Editing Very Large Telescope (section)

=== Telescopes ===

Each Unit Telescope is a [[Ritchey-Chretien]] [[Cassegrain telescope]] with a 22-tonne 8.2-metre [[Zerodur]] primary mirror of 14.4-metre focal length, and a 1.1-metre lightweight beryllium secondary mirror. A flat tertiary mirror diverts the light to one of two instruments at the f/15 [[Nasmyth telescope|Nasmyth]] foci on either side, with a system focal length of 120&nbsp;metres,<ref>{{cite web|url=https://www.eso.org/sci/facilities/develop/documents/VLT-SPE-ESO-10000-2723_is1.pdf | title = Requirements for Scientific Instruments of the VLT Unit Telescopes | publisher = ESO | access-date = 18 January 2018}}</ref> or the tertiary tilts aside to allow light through the primary mirror central hole to a third instrument at the Cassegrain focus. This allows switching between any of the three instruments within five minutes, to match observing conditions. Additional mirrors can send the light via tunnels to the central VLTI beam-combiners. The maximum field-of-view (at Nasmyth foci) is around 27&nbsp;arcminutes in diameter, slightly smaller than the full moon, though most instruments view a narrower field.{{Citation needed|date= April 2018}}

Each telescope has an [[Altazimuth mount|alt-azimuth]] mount with total mass around 350&nbsp;tonnes, and uses [[active optics]] with 150 supports on the back of the primary mirror to control the shape of the thin (177&nbsp;mm thick) mirror by computers.<ref>{{cite web |url=https://www.eso.org/sci/facilities/paranal/telescopes/ut/m1unit.html |title=The VLT primary mirrors: mirror production and measured performance
|first1=P. |last1=Dierickx |first2=D. |last2=Enard |first3=R. |last3=Geyl |first4=J. |last4=Paseri |first5=M. |last5=Cayrel |first6=P. |last6=Béraud |publisher=ESO}}</ref>