An introduction to Very Long Baseline Interferometry

Astronomers are constantly seeking ways to improve techniques of observing the universe. One way they do this is by building bigger and better telescopes to get a clearer and more detailed picture of the skies. However, there are physical limitations to the size of a telescope because it must be pointed very accurately and, for optical telescopes at least, the optics must be very rigid and precisely shaped. Radio astronomers, however, have an advantage since the accuracy of their telescopes is less critical. It is not surprising then to find that the world's largest telescopes are radio antennae. In fact, the most detailed picture of the universe is obtained by combining the signals from many radio telescopes. This technique, called VLBI, or Very Long Baseline Interferometry can achieve resolutions one thousand times better than that of the Hubble Space Telescope.

Resolution

The resolution of a telescope is a measure of the amount of detail it can pick out. Technically it is the minimum separation that two points in the sky have to be in order for the telescope to be able to distinguish them. Resolution is measured as an angle on the sky. The resolution that can be achieved with a particular telescope depends on two things; the diameter of the telescope and the wavelength of the light it is receiving. The bigger the telescope and the shorter the wavelength the better the resolution. Another thing that effects resolution is the motion of the atmosphere above the telescope which blurs the images and reduces the resolution that could otherwise be achieved. When designing a mirror for a large optical telescope the accuracy of the surface must be comparable to the wavelength of light that will be used. For example, to get a clear picture in optical light the surface accuracy of the mirror must be accurate to within several thousandths of a millimetre. But for radio astronomers, who work at wavelengths measured in millimetres or centimetres, the surface accuracy of their radio dishes is not that important. They can therefore build telescopes that are much larger than their optical counterparts. Another benefit that radio astronomers have is that the motions of the atmosphere do not significantly affect radio waves so blurring of images does not hamper them. On the down side, however, since the wavelengths are much longer, it would require dishes many hundreds of kilometres wide in order to achieve the resolution obtained from much smaller mirrors working in the optical region. However, not to be outdone, radio astronomers have devised sophisticated methods of improving and even surpassing the highest resolutions obtained at optical wavelengths.

Interferometry

The technique is known as interferometry. It involves linking small radio dishes together to act like a bigger dish. To understand how this works requires a knowledge of how a single radio dish operates. Radio waves are reflected by the dish to a focus where the radio signal is converted to an electrical signal that can be processed. Imagine the surface of the dish split up into separate imaginary segments. Each segment reflects the radio waves to the focus where they become superimposed. In effect the signal received is the combined signal from each of these segments. If we have only two segments we could simulate the effect of a larger dish by moving one segment around the other and adding together all the combinations of signals. Now imagine that instead of separate segments of a single dish we use separate dishes. If we move these dishes around and add together all the signals we could synthesise the signal that would be achieved with a single large dish. However, instead of moving each dish we can allow the rotation of the Earth to change their orientation with respect to the object we are observing. This technique is known as Earth-rotation aperture synthesis. The signals from each dish are multiplied and accumulated in a process called correlation. If we sampled the signal with dishes in every possible position we would produce the image that would be obtained with a completely filled-in dish. The result, however, is never that good because there are always gaps in the synthesised dish. However, the resolution, like other telescopes, is dependent on the size of the synthesised dish. This size is equal to the largest separation, or baseline, of all the individual dishes.

In an interferometer such as this we must combine the signals from many telescopes in a specially designed computer called a correlator. If the telescopes are sufficiently close we can pass the signals along cables or fibres to the correlator. In the US the Enhanced Very Large Array (EVLA) is a group of 27 antennae in a Y-shape whose signals are sent to a correlator along fibres. The maximum baseline of the VLA is 36 kilometres. In the UK the e-MERLIN instrument, operated by the University of Manchester at Jodrell Bank Observatory, consists of six telescopes dotted around the country giving a maximum baseline of 217 kilometres. MERLIN signals are also transmitted via dedicated optical fibre to a correlator at Jodrell Bank. In this kind of telescope astronomers have to ensure the signals from each telescope remain coherent. This means combining the signals arriving at precisely the same time at each telescope. In order to do this we need to know exactly how long the cables or transmission links are. This is easy to measure with short baselines but for baselines more than a few hundred kilometres it becomes extremely difficult.

VLBI

However, astronomers have extended the interferometer technique to baselines of many thousands of kilometres. The solution is simple but ingenious. In order to combine the signals from each separate antenna we need to know when they arrived at the telescope. So instead of correlating the individual signals as they arrive we can record them on tape along with the precise time. If the clocks at each antenna agree exactly then we can combine them at a later date by lining up all the times of arrival. This technique is known as Very Long Baseline Interferometry or VLBI for short.

VLBI was developed mainly in the US in the 1960's but with significant contributions from the UK, Australia and later Germany in the late 1970's. The technique requires independent telescopes situated all over the world and several networks of such telescopes now exist. In the US the Very Long Baseline Array (VLBA) extends from Hawaii to the Virgin Islands. In Europe the European VLBI Network (EVN) includes telescopes in the UK, Germany, Holland, Sweden, Poland, Italy and Spain. In the UK Jodrell Bank routinely participates in VLBI observations with one of several telescopes.

Technically VLBI is a very difficult technique. Each of the individual telescopes must observe the same object at the same time and at the same wavelength. The signals from each telescope are recorded on large magnetic tapes over three miles long which are similar to those used in high-quality TV video recording. Along with the radio signal the precise time is recorded on the tape. To allow the signals to be correlated later the time must be known to a very high level of accuracy. Most VLBI observatories use atomic clocks such as a hydrogen maser clock which gives the time to within several millionths of a second. Once the observations are complete each observatory sends their tapes to a central correlating centre where all the signals are lined up, combined and processed to reveal the final high-resolution image of the object.

Ground based optical telescopes such as the William Herschel Telescope (WHT) on La Palma can achieve resolutions of about 1 arcsecond (1 arcsecond = 1/3600th of a degree). The angular diameter of the Sun is about 1800 arcseconds. Because the Hubble Space Telescope is above the blurring atmosphere it can achieve resolutions of up to 50 milliarcseconds (1 mas = 0.001 arcseconds). Even at the much longer radio wavelengths the MERLIN interferometer achieves resolutions comparable to the Hubble and so is ideal for comparing objects seen in radio and optical light. With continental or global VLBI baselines the resolution can be as good as several milliarcseconds. This is equivalent to being able to see a small car at the distance of the moon. Ensuring the coherence of radio waves is possible simply because the separation of the individual waves is large (millimetres or centimetres). Coherence at optical wavelengths is much more difficult to achieve because of the shorter wavelengths and because the motion of the atmosphere destroys coherence. However, astronomers have recently begun to apply the interferometer technique at optical wavelengths but are not yet able to achieve coherence over baselines more than several tens of metres. So, at present, radio astronomers have the upper hand as far as achieving resolution goes.

Science with VLBI

The high resolution achieved with the VLBI technique has contributed in many areas of astronomical understanding. Topics routinely studied using VLBI include the energetic jets in distant quasars and radio galaxies, gravitational lenses, accretion-powered active galactic nuclei, binary stars where accretion onto compact objects takes place and the maser emission in star forming regions in our own Galaxy.

Radio interferometry has been used to study the Sun, planets, asteroids and comets. In 1994 observations revealed that the impact of Comet Shoemaker-Levy 9 with Jupiter severely disrupted the radio emission coming from the planet's radiation belts. Other planetary studies have included monitoring atmospheric water vapour on Mars and investigating the structure of Saturn's rings.

Further afield, all kinds of stars, from young to old, hot to cool, violent to passive are studied using VLBI. Stars emit radio waves through a variety of mechanisms depending, amongst other things, on the temperature of the material and the presence or absence of magnetic fields. Using VLBI observations astronomers can tell something about the origin of the radio emission in objects and their surroundings. Common stellar targets for VLBI observations are cool dwarf stars which show flaring and coronal emission much like the Sun, young pre-main-sequence stars and Wolf-Rayet binaries where the radio emission may be caused by the collision of energetic stellar winds. X-ray binaries are a class of object that contain a compact star, perhaps a neutron star or black hole, in orbit with another normal star. The compact object pulls material off the normal star into an accretion disk. As the material spirals onto the compact object energetic jets of hot plasma can be ejected at velocities close to the speed of light. VLBI observations have been crucial in our understanding of X-ray binaries.

Masers are sources of radio waves caused by the excitation of molecules in space. Molecules such as water (H₂O), hydroxyl (OH) and methanol (CH₃OH) are often found in outflow regions around star forming regions and in the thick circumstellar shells of evolved stars. VLBI observations have been used to study the physical conditions in these regions as well as the motions of the material. Some observations give information on the condition of the intervening interstellar medium.

A large proportion of VLBI observations are of the radio emission from active galactic nuclei including quasars, radio galaxies and Seyfert galaxies. It is believed that supermassive black holes exist at the centre of these objects. A mechanism similar to that in X-ray binaries creates extremely energetic jets of material racing in opposite directions from the core of the galaxy. Often these jets or beams interact with the ambient material of the galaxy creating complex radio knots and lobes either side of the optical image. The high resolution obtained with VLBI allows detailed study of the paths of jets and allows astronomers to examine the magnetic field strengths and particle densities. By observing at different times astronomers have also followed the motion of extragalactic jets. Often these features display superluminal motion. This is an optical illusion caused by material moving almost directly towards us at close to the speed of light. When this happens the material appears to move outwards at velocities greater than that of light.

There is another application of the VLBI technique that is not strictly astronomical. This is geodesy, the measurement of the Earth. By observing the times of arrival of radio waves from distant quasars it is possible to determine the position of a radio telescope very accurately; to within a few millimetres. Using an array of antennas the small changes in the telescope positions allow scientists to study tectonic plate motions and other geological movements of the Earth's crust.

Space VLBI

Not satisfied with baselines almost the size of the Earth radio astronomers have pushed the VLBI technique to unprecedented extremes. On February 12th 1997 the Japanese Institute of Space and Astronautical Science (ISAS) launched a satellite now known as HALCA into Earth orbit. HALCA consists of an eight-metre wire-mesh antenna and acts as one element of an interferometer with other antennae dotted all over the surface of the Earth. With a maximum baseline length of 200,000 kilometres the synthesised aperture is almost three times the size of the Earth. The resolution achievable with this system can be as good as about one tenth of a milliarsecond. This is at least a thousand times better than that of the orbiting Hubble Space Telescope at optical wavelengths. This is equivalent to being able to see a football at the distance of the moon. Since its launch over a year ago the HALCA satellite has operated very successfully. Because the satellite antenna is quite small the sensitivity of space VLBI measurements is low and astronomers have had to study only the brightest radio objects. These include quasars and radio galaxies and some galactic maser sources. The extremely high resolution has enabled astronomers to peer into the very heart of these objects and get a unique view of the most compact and distant objects in the universe. As is often the case in science, with the new leap in technology, astronomers now have more questions to answer about the universe at large.

Conclusion

Although VLBI networks consist of individual radio telescope around the world or in Earth orbit, they can rightly be regarded as the world's largest telescopes. Their resolution greatly surpasses that possible at shorter wavelengths and will continue to do so for the foreseeable future. VLBI has given astronomers an intriguing view of the details of many kinds of objects. In the future the technique will continue to be developed.