REPORT

Europe's new eye

Dome of the Great Canary Telescope (GCT) located 2 400 metres above sea level on the Spanish island of La Palma. © Miguel Briganti (SMM/IAC)
Dome of the Great Canary Telescope (GCT) located 2 400 metres above sea level on the Spanish island of La Palma.
© Miguel Briganti (SMM/IAC)
First image produced by the GCT instrument OSIRIS. This is M51, a pair of galaxies also known as the Whirlpool galaxy, located 23 million light years from Earth. The exposure time required with this instrument was two minutes. For a similar depth, from a telescope 1m in diameter, it would take more than three hours. © GTC-OSIRIS/ Daniel López (IAC-OT)
First image produced by the GCT instrument OSIRIS. This is M51, a pair of galaxies also known as the Whirlpool galaxy, located 23 million light years from Earth. The exposure time required with this instrument was two minutes. For a similar depth, from a telescope 1m in diameter, it would take more than three hours.
© GTC-OSIRIS/ Daniel López (IAC-OT)
The OSIRIS Instrument. © Miguel Briganti (SMM/IAC)
The OSIRIS Instrument.
© Miguel Briganti (SMM/IAC)
Inside the GCT dome. The light, collected by the mosaic of mirrors, is directed towards the secondary mirror on the top (small cylindrical structure), which will redirect it downwards, via the black tube, where the tertiary mirror is located, which is responsible for directing the light towards the various stations on which scientific instruments are mounted. © Miguel Briganti (SMM/IAC)
Inside the GCT dome. The light, collected by the mosaic of mirrors, is directed towards the secondary mirror on the top (small cylindrical structure), which will redirect it downwards, via the black tube, where the tertiary mirror is located, which is responsible for directing the light towards the various stations on which scientific instruments are mounted.
© Miguel Briganti (SMM/IAC)
Over time, dust collects on the primary mirror, which is why each segment is cleaned every two years in accordance with a realuminisation process. The fine layer of aluminium which covers the structure is removed and a new one is pulverised. © Miguel Briganti (SMM/IAC)
Over time, dust collects on the primary mirror, which is why each segment is cleaned every two years in accordance with a realuminisation process. The fine layer of aluminium which covers the structure is removed and a new one is pulverised.
© Miguel Briganti (SMM/IAC)

With a primary mirror 10.4 metres wide, the Great Canary Telescope (GCT) is the largest telescope in the world. Just a few months after its inauguration, its time is divided between scientific data collection and calibration operations. We visited this colossus, perched 2 400 metres above sea level on the Spanish island of La Palma in the Canary Islands.

It is not the best idea ever to arrive at the Roque de los Muchachos Observatory at night. A few kilometres outside of the capital of Santa Cruz, street lighting disappears – the island of La Palma is hardnosed as regards light pollution. Visitors have only their headlights to guide them along the 40 km of winding roads leading up to the site. Up there, you need a torch to reach the stargazers’ residence. The sky truly is studded with stars. Dazzling. We almost forget them in our over-illuminated cities.

During the day, the stars give way to no less fascinating a spectacle. On this early summer day, a pretty carpet of canary yellow broom flowers covers the volcanic mountain. From now on, there will be twelve telescopes, which much resemble giant mushrooms, here in this peaceful natural landscape. Located at the highest point on La Palma, Roque de los Muchachos offers a breathtaking view of the Caldera de Taburiente, an 8-km-wide volcanic crater.

Ideal conditions

La Palma was a natural choice for René Rutten, head of operations of the Great Canary Telescope (GCT), long-term resident of Roque de los Muchachos and former director of the Isaac Newton Group of Telescopes, ‘This is the only observatory of global stature in Europe. To avoid the air at ground-level heating up and creating atmospheric turbulence, the best sites are always located very high up, either on a mountain in the middle of the ocean, as is the case in La Palma or Hawaii, or on a mountain range very close to a coast, such as in Chile.’

The sky above La Palma and the part of Tenerife that hosts the Tiede Observatory have been protected by ‘Sky Law’ since 1988. Street lighting and the strength and orientation of light bulbs are strictly regulated in order to ensure that the sky above the observatories is very dark. Industrial activity and air traffic are also subject to limitations in order to reduce atmospheric pollution. Flights over La Palma, in particular, are banned without special dispensation. Finally, radio wave emissions are also restricted to prevent interference with scientific instruments.

Stable atmospheric conditions, mild weather and this unique law render this Canary Island mountain a favoured observation site, where 62 institutions from 19 different countries now converge.

Busy night and day

At eight o’clock in the morning, just as the GCT night-time team goes to bed, the daytime team is having breakfast at the residence, before getting into their cars and heading to the telescope site a few minutes away. Once there, it is impossible to look up at the impressive metallic dome, which is 45 metres high and 34 metres wide, without sunglasses. The material was chosen to reflect the light and hence to prevent the internal temperature from rising.

As of last March the telescope has been gathering scientific data, but half of its time is still dedicated to fine-tuning operations. ‘For a few days now, we have been working on the transit of an extrasolar planet, which will take place the day after tomorrow,’ states René Rutten enthusiastically.

Carlos Alvarez is one of eight astronomers responsible for supervising the observations requested at the GCT. ‘During the day, we carry out the necessary tests and fine-tuning for the nocturnal observations.’ At 4pm, when the telescope empties briefly, this smiling sportsman will run home to Santa Cruz, which will take him three and a half hours!

Plaques which trace the key construction phases of the facility hang on the walls leading to the telescope chamber. The first stone was laid in 2000 and the last seven years later. It cost EUR 104 million and was 90 % funded by the Spanish government, with support from the European Regional Development Fund (ERDF). Five per cent of funding also came from Mexico and 5 % from the University of Florida.

Objective: zero turbulence

In the world, there are about ten large telescopes with primary mirrors 8-10 metres in diameter. The GCT is probably the most recent example of this generation of telescope. In the years to come, very large telescopes with 30-metre mirrors will supersede them.

The larger the primary mirror, the better the quality of the images, and the further we can see into the universe. However, because of weight issues, in particular, it is very complicated to build a single-piece mirror of more than eight metres in diameter. This is why the large telescopes have segmented mirrors. The main device of the so-called ‘active optics’ system is the GCT’s mirror, which is made up of 36 1.9-metre hexagonal segments, and mounted on motors to ensure that they are perfectly positioned in relation to one another. Together, the segments form a concave surface of 81.9 m2, the equivalent of a 10.4-m-wide circular mirror.

For René Rutten, the comparison of the GCT with other large telescopes ends there, ‘The GCT is much more technologically advanced because it was built long after them’. One such technological advance is ‘adaptive optics,’ with which telescopes will be fitted in a few years’ time. This system corrects distortions caused by atmospheric conditions. As they penetrate our atmosphere, light waves, which are perfectly flat up until that point, are distorted as a result of turbulence. For this reason the stars seem somewhat blurred to us, like coins at the bottom of a pool of water.

At a rate of approximately 200 times per second, adaptive optics will determine and correct this distortion almost in real-time. The light of the European Regional Development Fund will in fact be redirected towards a small flexible mirror, 10 or so centimetres in diameter, which will move very rapidly so as to flatten the wavefront once again. In simple terms, the system will create observation conditions that rival those of space-based telescopes.

Ice palace

Under the dome, the prevailing cool quickly feels cold: all the installations are kept at temperatures close to those at night. That is around 10 degrees during the summer. ‘We open the dome half an hour before sunset so that the temperature inside can adjust to that outside,’ explains Carlos Alvarez. ‘At night, glycolised water is used to help remove the heat generated by the electronic parts. If the temperature of the mirrors were just one degree higher in relation to the air, a drop in quality would already be perceptible,’ adds René Rutten.

At the very top, right on the axis of the primary mirror, there is a smaller secondary mirror, which is convex in shape. It deflects the light gathered towards a third mirror, which is positioned in the centre of the mosaic of segments, serving to direct the beam towards one of the six foci stations, where the scientific instruments are installed. The GCT has only one such instrument at present. It bears the name of an Egyptian god: OSIRIS, which stands for Optical System for Imaging and Low Resolution Integrated Spectroscopy. And so, it is in this large blue box, fed by an incalculable number of cables, that light, which has travelled for thousands of years, finishes its course. It becomes an image and, most importantly, scientific data.

From gamma-ray bursts to extrasolar planets

OSIRIS operates over a range of wavelengths, from infrared to near ultra-violet. As well as producing images, it allows for multipleobject spectroscopy, the simultaneous analysis of 30 or so celestial objects, which is not insignificant for a telescope of this size. But what really makes this instrument unique, according to the director of operations, is its tuneable filter, which can select an extremely small part of the spectrum. ‘Teams from the University of Mexico used it for the first time yesterday to observe the formation of stars and the movements inside an active galaxy.’

The filter will also make it possible to determine the level of redshift of cosmic bodies. The term ‘redshift’ refers to the shift of a light wave towards the red end of the spectrum, caused by the observer moving relative to the source of the wave. The greater the speed of movement, the greater the redshift, the further away the object will be. ‘By applying the filter to an image, we will see galaxies appearing at different wavelengths, which will enable us to measure their individual speeds.’ And ultimately, we will learn more about the expansion of our universe.

Other instruments will gradually fill the different stations. CanariCam, an infrared camera with spectroscopic, polarimetric and coronagraphic capabilities, should be installed next spring. The polarimeter will mainly be used to measure the polarisation of radiation emitted by the coldest objects in the universe, such as forming stars or extrasolar planets. As for coronography, this technique creates artificial eclipse around a star, so as to expose not only the star’s corona, but also the objects which orbit around it.

The EMIR, a multi-object spectrograph that will work on the near-infrared range, is also planned, as is FRIDA, which has been specially designed to work with the GCT’s adaptive optics. The instrument will serve to perform ‘3D spectroscopy’ which allow for high-resolution observation of very distant objects, among other things.

Sharing precious time

The telescope’s schedule reflects its financing: Spain has access to 90 % of its time, while Mexico and the University of Florida each have 5 %.

Requests submitted by research centres are prioritised by an independent committee in relation to their scientific interest. René Rutten then divides the proposals into different categories, according to the observation conditions they require. ‘Use of the telescope is still a very complex matter. In order to be able to concentrate on the scientific operations, the first year we decided to lead the observations ourselves, with people who knew exactly what they were doing’.

At night, the astronomer on duty manages the operations alone, assisted by a technician who is responsible for operating the telescope. ‘We draw from a list of possible observations in order to establish a scientific programme which corresponds to observation conditions,’ explains Carlos Alvarez. Until now, teams have mainly looked for brown dwarfs (sort of missing links between the stars and the planets) and gamma-ray bursts (violent explosions linked to the formation of black holes or the merger of neutron stars). ‘We have also spent quite some time on star clusters in very distant galaxies,’ finishes René Rutten. ‘But the GCT is a multipurpose tool which could be used for all sorts of research’.

Of course, at this point, we still do not know what will come of data circulated to the scientific community. But in the eyes of Carlos Alvarez, astronomers have many reasons to be satisfied. ‘On the same night, you can look for an extrasolar planet, then a galaxy and, after that, a quasar located very close to the source of the universe. Our work affects numerous fields. When you carry out your own research, you concentrate on a particular field, which will make you an expert after a few years, but that’s more limited. This profession is more general, which is precisely what I like about it.’ In a few minutes, the sun will set on a sea of clouds. The GCT opens its dome in quite surprising silence. Soon, data will begin to rain down on the row of screens lining the control room. The hunting season is open.

Audrey Binet & Laurence Buelens

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A major gain for Europe

Ninety per cent of the Great Canary Telescope (GCT)’s time will be offered to Spain, its main financial backer, and 5 % to Mexico and the University of Florida respectively. And yet, there is no doubt that the data gleaned from this telescope will travel beyond these boundaries. Within the European Research Area (ERA), ‘The GCT only began producing data very recently, which is why no project directly related to its use received funds from previous and current Framework Programmes (FP) yet. Of course, certain researchers who have been allocated observation time are part of research networks funded by FP6 and FP7,’ maintains Jesús Burgos, Director of the Offices for the Transfer of Research Results (Oficinas de Transferencia de los Resultados de Investigación – OTRI) of the Astrophysics Institute of the Canary Islands (IAC).

What’s more, the teams that participated in the GCT’s construction will be in a position to share their know-how with several European projects, including the European Extremely Large Telescope, the E-ELT, and the European Solar Telescope (EST) construction projects.

In the light of an eclipse

The Roque de los Muchachos Observatory, which has been located on the hills of La Palma since 1985, today houses 12 telescopes and tallies several astronomical discoveries on its record of achievements – both in the literal and figurative senses. Among the most recent is a study carried out by researchers from the Astrophysics Institute of the Canary Islands (Instituto Astrofísica de Canarias – IAC) which, at the end of 2008, revealed the existence of napthelene in the middle of a cloud of interstellar medium in the Perseus constellation, located 700 light years from Earth. When subjected to ultraviolet radiation and combined with water and ammonium, both of which are (relatively) abundant in the interstellar medium, this molecule is capable of producing a wide range of amino acids, which play a fundamental role in the development of life. This discovery allows us to better understand the environment in which Earth was immersed when our solar system formed, then enabling life to emerge.

More recently, in June 2009, another team of IAC researchers managed to analyse the composition of Earth’s atmosphere in detail, thanks to a lunar eclipse. The results of their study were published in the magazine Nature. The principle is simple: sunlight crosses Earth’s atmosphere, is reflected on the surface of the Moon and then directed back to Earth, where it is picked up by a telescope. This new technique allows Earth’s atmosphere to be analysed as if it were a distant planet. Scientists have thus been able to determine the biological markers of our atmosphere, information that should enable us to evaluate whether other planets possess all the necessary atmospheric conditions to harbour life.


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