Giant telescope's double vision


Nov 1, 2007

Almost 20 years after it was first conceived, what will become the world's most powerful optical telescope is about to open its eyes.

Lying beneath the clear skies of Arizona, the $120m (£55m) Large Binocular Telescope will allow astronomers to probe the Universe further back in time and in more detail than ever before.

"The LBT is a very exciting step forward for astronomy," said Professor Gerry Gilmore of the Institute of Astronomy at the University of Cambridge, UK.

"Not only is it big, but it is proving the practical implementation of some of the new technologies which will be critical for all next-generation large telescopes."

Unlike most telescopes today, which consist of one light collecting mirror, the binocular telescope will consist of two 8.4m (27.5ft) discs used in tandem.

"Astronomers are looking for two things in a telescope," explained Dr John Hill of the University of Arizona and the technical director of LBT.

"They want a big collecting area so they can look at really faint objects far, far away; and they want high resolution images because they want sharp images of those faint fuzzy things far, far away."


Hence, astronomers crave the largest mirrors they can get their hands on, as the larger the disc, the more light they are able to gather.

But constructing these giant reflectors is a difficult, expensive and time-consuming task.

With present technology, the largest mirrors that can realistically be constructed are about 8m (26ft). And even they come with their own difficulties.

"Handling and shipping become a big problem," said Dr Hill. "Already an 8.4m mirror in its box takes up two lanes of an interstate highway."

As a result, scientists have had to come up with clever ways of maximising the potential of today's mirrors.

One way of doing this is by using multiple reflectors in tandem.

This has been done before. For example, the now retired Multiple Mirror Telescope, also in Arizona, contained six mirrors each with a diameter of 1.8m.

However, scientist have never tried to combine them on the scale of the LBT.

"Everything about this is a first time experiment," said Dr Richard Green, director of the instrument.

Using two mirrors will give LBT the equivalent light-gathering capacity of a single 11.8m (39ft) instrument and the resolution of an even bigger telescope.

"It acts like a single telescope of 22.8m (75ft) in diameter," said Dr Green.

"That will give us a resolution - a sharpness on the sky - which is 10 times greater than Hubble."

The space-based Hubble telescope has taken some of the clearest images yet of the Universe with its 2.4m (8ft) mirror.


But Hubble has a significant advantage over ground-based instruments.

"The bubbling and windy air in the Earth's atmosphere creates distortions and blurs the stars," said Dr Green. "We need to un-blur them in order to combine both sides and make them work like one unit."

To do this, the telescope has two secondary bowl-shaped mirrors, each 1.6 mm thick and covered with 672 magnets on the back.

A computer analyses the light from a reference star and works out by how much the light has been distorted.

The magnets then kick into action, changing the shape of the secondary mirror 1,000 times a second to correct for the atmospheric distortions.

The secondary mirrors will be delivered in 2008 and instruments that will take advantage of this so-called adaptive optics system will be up and running the following year.

One of those will be the LBT inferometer (LBTI).

"It's designed as what's called a nulling inferometer that lets you reverse the phase of the light when it's combined so you cancel the light from a bright star and look for a faint planet next to a star," explained Dr Hill.

The instrument should allow astronomers an unprecedented ability to image these extrasolar planets - bodies thought to be good candidates for looking for extraterrestrial life.

Another inferometer will be used to create the crystal clear images astronomers want.

"What that super sharpness will allow is to look at planets and forming planet systems in nearby stars," said Dr Green.

"If you can concentrate that bright star in the middle to a very small area, you can then look around it at high contrast and see the discs of dust and gas that are forming into planets."

Other researchers, he said, would focus on probing the hearts of nearby galaxies to understand supermassive black holes.


Although, scientists cannot combine the image from both mirrors just yet, the flexible design of the telescope means that useful science can still be done using each mirror individually.

"We are already doing imaging projects for everything from looking for asteroids in our Solar System to doing photometry of stars in nearby galaxies," said Dr Hill.

This work is currently being done with a wide-field ultraviolet and blue spectrum camera, known as the Large Binocular Camera (LBC), situated above the left-hand primary mirror.

Using this instrument, LBT achieved "first light" on 12 October 2005 when it imaged a spiral galaxy in the constellation of Andromeda.

But LBT will not truly have arrived until it opens both of its eyes and achieves "second light" - taking images simultaneously with instruments positioned above both mirrors.

This is scheduled to happen "sometime this winter" according to Dr Hill when the observatory takes delivery of another LBC able to take images in the infrared.

"It will work like two 8m telescopes working together to look to the very ends of the Universe," said Dr Green.

Science hopes

Dr Xiaohui Fan of the University of Arizona is a cosmologist who is eager to work with the instrument when it is up and running.

He looks for and studies the most distant objects in the Universe, at about 900 million years after the Big Bang.

"It appears to be an exciting era for the Universe," he explained.

"At this time, the first generations of galaxies and quasars and black holes were forming."

The power of LBT combined with its ability to image multiple objects at once and in multiple spectrums would allow him to search more objects, more quickly and in more detail, he said.

"My goal is to push that distance limit to even further away - to something like the first 800m years or less," he said.

"That is where I think LBT is going to be very important."

But others are less sure about what LBT will allow, although they are confident that it will push the boundaries of astronomy.

"The exciting thing about building a big telescope is that you never know what kind of discovery you will make," said Dr Hill.

"It will probably discover things that people haven't even thought of yet."