Jodrell Bank

Interferometers

The smaller Mark II radio telescope, with its 25 x 38-m elliptical bowl, can be connected to the Lovell Telescope to form an interferometer

A big telescope like the Lovell has a very large collecting area. It is therefore highly sensitive and is ideal for observing radio sources which are weak or which fluctuate rapidly. A sensitive telescope can make observations faster than a less sensitive telescope, and so observe more objects in a shorter time. It can also detect objects that are fainter and therefore further away. About half the observing time of the Lovell Telescope is spent working alone as a `single-dish' telescope on projects where sensitivity is critical.

But astronomers are not only interested in sensitivity. They also want to be able to see the detailed structure of astronomical objects. Here again, the bigger the telescope, the sharper the images it can produce. But because radio waves are so much longer than waves of visible light, even the Lovell Telescope cannot match optical telescopes in `resolution' - the ability to discern fine detail in the sky (see box below).

To get around this handicap, astronomers have developed the technique of radio interferometry, in which telescopes are connected together to work as a single , much larger telescope. The resolution depends on the distance between them (the `baseline'), so can be greatly increased simply by placing the telescopes further apart. Interferometry gives radio astronomers the means to achieve high resolution without building impossibly large telescopes.

About half the observing time of the Lovell Telescope is spent working with other telescopes to combine the advantages of the Lovell's large collecting area with advantages of wide separations. The technique of bringing the radio signals from the telescopes differs depending on how far apart they are, but the principle is always the same.

Modern radio interferometry would not be possible without formidable computing power and sophisticated software. Jodrell Bank scientists have contributed greatly to the development of image processing techniques required to turn complex radio signals into the beautiful images seen in these pages.

Lovell-Mark II interferometer

In the simplest interferometer, two telescopes are connected. The other large telescope at Jodrell Bank, the 25-m Mark II, is occasionally linked to the Lovell Telescope to form an interferometer with a baseline of 425 metres. The telescopes are connected by cables, with the radio signals from both brought together in the Lovell Telescope observing room. With a resolution of about 0.5 arcmin the interferometer combines sensitivity with precision and is useful for survey work and for measuring accurate positions of faint objects.

One of the distant elements of MERLIN - the 32-m telescope at Cambridge

MERLIN

Larger interferometers need more sophisticated techniques. MERLIN, the Multi-Element Radio-Linked Interferometer Network, consists of seven separate telescopes: the Lovell or Mark II at Jodrell Bank, the Mark III telescope and two 25-m telescopes elsewhere in Cheshire, another two 25-m telescopes in Shropshire and Worcestershire, and the far-flung 32-m telescope at Cambridge.

With baselines up to 217 km, it is impractical to connect the telescope by cable, Instead, the signals from each telescope are transmitted by microwave relay stations back to Jodrell Bank where they are combined (or `correlated') and then analysed by computer to produce detailed images of radio sources.

MERLIN has a resolution of around 0.05 arcsec; about the same as the Hubble Space Telescope at visible wavelengths. The Lovell Telescope is an important component of MERLIN, since its large collecting area provides the sensitivity that the smaller telescopes lack. More information may be found in the Jodrell Bank booklet, Observing the Universe with MERLIN.

VLBI

In a technique called VLBI (Very Long Baseline Interferometry) very widely spaced telescopes observe the same object at the same time. Signals are recorded on magnetic tapes, which are then played back together at a later stage. Provided that accurate timing is available, it does not matter that the radio signals are coming from a tape recording rather than a 'live' telescope. VLBI can achieve resolutions of 0.001 arcsec, far better than in any other branch of astronomy.

An observer in the VLBI room. The tape deck in the background is recording the radio signals for future analysis.

Together with large telescopes in Germany and the Netherlands, the Lovell Telescope forms the core of the European VLBI Network (EVN) which has regular programmes of collaborative observing. On a larger scale still, the Lovell Telescope routinely works with radio observatories all over the world to create a telescope the size of the Earth.

Space VLBI

Interferometers are not limited by the size of the the Earth. The Lovell Telescope will play a major role in space VLBI, where a telescope on an orbiting satellite can extend baselines to tens of thousands of kilometres. Two such satellites are planned in the next few years, Radioastron (Russia) and VSOP (Japan). Each satellite will form an interferometer with existing VLBI networks, including the Lovell Telescope.

Angular sizes and resolution

Since the sky appears to use us as the inside of a sphere, it is convenient to measure the apparent sizes of astronomical objects in angular measure. So Orion's belt is about 2.5 degrees long, the Sun and the Moon are each about 0.5 degrees in diameter, and the planet Venus at its closet is about 1 arcminute in diameter. The stars are so far away that even the biggest and nearest are much less than 1 arcsecond in diameter.

The resolution of a telescope is a measure of the fineness of angular detail it can make out in the sky. It is proportional to the wavelength divided by the width of the telescope. For the Lovell Telescope observing the 21-cm hydrogen line, the resolution is about 0.2 degrees - much poorer than the human eye. This is because radio waves are so much longer than visible light. Until interferometry was invented, radio astronomers had no practical way of making high-resolution observations of the the sky. With VLBI, resolutions measured in thousandths of an arcsecond are now possible - equivalent to seeing a penny coin a distance of 4000 km - far better than the best ground-based optical telescopes.

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