Astrophysics Group

Cavendish Laboratory

COAST A Gentle Introduction

Building the world’s largest optical telescope

Ever since Galileo first turned his telescope towards Jupiter in 1609, astronomers have wanted bigger and bigger telescopes. Larger telescopes bring two advantages. First, the possibility to study fainter and more distant objects, and second, the ability to study finer and finer detail. The limitations of current technology mean that the largest optical telescopes produced today have collecting mirrors up to 10m in diameter. This is a far cry from Galileo’s telescope, but not nearly large enough to explore some of the most exciting phenomena occurring in our Galaxy and beyond, too small to be seen with telescopes of even this size. Surprisingly the way ahead is to use arrays of small telescopes!

Conventional telescopes reveal fine detail by bringing together light from different parts of the collecting mirror to a single focus. The larger the mirror, the further apart these parts can be, and the sharper the resulting image. One of the most amazing developments in optics this century was the realization that not all the pieces of the telescope mirror need be present at the same time! We can see how this works in the following way:

Take a large telescope and mask out most of its mirror apart from a few small patches. These still direct light to the focus where it can be recorded with a detector. Then repeat this procedure, but using different patches of the mirror that were previously covered. If we continue this process, using different parts of the mirror each time, it turns out that we can analyse all the separate recordings to produce one picture with the same detail as though we had used the whole mirror at once. Astronomers call this technique “aperture synthesis”, because we synthesise a large telescope by using much smaller pieces of it at any one time.

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Figure 1: Close-up of one of the COAST telescopes. The beam combination building, together with one of the other telescope covers, can be seen in the background.

In the example above we considered using different parts of a single large mirror sequentially. But the small pieces of mirror used at any instant need not be part of a much larger one. An array of small telescopes will do the job just as well, while being far easier and cheaper to build, but with one problem: the telescopes have to be moved round on the ground, so as to trace out the larger mirror they are trying to synthesise. This sounds like hard work, but fortunately we can let the rotation of the earth move the telescopes instead, as long as we are willing to wait a bit!

The big advantage of using individual small telescopes is that they can be separated as far apart as we like. Since the finest detail in an image depends on the maximum separation between the mirrors collecting the light, this implies that we can make images of stars as though we had telescopes with mirrors hundreds of metres in size. It would be impossibly difficult to build a conventional telescope as large as that, so aperture synthesis is the only way to achieve the sharp images that astronomers really want.

Although the principles of aperture synthesis have been known for many years, and are used regularly in radio astronomy, astronomers had thought that building an optical telescope using this principle was just too difficult. Light waves are about a million times shorter than radio waves, so the precision required in building such a telescope is a million times more challenging than for radio synthesis instruments.

Our team in Cambridge has, however, recently commissioned the world’s first optical aperture synthesis telescope. The instrument, called COAST, currently incorporates five 40cm telescopes arranged in a “Y” configuration, with a maximum telescope-telescope separation of approximately 22m. Up to four of the telescopes are used simultaneously. COAST already produces images showing several times more detail than the Hubble Space Telescope at less than a thousandth of the cost, and has been designed to operate with the telescopes up to 100m apart allowing another factor of 10 improvement in the sharpness of its images.

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Figure 2: Schematic view of COAST, showing four telescopes and the light paths from the source, via the telescopes into the beam-combination laboratory.

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Figure 3: A view inside the COAST beam combining laboratory. The movable mirrors that ensure equal path lengths for each of the four telescope beams can be seen on railway tracks in the distance.

In COAST, light from the star is first collected by four telescopes. These send four parallel beams into a beam-combination laboratory via guiding mirrors located at the rear of one of the central telescopes. Once in the laboratory the light from each telescope passes between a series of movable mirrors so that the total distance travelled by each beam is precisely the same. The beams are then combined using more optics and detected by a set of low-noise fast-readout detectors. The signals from these are recorded in computer memory.

During the night, the motion of the earth moves the telescopes as seen from the star, tracing out the imaginary giant mirror COAST is synthesising. Data collected over many hours are then processed to make a high-quality map of the source being observed.

The design of instruments like COAST makes it possible to overcome another of the fundamental problems of ground-based telescopes — the effects of the earth’s atmosphere. Even if extremely large telescopes could be built, the quality of the images they would produce would be far poorer than expected on the basis of their optical quality.

Because of the effects of turbulence, looking through the earth’s atmosphere is like looking through a layer of water with ripples on its surface. One bad effect of this is the apparent “twinkling” of the stars. Another is that images taken with conventional ground-based telescopes are never as sharp as expected, usually up to 50 times poorer than could be achieved if the atmosphere were not present.

Because it uses relatively small telescopes, as well as active control of some of its optical surfaces, COAST can successfully overcome these atmospheric fluctuations. A good example of this is shown below, where the very first images obtained with COAST are displayed. When the images were taken, the separation of the COAST telescopes was comparable to that of the largest telescopes now in existence. However, the detail seen in the map is over 50 times finer than could have been seen with these telescopes, essentially because their imaging quality is limited by the atmosphere, whereas COAST’s is not.

Figure 4: The first pair of COAST images, of the close binary star Capella. In a conventional telescope this star looks as though it is a single object. With COAST it is seen to be a binary system with a separation of 1/20 arcsecond, with the two components orbiting each other every 104 days. The two images were taken two weeks apart and clearly show the orbital motion of the pair about their common centre of mass.

Figure 4: The first pair of COAST images, of the close binary star Capella. In a conventional telescope this star looks as though it is a single object. With COAST it is seen to be a binary system with a separation of 1/20 arcsecond, with the two components orbiting each other every 104 days. The two images were taken two weeks apart and clearly show the orbital motion of the pair about their common centre of mass.

The next aim of COAST will be to image the surfaces of nearby evolved giant stars. The energy output of these stars is so great that much of their interior is “bubbling” like a pot of boiling water. Their surfaces show bright features where we believe hot gas from their interiors has burst out, boiling off their atmospheres and spewing material into outer space.

The fantastic detail made possible by aperture synthesis means that now astronomers will be able to probe some of the most exciting objects in the sky. These include pulsating variable stars, which shed layers of material as they pulse in and out, interacting binary stars, where matter can be sucked off one star and fall into the gravitational well of a companion, and even black holes as they relentlessly grab all the matter around them. Many other alternatives are possible too, such as monitoring the motions of stars in the centre of our galaxy, or even exploring the nuclei of the nearest active galaxies, which harbour massive black holes in their centres.

What can we expect in the future? COAST is the first telescope of its type to operate successfully anywhere in the world. Others are planned or under construction in France, Chile, Australia and the USA, although few of these will have the same capabilities as COAST. In as short as 10 years we can expect arrays with many more telescopes than COAST, much larger collectors, and maybe even synthesising telescope apertures up to a kilometre in size. Telescopes of this kind will certainly play a major role in the future evolution of astronomy, and who knows what scientific treats are in store!

Funding for COAST was provided principally by the Particle Physics and Astronomy Research Council (PPARC) with further support from the University of Cambridge, of which both the Mullard Radio Astronomy Observatory at the Cavendish Laboratory, and the Institute of Astronomy are parts. The total cost of the telescope was £850,000.