By Paul Rincon
Science reporter, BBC News
On a dusty mountaintop in the Atacama desert of northern Chile, a large telescope funded and led by Britain is taking shape.
Vista was a payment in kind for the UK joining Eso
The £36m facility is gradually being assembled near the Chilean peak of Cerro Paranal.
Vista (Visible and Infrared Survey Telescope for Astronomy) will be part of the European Southern Observatory (Eso).
Eso is a research organisation comprising 13 member countries - including the UK.
The telescope will use infrared wavelengths to detect objects that are too distant or too cool to be seen using the visible spectrum.
Vista's task has been likened to looking for needles in astronomical haystacks.
Its solution is to combine a telescope with a moderately large (4m) mirror to detect distant objects, with a large field of view, so it can survey a large area.
Most telescopes of 4m or above have a narrow field of view; Vista has the widest field of view for a telescope of its size.
Getting the best quality pictures from a relatively wide field telescope required a careful balancing of different factors when it came to designing the telescope.
4m telescope will survey the Southern Hemisphere sky
Widest field of view for a telescope of its size
Near infrared camera with 67 million pixels of mean size
Interior of camera must operate in a vacuum at -200C
Camera weighs about as much as an Indian elephant
Vista's design, which uses two mirrors, produces a large field of view but at the expense of some residual image distortions.
These distortions can be corrected by lenses. But if too much bending of light is done by lenses, they make different wavelengths of light focus in slightly different places. This blurs the resulting image.
Mirrors, on the other hand, bend all wavelengths of light the same.
So Vista's designers came to the following compromise: most of the light coming into the telescope would be bent by the mirrors. Residual distortions would then be corrected by a series of three lenses inside the telescope's camera.
"Vista is very much a system. You won't get a nice image from the telescope alone, you need the telescope and the camera working together," says Jim Emerson, Vista project leader, from Queen Mary, University of London.
Vista's 3m-long, 2.9 tonne camera - the biggest of its kind in the world - is another impressive feat of engineering.
"It's a huge beast," says Kim Ward, Vista's camera manager, "It has the largest array of infrared detectors and everything about it is just massive. Part of this has come about through having to make it earthquake-proof."
The Vista site is on an active seismic zone. Thus, the camera had to be more sturdy than would normally be necessary. It must be able to shrug-off weak and medium-strength earthquakes and survive largely intact through strong ones.
The camera may be the biggest of its type anywhere
The sensitive infrared detectors are required to work at a temperature of -200C, so the camera has a cryogenic system to keep the interior cool; it must also operate in a vacuum.
"We have pumps and coolers attached around the camera to maintain the temperature and the vacuum," says Kim Ward.
The camera, he says, is a bit "like an inside-out spacecraft."
Vista will be able to see through the smog of interstellar dust clouds that obscure much of our galaxy. It is designed to detect very faint galaxies and will be able to detect very cool cosmic objects that do not emit much visible light.
The data will tell astronomers a lot more about these objects. But in some cases, the telescope will identify faint astronomical bodies for follow-up observations using other observatories such as the Hubble Space Telescope.
In one of the first surveys planned for Vista, astronomers will look at an area of the sky known as the Cosmos Field.
Eagle Nebula: In the infrared (L) and at Hubble's visible wavelengths
By looking deep into the Cosmos Field, Vista will also be looking a long way back in time, to the Universe as it was in its early years. The aim will be to understand how galaxies - like the Milky Way - are assembled from their constituent parts.
"Vista will be looking for old galaxies in two square degrees of sky. The idea is that galaxies are assembled bit-by-bit. Galaxies accrete more and more matter with time," says Jim Emerson.
"So galaxies should look reasonably chaotic a long time ago. As other pieces get added, they should start to look statistically more like galaxies do nowadays."
At the other end of the spectrum of tasks Vista will carry out is a project called the Vista Hemisphere Survey. This will aim to survey the entire southern hemisphere.
By contrast with the Cosmos Field survey, which goes very deep but only surveys a small volume of the sky, the Hemisphere Survey will not peer very deep into space, but will survey a large volume of sky.
Astronomers will carry out lots of short exposures of one part of the sky and then carry on to the next part.
The Atacama is one of the best places to do astronomy in the world
Even so, Jim Emerson explains: "It will be about four magnitudes deeper than the previous all-sky survey in the infrared. And it will be in more wavelength bands."
How objects in the sky look at different wavelengths in the electromagnetic spectrum will help astronomers classify them. They will then hone in on rare objects.
"You can do a lot of science on these objects if you can pin down their properties precisely. You can also find exciting new objects such as very high-Redshift quasars," says Jim Emerson.
Quasars are compact, luminous objects thought to be powered by supermassive black holes and are thought to have played an important role in the evolution of the Universe. Those at the highest "Redshifts" are the most distant, and therefore the youngest, class of quasars.
"You don't know where rare objects are to start with. So you have to observe the whole hemisphere to find these things, and you need to find several as finding one doesn't give you a statistical sample of these objects."
Other Vista projects include:
- an effort to map the "bulge" of the Milky Way which will help give a better idea of our galaxy's shape
- a study of the Milky Way's galactic plane, which will search for star-forming regions
- a survey of the Magellanic Clouds, the two nearest galaxies to the Milky Way
- using the colours of galaxies in order to measure their distances and to map their 3-D distributions
These surveys will take about five years to complete. Some projects can only be undertaken at certain times of the year. Others are dependent on having certain atmospheric conditions.
"All the projects should produce science results that help answer questions we already have. But the most exciting thing may be something we have no idea about that we discover not having set out to discover it," says Professor Emerson.
When the UK was negotiating to join Eso, Vista became an in-kind payment towards the subscription for joining.
All the crucial elements of the telescope are now in place except for the main mirror.
The Russian company polishing the primary mirror needed more time to complete the task, delaying its arrival in Chile.
"Normally, telescope mirrors are subtly curved. This one looks a bit more like a saucer. It is curvier than usual and comes up a lot at the edges," says Professor Emerson.
"No one has polished a mirror this big of this shape before."
The secondary mirror is now complete and in Chile. The telescope's primary mirror will be finished in July. It will be flown to Chile on an Antonov transport plane from Moscow.
It is hoped that Vista will be able to take its first pictures towards the end of 2007.
VISTA'S WIDE FIELD OF VIEW
Telescopes of 4m or above generally have a narrow field of view. The 8.2m Very Large Telescope (VLT) falls into this category
The 4m Vista telescope is an exception. It will have the widest field of view for a telescope of its size, allowing it to survey a large area