Astronomy Principles and Practice Fourth Edition.pdf

Aperture synthesis 367
a 1
a 2
a 3
a 4
Figure 21.16. The interferometric elements a 1 − a 2 and a 3 − a 4 within a large array carry identical information.
full aperture—all orientations of linking axes are possible. There may be a fair degree of redundancy in
the information. For example, in figure 21.16, the combination of a 1 and a 2 supplies the same pattern
and, hence, identical information as the combination of a 3 and a 4 , their separations and orientation
being identical.
At the expense of not achieving the flux gathering power of the single large aperture, it is possible
to devise an array with the right spacings and axis orientations so that it is equivalent to a single large
aperture in terms of its angular resolving power. Such a notion is the basis of the skeleton radio
telescope.
Consider an array configuration in the form of a ‘T’ with the row comprising 2 √ N elements and
the column with √ N elements. With this pattern, there are approximately 2N different interferometer
pairs that can be made by combining one element from the vertical strip with an element from the
horizontal strip. These 2N interferometer pairs represent all the possible spacings and distances found
in the N(N − 1)/2 equivalent pairs in the complete filled aperture given by √ N × √ N shown in
figure 21.17.
On the assumption that a radio source does not vary from day to day, the brightness distribution
over the source can be derived from the elementary interferometer patterns taken one at a time. It is
not necessary to have all the elements present simultaneously—their equivalent contributions can be
generated sequentially.
In the ‘T’ formation, it is usual to have two elemental telescopes on the top arm running east–
west, acting as a simple interferometer, with a third telescope on the north–south track. After setting
the dishes for a particular declination, a series of recordings is made, with the separation of the
interferometer and moveable telescope adjusted every 24 hours. From analysis of a complete series of
recordings, it is possible by computer to synthesize the observations as though they had been obtained
by an aperture given by the area described by the ‘T’. There are 2N separate patterns containing all
the information that would be present in the field aperture. Hence, if each of the recordings are made
for 2N times longer, observations can be made with the same sensitivity and resolution as though they
were made by a single large aperture. By progressively changing the declination, the whole of the
available sky can be mapped at a particular frequency. The now-famous Third Cambridge Catalogue
(3C) of 328 sources measured at 178 MHz was produced by the aperture synthesis method using a ‘T’
configuration.
Another well-established aperture synthesis system is the Very Large Array (VLA) at the US
National Radio Astronomy Observatory at Socorro, New Mexico. It comprises 27 antennas, each in
the form of a 25 m diameter dish. The elements are set out in a ‘Y’ configuration and the arrangement
provides a resolution equivalent to a single antenna 36 km across with a sensitivity equivalent to a dish