2.2 Principles of Radio Telescopes
49
lines. The large distances between the radio telescopes enable very high angular
resolutions to be achieved. The synthesized beams with less than 1 milli-arc-second
are possible at high frequencies.
There are two pre-eminent VLBI arrays operating today. One is the Very Long
Baseline Array, where the radio telescopes are located across North America. Another
is the European VLBI Network, where the radio telescopes are located, respectively,
in Europe, China, South Africa and Puerto Rico, also referred to as Global VLBI.
Since its inception, recording data onto hard media was the only way to bring the
data recorded at each telescope together for later correlation. Today, the availability
of worldwide, high-bandwidth networks makes it possible to do VLBI in real time.
This technique is also referred to as e-VLBI, originally pioneered in Japan and more
recently adopted in Australia and in Europe.
2.2.3 Synthetic Aperture Radio Telescopes
The two-element radio interferometers can only improve the resolution of radio
sources at one-dimensional direction, and do not give two-dimensional visual image
like optical telescopes. In 1952, Ryle at Cambridge University presented a radio
imaging technique, called synthetic aperture radio telescope. Its basic principle is:
any image can be decomposed into sine and cosine components of lots of luminance
distributions; on the contrary, if the luminance distributions with sine and cosine
shapes are known, the original images can be restored synthetically. The synthetic
aperture radio telescope needs at least two radio antennas, similar to the two-element
radio interferometer. Nevertheless, the basic method of the synthetic aperture is: one
antenna is fixed as the center of circles and the circles are drawn at any distance as
radius; another antenna is moved in turn to various locations on the different radial
circles to perform the radio interferometry. A schematic diagram of the synthetic
aperture radio telescope is illustrated in Fig. 2.5, where the actual operation processes
are.
(1) Supposed that A and B, respectively, stand for two antennas, where A is a fixed
antenna and B is a movable one.
(2) At any time, the two-element radio interferometer is composed of antennas
A and B. The correlated amplitude and phase information can be obtained by
correlation processing of the receiver, and thereby the sine and cosine components of the luminance distributions can be extracted from the information.
(3) Antenna A is fixed and antenna B moved in turn to various locations on the big
circular plane to perform the radio interferometry. Considered the symmetry
of circular plane measurement, generally it is only needed to measure half of
the equivalent circular plane. And thus, the sine and cosine components of the
luminance distributions of all directions and distances on the big circular plane
are obtained completely.
49
lines. The large distances between the radio telescopes enable very high angular
resolutions to be achieved. The synthesized beams with less than 1 milli-arc-second
are possible at high frequencies.
There are two pre-eminent VLBI arrays operating today. One is the Very Long
Baseline Array, where the radio telescopes are located across North America. Another
is the European VLBI Network, where the radio telescopes are located, respectively,
in Europe, China, South Africa and Puerto Rico, also referred to as Global VLBI.
Since its inception, recording data onto hard media was the only way to bring the
data recorded at each telescope together for later correlation. Today, the availability
of worldwide, high-bandwidth networks makes it possible to do VLBI in real time.
This technique is also referred to as e-VLBI, originally pioneered in Japan and more
recently adopted in Australia and in Europe.
2.2.3 Synthetic Aperture Radio Telescopes
The two-element radio interferometers can only improve the resolution of radio
sources at one-dimensional direction, and do not give two-dimensional visual image
like optical telescopes. In 1952, Ryle at Cambridge University presented a radio
imaging technique, called synthetic aperture radio telescope. Its basic principle is:
any image can be decomposed into sine and cosine components of lots of luminance
distributions; on the contrary, if the luminance distributions with sine and cosine
shapes are known, the original images can be restored synthetically. The synthetic
aperture radio telescope needs at least two radio antennas, similar to the two-element
radio interferometer. Nevertheless, the basic method of the synthetic aperture is: one
antenna is fixed as the center of circles and the circles are drawn at any distance as
radius; another antenna is moved in turn to various locations on the different radial
circles to perform the radio interferometry. A schematic diagram of the synthetic
aperture radio telescope is illustrated in Fig. 2.5, where the actual operation processes
are.
(1) Supposed that A and B, respectively, stand for two antennas, where A is a fixed
antenna and B is a movable one.
(2) At any time, the two-element radio interferometer is composed of antennas
A and B. The correlated amplitude and phase information can be obtained by
correlation processing of the receiver, and thereby the sine and cosine components of the luminance distributions can be extracted from the information.
(3) Antenna A is fixed and antenna B moved in turn to various locations on the big
circular plane to perform the radio interferometry. Considered the symmetry
of circular plane measurement, generally it is only needed to measure half of
the equivalent circular plane. And thus, the sine and cosine components of the
luminance distributions of all directions and distances on the big circular plane
are obtained completely.