|Credit: Iztok Boncina/ESO|
|A map of the Paranal site, from the Residencia up to the VLT platform.|
The four UTs, Antu, Kueyen, Melipal, and Yepun, each stand over 28.5 m high, each containing the 22-tonne 8.2-m diameter primary mirrors - single, monolithic mirrors, as large as we can build them with our technology today, made from a special glass-ceramic with almost no thermal expansion, called Zerodur. These are mountain on a 350-tonne alt-azimuth mount, with a 1.1-m diameter beryllium secondary mirror to reflect the light back to the instruments at the Cassegrain focus. Active optics with 150 supports control the shape of the thin (177 mm thick) primary mirror. We got to go inside UT3 just as the sun was setting, and seeing this huge structure moving and rotating with very little sound was quite incredible. We stood in total darkness, until suddenly the huge dome doors opened up, letting the moonlight flood in. The wind shields behind the dome doors then opened one by one, as well as the vents all around and below the telescope - this lets the air in to keep the optics down at the ambient temperature of the mountaintop. The dome rotates silently to align with the chosen target, and the alt-azimuth mount slews to the right elevation. This is astronomy on an industrial scale.
Watching these huge UTs move around was an absolute privilege, especially with the golden glow of sunset in the background, Venus setting, and Jupiter rising in the east. By the time we'd finish each night, Saturn was also high up in the sky.
The four UTs aren't the only things up on the platform - there are four additional 1.8-m telescopes called the ATs (auxiliary telescopes). These are dedicated to full time interferometry, being able to move the ATs around on rails to produce a variety of different baselines (the further apart they are, the finer the scales they probe). The UTs can also be used for interferometry, increasing the light collecting power, but it means that they're all being used at the same time on the same object, and no one else can use the instruments. The beams from each telescope are combined in "delay lines" beneath the platform, where they are fed into different instruments.
While we've been here at Paranal, UT1 celebrated a very special anniversary - it achieved first light exactly 20 years ago, on May 25th 1998, and in excess of 330 million EUR were spent in ESO Member States for the construction of the VLT. The other UTs followed:
UT2, Kueyen: 1 March 1999
UT3, Melipal: 26 Jan 2000
UT4, Yepun: 4 September 2000
The annual budget is 16.9 million EUR without personnel costs, You can see recent 360-degree images of the summit here, and take a virtual tour here. This website showing the current conditions is extremely useful.
Our final night observing Jupiter on #VLT, and a final walk around on the platform by moonlight. This is such a special place. UT3 is there, pointing towards everyone's favourite gas giant. Tomorrow, its back to Santiago. pic.twitter.com/FOjt1XqGSj— Leigh Fletcher (@LeighFletcher) May 27, 2018
"There's no cause for alarm.... but there probably will be."