Arrays of telescopes Multiple baselines

The utilization of multiple telescope spacings and orientations will improve image quality by removing artifacts. For our observation near the North Pole, the addition of different spacings obtained with additional telescopes would yield additional rings at different radii in the image, in effect flattening the region surrounding the point source. Observations with shorter spatial wavelengths (larger b) would also serve to restrict the angular size of the point source. As the Fourier plane is filled with more projected baselines b, one fills in more and more pieces of a hypothetical large single telescope, and the Fourier synthesized map becomes closer and closer to the actual sky.

In general, the baseline b of a single telescope pair tracks out an elliptical path on the u,v plane as the earth rotates. Our two examples in Figs. 2 and 3 were special limiting cases: a line and a circle respectively if the observations had been continuous in time. An array of n telescopes will contain n(n — 1)/2 pairs of telescopes and hence that number of baselines. The optimum arrangement is one wherein none of these pairs have the same spacing and orientation, i.e., so that none have the same projected baseline b. Also, the Fourier plane should be more or less uniformly sampled by the tracks of these vectors as the earth rotates.

Radio arrays

There are currently a number of radio interferometric arrays in operation. There are several arrays containing at least eight telescopes extending over a few kilometers or more. Arrays of this size yield angular resolutions of down to 0.1" -1". Examples are the Westerbork array in the Netherlands, the Very Large Array (VLA) in the US (New Mexico), the MERLIN array in England, and the Australian Telescope (AT). These operate at centimeter and longer wavelengths.

The VLA in New Mexico has 27 telescopes, each of 25-m diameter. It can operate on frequencies ranging from 74 MHz to 50 GHz (4 m to 7 mm). The antennas are arranged on three arms, each of length 21 km (Fig. 9a). The telescopes are on tracks so the arm length can be reduced to 600 m for lower-resolution studies. The number of telescope pairs in the VLA is 27(27 — 1)/2 = 351. A similar array with larger dishes and optimized for long wavelengths (> 1 m), a Giant Meter-wavelength Radio Telescope (GMRT), has more recently begun operations in Pune, India.

Figure 10 is a radio image taken with the VLA of the famous twin quasars (labeled A and B). The discovery of these two identical objects at optical wavelengths was the first demonstration of gravitational lensing. The light rays from a single distant quasar are bent by the gravitational pull of an intervening galaxy. They arrive along two different paths, so two apparent quasars (A, B) are seen. The radio image seen here also shows lobe structure (C, D, E; matter ejected from the quasar) as well as the closer lensing galaxy (G). Note the high resolution of this image, ~0.4". The two images A and B are separated by only 6".

Figure 11 shows a VLA "image" of the famous radio galaxy Cygnus A. This image was constructed from an unusually wide range of projected baselines that allowed the observers to extract amazing detail in this extended source. The galaxy

Figure 7.9. (a) The Very Large Array (VLA) in New Mexico with three arms, each of 21 km length. The 27 individual telescopes are each 25 m in diameter. (b) Locations of the 12 telescopes of the Very Long Baseline Array (VLBA). They span 8000 km and operate up to 43 GHz (7 mm). [(b) Courtesy S. Olbert]

Figure 7.9. (a) The Very Large Array (VLA) in New Mexico with three arms, each of 21 km length. The 27 individual telescopes are each 25 m in diameter. (b) Locations of the 12 telescopes of the Very Long Baseline Array (VLBA). They span 8000 km and operate up to 43 GHz (7 mm). [(b) Courtesy S. Olbert]

Figure 7.10. The twin quasar 0957+561 viewed at radio wavelengths with the VLA. It shows the two quasars (A and B), lobes (C, D, E), and the intervening lensing galaxy (G) with a resolution (FWHM beam size) of 0.4". [From Roberts, et al, Astrophys. J. 293, 356 (1985)]
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