp;cm f/4 mirror. This is a relatively slow primary compared to similar instruments; the Hale is a 5&nbsp;m f/3.3. The telescope optics are a [[Cassegrain reflector#"Classic" Cassegrain telescopes|Cassegrain]]  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,<ref name=sky/> about the same as the [[Cassegrain reflector|Cassegrain]]ian focus of a 4&nbsp;m telescope.
When working at the prime focus, a [[Ross corrector|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.<ref name=six/>
BTA-6 is enclosed in a massive dome, 53&nbsp;m tall at the peak, and 48&nbsp;m tall from the cylindrical base it sits on.<ref name=six/> The dome is much larger than required, and there is a gap of 12&nbsp;m between the telescope and dome.
==Comparison==
<!-- 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.<ref>[http://www.astro.caltech.edu/palomar/hale.html The 200-inch Hale Telescope]</ref> -->
{| class="wikitable" style="text-align: center; font-size: 0.9em;"
|+Largest optical astronomical telescopes in the late 1970s
|-
! width=18 | #
! Name /<br />observatory
! style="width:40px;" | Image
! Aperture
! width=60 | M1<br />area
! Altitude
! width=60 | First<br />light
! Special advocate
|-bgcolor=#edf3fe
| {{small|1.}}
| align=left | '''BTA-6'''<br />{{small|([[Special Astrophysical Observatory of the Russian Academy of Science|Special Astrophysical Obs]])}}
| bgcolor=#ddd | [[File:Big asimutal teleskop.jpg|80px]]
| 238&nbsp;inch<br />605&nbsp;cm
| 28&nbsp;m<sup>2</sup>
| {{convert|2070|m|ft|abbr=on}}
| 1975
| [[Mstislav Keldysh]]
|-
| {{small|2.}}
| align=left | [[Hale Telescope]]<br />({{small|[[Palomar Observatory]]}})
| bgcolor=#ddd | [[File:P200 Dome Open.jpg|80px]]
| 200&nbsp;inch<br />508&nbsp;cm
| 20&nbsp;m<sup>2</sup>
| {{convert|1713|m|ft|abbr=on}}
| 1949
| [[George Ellery Hale]]
|-
| {{small|3.}}
| align=left | [[Nicholas U. Mayall Telescope|Mayall Telescope]]<br />{{small|([[Kitt Peak National Observatory|Kitt Peak National Obs.]])}}
| bgcolor=#ddd | [[File:Kittpeakteliscope.JPG|80px]]
| 158&nbsp;inch<br />401&nbsp;cm
| 13&nbsp;m<sup>2</sup>
| {{convert|2120|m|ft|abbr=on}} <!-- alt. for telescope not obs! -->
| 1973
| [[Nicholas Mayall]]
|-
| {{small|4.}}
| align=left | [[Víctor M. Blanco Telescope]]<br />{{small|([[Cerro Tololo Inter-American Observatory|CTIO Observatory]])}}
| bgcolor=#ddd | [[File:4m-Victor M. Blanco Telescope cropped.jpg|80px]]
| 158&nbsp;inch<br />401&nbsp;cm
| 13&nbsp;m<sup>2</sup>
| {{convert|2200|m|ft|abbr=on}}
| 1976
| [[Nicholas Mayall]]
|-
| {{small|5.}}
| align=left | [[Anglo-Australian Telescope]]<br />{{small|([[Siding Spring Observatory]])}}
| bgcolor=#ddd | [[File:Anglo-Australian Telescope dome.JPG|80px]]
| 153&nbsp;inch<br />389&nbsp;cm
| 12&nbsp;m<sup>2</sup>
| {{convert|1742|m|ft|abbr=on}}
| 1974
| [[Charles, Prince of Wales|Prince Charles]]
|-
| {{small| 6.}}
| align=left | [[ESO 3.6 m Telescope]]<br />{{small|([[La Silla Observatory]])}}
| bgcolor=#ddd | [[File:Wallpaper of 3.6-m Telescope at La Silla.jpg|80px]]
| 140&nbsp;inch<br />357&nbsp;cm
| 10&nbsp;m<sup>2</sup><!-- 8.8564 m2 -->
| {{convert|2400|m|ft|abbr=on}}
| 1976
| [[Adriaan Blaauw]]
|-
| {{small|7.}}
| align=left | [[C. Donald Shane telescope|Shane Telescope]]<br />{{small|([[Lick Observatory]])}}
| bgcolor=#ddd | [[File:Shane dome.JPG|80px]]
| 120&nbsp;inch<br />305&nbsp;cm
| ~7&nbsp;m<sup>2</sup>
| {{convert|1283|m|ft|abbr=on}}
| 1959
| [[Nicholas Mayall]]
|}
{{clear}}
;Graphic
{{comparison optical telescope primary mirrors.svg|left}}
{{clear}}
== See also ==
* [[List of largest optical reflecting telescopes]]
* [[List of largest optical telescopes historically]]
==References==
{{reflist}}
==Further reading==
*{{cite book |vauthors=Ioannisiani BK, Neplokhov EM, Kopylov IM, Rylov VS, Snezhko LI | title= Instrumentation for Astronomy with Large Optical Telescopes | chapter= The Zelenchuk 6M telescope (BTA) of the USSR Academy of Sciences | year=1982| volume= 92| pages= 3–10 | bibcode=1982ASSL...92....3I | doi=10.1007/978-94-009-7787-7_1| series= Astrophysics and Space Science Library | isbn= 978-94-009-7789-1 }}
==External links==
*[http://www.sao.ru/Doc-en/index.html Special Astrophysical Observatory (SAO)].
*[http://www.sao.ru/tb/tcs/ BTA control info and webcams]. (''Webcams, telescope aiming, outdoor/indome/main mirror temperature, etc.''). Retrieved 13 December 2010.
*[http://www.sao.ru/zserv/webcam/z_webcam_js.html?t=20 BTA dome from '''Zeiss-1000''' webcam]. Retrieved 14 December 2010.
*[http://www.sao.ru/Doc-en/Telescopes/ 6m telescope] and other SAO telescopes. (''Instrumentation, observation schedules, request submission, observation reports, etc.''). Retrieved 14 December 2010.
*[https://web.archive.org/web/20130927004740/http://www.sao.ru/acs/ BTA local web site] (in Russian). (''Staff, equipment, climate parameters, etc.''). Retrieved 14 December 2010.
*[http://www.sao.ru/tb/ BTA "indome" main server computer]. (''Manuals, software documentation''). Retrieved 13 December 2010.
{{DEFAULTSORT:Bta-6}}
[[Category:Reflecting telescopes]]
[[Category:Astronomical observatories in Russia]]
[[Category:Astronomical observatories built in the Soviet Union]]
[[Category:Institutes of the Russian Academy of Sciences]]
[[Category:LOMO]]