Astronomy Principles and Practice Fourth Edition.pdf

366 Radio telescopes
The record from the arrangement is depicted in figure 21.15(ii) displaying that the fringe amplitude has
doubled and that the effects of P B and P S have been removed.
Problems of source identification, occurring with the two-beam interferometer as a result of the
strong secondary lobes, are generally reduced by using multi-element interferometers. The designs
normally allow the separations between the elements to be adjusted from day to day. For example, the
Ryle Telescope W21.3 at the Mullard Radio Astronomy Laboratory of the University of Cambridge, is an
eight-element interferometer operating at 15 GHz. The components are 13 m Cassegrain antennas on
an east–west baseline. Four antennas are mounted on a 1·2 km rail track with the others fixed at 1·2km
intervals. Baselines between 18 m and 4·8 km are, therefore, available in a variety of configurations.
Long baseline interferometry (LBI) is also possible by linking the outputs from individual
elements located at separated operating stations in such a way that the phase information is preserved.
An LBI array known as MERLIN (Multi-Element Radio-Linked Interferometer Network) uses
microwave communication links to send signals from its seven radio observatories in England to a
computer system at Jodrell Bank, Cheshire, to make detailed maps of radio sources. With some 15
baselines ranging from 6 to 217 km in length, it can achieve a resolution of 0·05 arc sec.
21.6.3 Very long baseline interferometry
In principle, and practice, very large distances between two or more radio telescopes can be used to
obtain high resolution in measuring a radio source’s coordinates without the stations having direct
or real time links. This is done by the principle of Very Long Baseline Interferometry (VLBI).
Baselines between the radio telescopes have been extended in some arrays to thousands of kilometres
by employing simultaneously several radio astronomy observatories.
With very accurate timekeeping methods available, the radio telescopes’ signals are recorded
individually but synchronously with accurate time markers added as the sky moves across each of
the apertures. By adding the recorded signals together at some future convenient time, keeping their
recorded times accurately matched, an interference pattern is produced, corresponding to the one that
would have been produced if the signals had been combined at the actual time of the observations.
The Australia Telescope National Facility (ATNF) uses eight radio antennas located at three
observatories in New South Wales. It has a baseline of 300 km. Like MERLIN, the antennas can be
used individually or in various combinations and combines the principles of both VLBI and aperture
synthesis.
The Very Long Baseline Array (VLBA) operated by the US National Radio Astronomy
Observatory with headquarters at Socorro, New Mexico, consists of an array of ten radio telescopes
situated with all but two in the continental USA, the others being on Hawaii and the Virgin Islands.
With a maximum baseline of 8000 km, the VLBA can achieve a resolution of 0.001 arc sec, some 50
times better than the Hubble Space Telescope.
In principle, putting a radio telescope in a spacecraft or on the Moon to be used in conjunction
with one or more situated on Earth should give baselines of hundreds of thousands of kilometres with
resolutions orders of magnitude better than those obtained by the VLBA.
21.7 Aperture synthesis
Any large telescope aperture can be considered as being made up of a grid of small elements, each
collecting energy and transferring it to a common receiver. At this point, all the contributions are
added according to the phase.
Suppose that an aperture is taken as comprising N elements of equal area. The contribution from
each element will produce an interference pattern with each of the other N − 1 elements. The overall
effect is equivalent to the resultant of N(N − 1)/2 superposed interference patterns from pairs of
elements. The spacing of the interferometers ranges from adjacent elements to a maximum across the