The world's largest telescope – the Extremely Large Telescope (ELT) – is under construction in Chile. When it captures its "first light", sometime in 2027 or 2028, Irish adaptive optics technology will be there to ensure it sees further and with greater clarity than any telescope in human history.
The opportunity for Irish astronomers to take part in the ELT project arose when the government decided to join the European Southern Observatory (ESO) – the top intergovernmental astronomy organisation in Europe – in 2018. Membership cost €14.66 million, with an annual fee of €3.5 million.
A team of researchers at NUI Galway, led by Dr Nicholas Devaney, with expertise in adaptive optics are involved in the ELT project as part of a consortium also involving the Grenoble Institute for Planetary Sciences and Astrophysics and the National Institute of Astrophysics (INAF) in Italy.
The consortium will design and manage the construction of an instrument on the ELT, called multi-conjugate adaptive optics relay (MAORY), which corrects image distortion due to atmosphere blurring. The NUIG team were invited to join the MAORY project based on their scientific reputation.
"The Galway team is responsible for the device we call the test unit that is needed to pass all the performance on this domain here in Europe and then also when we arrive on the mountains in Chile," says Paolo Ciliegi, an astronomer at INAF; the overall principal investigator of MAORY.
“They put on the table their expertise in adaptive optics and also the construction of this test unit,” Ciliegi adds.
The construction of the ELT at an altitude of some 10,000 feet on top of a mountain called Cerro Amazones has halted due to the Covid situation in Chile. The site is in the Atacama Desert, a high plateau covering an area slightly bigger than Ireland, and made up mostly of stones, salt and sand.
The altitude puts it above the cloud line, so there is very little precipitation, which can distort telescope images of space. That dryness – this is the driest desert on the planet outside the poles – make it an ideal location for astronomers to view the heavens. Yet the ELT must still peer up and out through about 480km of atmosphere, with the distortion that this brings.
“When you feel the bumpiness in an airplane that’s the atmospheric turbulence,” says Devaney. The turbulent atmosphere, he says, is made up of bubbles of air with differing temperatures. The speed of light through air varies slightly with the temperature of the air through which it travels.
The net effect of this is to reduce the sharpness of images from space that a ground telescope can gather. “That introduces distortions in the light which leads to a blurry image instead of a sharper image,” he adds.
Adaptive optics technology works hard to overcome such atmospheric distortion. This task is akin to gathering light that has been bent and scattered in water and rebuilding it into its underformed original form. This is the job that the MAORY instrument will be performing for the ELT.
A limitation of adaptive optics technology up to now has been that it relies on a natural constellation of bright stars to sharpen distorted images from an optical telescope viewing a big area of sky, but such constellations are not always available. In order to get over this issue scientists use guide stars.
The ELT is going to generate six artificial laser-generated guide stars which will act like a natural constellation of six bright stars to facilitate adaptive optics to work wherever the ELT is pointing towards in the sky. It has proved a huge challenge over decades to get the lasers up to sufficient power to produce bright enough guide stars to facilitate adaptive optics.
After much research scientists decided to use a sodium wavelength for producing guide stars. This is because there is a natural layer of charged sodium ions in the Earth’s atmosphere at an altitude of 90km, which can be excited and energized by a laser so that it looks just like a natural star.
“This is perfect for astronomers,” says Devaney. “It’s like the ions were put out there specifically for that purpose. It means that it is possible to make constellations of artificial guide stars using the six lasers on the ELT.”
An optical telescope works by gathering light through mirrors. The bigger its mirrors the more light the telescope can gather and the farther it can see. The main mirror of the ELT will be an enormous 39 metres ( 127.9ft), in diameter. That’s roughly equivalent to 21 men, six feet tall, lying head to toe.
The designers knew that technically it wasn’t possible to construct the main mirror as one piece. They also knew that it would be difficult to carry large mirror segments to a mountain top. A decision was therefore made to separately make 798 hexagonal-shaped segments; each 1.5 metres wide weighing 250kg, which, when aligned carefully together, would make up the main ELT mirror.
The mirror segments had to be aligned with nano-metre precision, and that alignment has to be maintained as the telescope moves and tracks objects. There are some 9,000 tiny sensors arranged around each segment so that any kind of motion in one segment with respect to another is accounted for.
There are also actuators that bend the mirrors into optimum shape. The biggest optical telescopes today have three mirrors. The ELT will have five.
In return for Devaney's team working on the adaptive optics on the ELT his astronomer colleagues at NUIG are to be offered ELT observation time. One of those scientists hoping to use the ELT to advance his work is physicist Dr Matt Redman, director of the centre of astronomy at NUIG.
Redman is interested in planetary nebulae. These are badly named celestial objects as they have nothing to do with planets. They looked like planets when viewed by the first telescopes so that’s how they got the name. They might better be described as the glowing shell of gas ejected from a dying star.
These nebulae are observed in a variety of shapes including butterfly-shaped, elliptical, spherical, ring-shaped, bi-polar, cylindrical and round.
“The big mystery is that the Sun is round, spherical and will turn into one of these objects, and these objects are not round and spherical,” says Redman. “The most likely idea is a companion star, or even a companion planet, disturbing the material as the dying star throws it off,” he explains.
“I am hoping the MAORY will be able to get right into the centre of these objects and we might even see that shaping mechanism happening,” he adds.
There are some who question the economic and scientific logic of building expensive telescopes on the top of Chilean mountains in order to see through atmospheric distortion when it is possible to put a space telescope, like the Hubble telescope, into orbit up where atmospheric distortion is not a factor.
The justification lies in the cost of getting telescopes into orbit against building them on Earth. The Hubble Space Telescope, which had a primary mirror 2.4metres wide, cost €2.5 billion (today equivalent) to get into orbit and operational. The ELT will cost some €1.3 billion; about half the price.
This point of view holds that although they do different things, ground-based telescopes like ELT give more scientific bang for your buck than space telescopes. The James Webb Space Telescope (JWST), set to launch in November, will cost €8.2 billion.
The ELT sees farther, clearer. “You are able to collect a lot more, like with a 39-metre mirror,” says Devaney. “You are able to see further away and see things that are much fainter, such as really faint galaxies. The ELT will be able to see things that are fainter than was possible with the Hubble.”
The huge jump in astronomical capability that the ELT will provide is likely to trigger a round of unexpected scientific findings that will change our understanding of the Universe and how it was formed in its earliest days.
We’ve seen it before. For example, in 1998 data from the Hubble led scientists to conclude the universe was expanding at an ever accelerating rate. “Each time there is a big step forward like this it leads to a huge mushrooming of astronomical activities and discoveries,” says Devaney.