Paranal Visit

Me and the Very Large Telescope

To say I was excited about the prospect of visiting Paranal would be an understatement. I'm not sure exactly why this visit in particular held so much excitement for me. Perhaps it was because until I had actually started my trip in Chile, I thought I was only going to be able to visit as part of one of the tourist tours. Instead, as things turned out, I was able to have an overnight stay at the infamous Paranal Residencia.

Unfortunately, it was cloudy and extremely windy on the night I stayed over, so I didn't get to see the telescopes opening and operational at night - I barely even saw the stars at night - but still, that didn't stop the visit from being thoroughly enjoyable and hugely interesting.

My host, Laura, whom I had met at the ESO offices in Santiago earlier in my trip, was hosting a French film-making team, which was why I was able to go up and visit too. The French team had been commissioned to make a film for planetarium use, about an astronomer using the VLT and E-ELT to find out more about exoplanets. Specifically, using the E-ELT to try and detect water on an exoplanet.

I spent the night before my visit in a hotel in Antofagasta, and was picked up by the Paranal bus from outside the hotel. There were probably around 10 - 15 people also getting the bus there - mostly engineers.

Life at Paranal

Paranal Residencia

It seems fitting to start with some information about the Residencia, which in itself is a pretty amazing place. You may recognise it from a certain James Bond film, Quantum of Solace - its the building which gets blown up a bit at the end of the film! But enough about that for now.

The Residencia was finished and opened in 2001. It was designed especially so that it would not intrude too much on the surrounding landscape - in fact, when you first arrive on-site, you can barely see it as you're on the roof of it. The colour of the building is not down to paint, instead, pigments have been added to the building materials to blend in with the nature around. It has also been designed to withstand a Richter Scale 10 earthquake, which luckily it hasn't had to stand up to so far, but it has seen some big ones.

Inside the Residencia, the first thing you notice is the swimming pool, and tropical looking garden around it. It looks very luxurious. Not only is the pool there as a health and leisure facility for those working at Paranal, but it also serves a purpose in maintaining the humidity and temperature of the complex. Outside, there is typically 5-10% humidity but inside, thanks to the pool, there is around 30% humidity. By helping to maintain a constant temperature, it also serves for energy conservation - there is very little air conditioning or heating needed in the Residencia. Additionally, the pool helps to maintain the plants which surround it.

Laura explained that often, people who come to visit Paranal think the Residencia is a bit over the top for a workers' hotel. However, when you consider that people generally work shifts (8 days on, 6 days off, or similar) and they are 110km from the nearest small town (Taltal), having a good variety of leisure facilities and as comfortable a residence as possible is necessary.

The Residencia has also been designed to limit light pollution from it as much as possible. As such, the windows are all very small and have blinds on them. Also, the glass doors to the balcony areas have wooden sliding doors behind to be closed at night. Also, the roof above the swimming pool has a cover which automatically closes on sunset and reopens on sunrise.

Typically, there are around 120 people working on-site at any one time. Of those, only about 10% are astronomers, around 40-50 are engineers, and the remaining are contractors who look after the catering, maintenance, logistics, cleaning etc.

Everything has to be brought in from off-site, in particular, water. Around 60,000 litres of water are brought in per day, in 20,000 litre capacity tankers.

One really lovely touch in the Residencia which I particularly liked were the hand-made rugs you can see dotted around the communal areas. The rugs have images of the first science results from the VLT. 

I also particularly liked the fact that all the crockery is branded with 'ESO Paranal' - and the towels too!

It was interesting to hear from Laura that many of the temporary buildings currently being used on the ALMA OSF site for accommodation and facilities, were also used during the construction phase of Paranal.

Why Here?

So why exactly was Paranal chosen as an optimum site for a professional observatory? The Humboldt Current, which comes in from the Pacific Ocean, helps to maintain the excellent atmospheric conditions of the area. The Current maintains the sea temperature at around 14 C, and creates a thermal inversion layer at around 1100m, meaning that normally, the VLT sits above the cloud layer and the atmosphere is very still.

Layer of cloud at 1100m and abnormal clouds higher up!

The Telescopes

After a good look around the Residencia, getting settled in my room and having some lunch, we headed off up towards the telescopes. The film crew were looking for a good spot to film the sunrise the following day, and Laura suggested up near VISTA might be good. VISTA is on a hill which is referred to as the NTT Peak (NTT = New Technology Telescope) as originally the NTT was going to be situated at Paranal. Instead it is at the La Silla Observatory...so really the hill should be renamed the VISTA Peak. That has a much nicer ring to it!

Anyway, whilst the film crew were checking out the view and conditions, Laura took me in to see VISTA.

VISTA - Visible and Infrared Survey Telescope for Astronomy

VISTA is a 4.1m survey telescope, which the UK ATC project managed. It's one of the first projects I remember being completed in my time working at the Royal Observatory Edinburgh. First light for VISTA was in December 2009.

It is a very compact telescope, with not much space between the primary and secondary mirrors - in other words, it has a very short focal length. It has a large field of view, of 1.5 square degrees (roughly the area the full Moon covers) and, due to the size of the primary mirror, can see faint objects too - so a perfect survey telescope.

The primary mirror on VISTA is quite special - it is the most highly curved mirror of this size ever made for a telescope. Also, any deviations on the surface are less than 1/3000th of the thickness of a human hair. As I mentioned in my ALMA OSF post, the precision in the surface of mirrors for infrared telescopes needs to be much greater than it does with ALMA. The level of precision correlates with the wavelength the telescope works at.

The original coating on the VISTA mirror was silver, chosen, as with Gemini, for its good reflectivity in the infrared. However, its durability is not so great and so now the VISTA M1 is coated with aluminium. VISTA has its own coating plant right there in the building - so the mirror doesn't have to be transported far to be re-coated.

The VLT - Very Large Telescope

After some time having a look at VISTA, Laura took me up to the VLT platform. On the platform there are not just the four 8.2m telescopes of the VLT, but also the 4 Auxiliary Telescopes (ATs) used for interferometry and the VLT Survey Telescope.

It was extremely windy by this time and very shortly after we arrived Laura got a message over the radio system saying we were no longer allowed to be out on the platform. Already at this stage, even with a good 3 hours til sunset, the opening of the telescopes was not looking hopeful. Firstly, the cloud layer wasn't looking like it was moving anywhere. Secondly, the gusts of wind were reaching higher than 18m per second - the maximum wind speed of operation.

So, we went indoors and to see UT1, or Antu - the telescope where the UK ATC instrument KMOS will be going. I had specifically asked Laura if I could see this telescope. There is a tunnel system underneath the platform which allowed us to still be able to enter the telescope. The tunnel system, as well as being a passageway for workers, is also where all the cables are for combining the light from the ATs for interferometry.

The Unit Telescopes

Each one of the Unit Telescopes of the VLT has a primary mirror of 8.2m diameter and 17cm thickness - 23 tonnes of glass ceramic! The material it is made from is called 'Zerodur'. This is a very stable material but as the telescope moves, gravity acts on it so active optics compensate for these deformations. There are 150 actuators on the primary mirror, and they move to make corrections roughly every 30 seconds.

The primary mirrors are re-coated roughly every 18 months, and unlike the VISTA mirror, they have to be transported down to base camp for this to be done. This is a particularly delicate operation - after all, it involves handling the most expensive and delicate part of the telescope. There are no spare primary mirrors! So, the mirror is transported down the hill, travelling at 5km/hr. The whole process takes around 1 week from when the mirror is removed from the telescope to when it is replaced.

The mirror of UT1 was covered when we were in to see it - to protect from any dust which might have been blowing around in the high winds. When dust accumulates on the mirror, it may only reduce the reflectivity by a couple of % but every photon is valuable, to preserve the quality of the image. To protect the mirror, when the dome is opened at night, the telescope is tilted to 90°, to prevent anything falling on it.


The tower of the secondary mirror on these telescopes is mechanically the most complicated part of the telescope, as it has to move a lot to maintain the focus of the image. The mirror itself is made from beryllium - making it both light and stiff.

The telescopes are built on altazimuth mounts rather than equatorial. This has the advantage that the enclosures are much smaller than they would be with an equatorial mount. With an altazimuth mount, it does mean the telescope has to move in 2 directions at different speeds to track an object. The NTT (New Technology Telescope - the name makes sense to me now) was actually a prototype for the VLT unit telescopes. It was the first large telescope to be built with an altazimuth mount. It was also a prototype for the active optics on the primary mirror.

Instruments

Tertiary mirror on UT1

Each of the Unit Telescopes has 3 different instruments and the tertiary mirror sends the light to the chosen instrument. On UT1 there are currently only 2 instruments attached as KMOS will be fitted shortly. In fact, KMOS is getting packed up and shipped on Monday from Edinburgh. Watch this blog for more information on that.

The instruments UT1 has are CRIRES, on Nasmyth A and FORS 2 on the Cassegrain - both different types of spectrograph. KMOS will be fitted to the Nasmyth B position.

The VLT is now onto its second generation of instruments - typically an instrument will last/be used for around 10 years.

Interferometry

The VLT was built with the objective of combining the 4 telescopes together for interferometry. In practise, around 20% of the Unit Telescope time is spent on interferometry, but the Auxiliary Telescopes are used for interferometry all the time.

There are 30 different stations where the ATs can be positioned on the platform, with a maximum distance of 120m, which, by the process of interferometry, means the equivalent of a 120m diameter telescope.

In the interferometer, the combined light has a very complicated path to follow, bouncing off 17 mirrors on its travels. This means that most of the light is lost - only around 30% of the original light is received. Because of this, typically, the interferometry is only done for objects which are visible to the naked eye (approximately magnitude <6).

On the platform, there are certain areas where you are not allowed to walk, as even the smallest vibration can cause disruption to the interferometer.

The Control Room

UT1 control station

We headed back to the control room afterwards and spoke with a couple of the engineers and astronomers there.

Inside the control room, there are separate control stations for each one of the Unit Telescopes of the VLT, the 2 survey telescopes, and the VLT Interferometer - making a total of 6 control stations. There is also a control station already set aside for the European Extremely Large Telescope, but for now all the screens are showing are the weather monitors.

Each observing night, there would normally be 2 people at each control station: one astronomer and one telescope and instrument operator. Sometimes there will be extra people, for example visiting astronomers, or new staff.

On any given night, the number of observations can vary. One observation might be for 6 hours of the night, others can be very brief. To change between the different instruments on the telescopes takes a maximum of 10 minutes, depending on the position of the telescope. The telescope needs to be in the stand-by position to change instruments (i.e. pointing directly upwards) so the further from this position it is, the longer it takes to change instrument.

You could tell there was a mixed sense of disappointment and relief in the control room, since it was looking unlikely the telescopes would be operating. It must be tough working shifts like that - getting a night off must be a welcome relief at times. Although, I'm not sure exactly whether they would really have the night off, or be on-call just in case the weather improved.

Maintenance Buildings

AT1 in for maintenance

One of the ATs was in the maintenance building for routine checks and to have some of its mirrors re-coated. The primary mirrors on the ATs are only re-coated every 5 years. This one was having M2, M3 and M4 re-coated. This was the second of the ATs which they had worked on this year, and in September they would be re-coating the same mirrors on one of the other AT's. I think, by the end of the year, they would have worked on all 4 of them.

Usually, the re-coating is done with the large re-coating machine which is in the maintenance building, but at the moment, they're having some technical problems with this, so the mirrors of the AT were transported up to the VISTA coating plant.

European Extremely Large Telescope

Me and Cerro Armazones

On the Tuesday, the film crew wanted to get some footage of their 'star' astronomer driving through the desert on the way to the E-ELT, so we headed off on the dirt road to Cerro Armazones.

Once built, the E-ELT will be controlled from Paranal - although it is on a mountain around 20km away from Paranal, it is still going to be a Paranal Observatory telescope. 

I still can't quite imagine just how big the E-ELT is going to be - it seems impossible to think of building something that big, having already thought the ALMA dishes were huge!

The E-ELT primary mirror will be made of segments and because there are so many of them (around 800), the re-coating process will be practically continuous. One segment will be re-coated each day, so it is very likely that the E-ELT will have its own coating plant on-site - it wouldn't be very practical to have to transport a segment 20km and back each day!

Although I didn't get to see the opening of the telescopes at Paranal, I still had an amazing time. I think having been to ALMA first, which is still under construction, and then to Paranal, really helped me to imagine both Paranal under construction and what the ALMA site will be like when complete. Just need to make sure I can get out here again to see the E-ELT being constructed!