48
2 Observations of Radio and X-ray Pulsars
A 1 is earlier than that at antenna A 2 , and then the path difference between both can
be expressed as
d = D sin α.
(2.3)
When d is an even multiple of half of the operating wavelength of the radio
telescope, the radio waves with the same phases received by the two antennas will
be added to each other and further strengthen. When d is an odd multiple of half of
the wavelength, the two waves with the opposite phases will cancel each other out
and further weaken to reach the minimum. Since the radio source generates diurnal
apparent motion with the Earth rotating, α is always varied so continuously that the
receiver outputs the periodic superposition signals with strong and weak intervals to
form interferometric fringe pattern. The angular resolution of the two-element radio
interferometer is still expressed as formula (2.2), but hereD no longer denotes the
aperture of the single radio antenna other than the distance between the two radio
antennas. Obviously, the resolution of the radio interferometer is equivalent to a radio
telescope whose aperture is length of the baseline. In other words, the observational
effects of the single-antenna radio telescope with large aperture can be achieved by
using two small aperture radio telescopes.
Modern radio interferometers consist of widely separated radio telescopes
observing the same objects that are connected together using coaxial cables, waveguides, optical fibers or other types of transmission lines. This not only increases the
total signal collected, but is also used in an aperture synthesis to vastly improve the
resolution. In order to produce a high-quality image, a lot of separations between
different telescopes are required. Although the resolution of radio interferometer can
be improved by increasing the length of baseline, there are some technical difficulties
in practice. For example, the transmission lines are so longer that the phase difference
between the signals from various paths will be brought up and the quality of signals
affected.
Beginning in the 1970s, with the stability of the radio telescope receivers
improving, a new technique of Very Long Baseline Interferometry (VLBI) was
introduced to combine the radio telescopes from all over the world, and even in
the Earth orbits. In the VLBI, the signals from a celestial radio source are collected
at multiple radio telescopes on the ground; the distances between the radio telescopes
are then calculated using the time differences between the radio signals arriving at
the different telescopes; and thus, this allows observations of a celestial object that
are made simultaneously by combining many radio telescopes. Instead of physically
connecting the antennas, the data received at each antenna in the array is paired with
timing information, usually from a local atomic clock, like a hydrogen maser, and
then stored for later analysis on magnetic tape or hard disk. At that later time, the
data is correlated with that from other antennas similarly recorded, to produce the
resulting images. Using this method, it is possible to synthesize an antenna with
the equivalent size of the Earth. The VLBI technique enables the distance between
telescopes to be much greater than that possible with conventional interferometry,
which requires antennas to be physically connected by various types of transmission
2 Observations of Radio and X-ray Pulsars
A 1 is earlier than that at antenna A 2 , and then the path difference between both can
be expressed as
d = D sin α.
(2.3)
When d is an even multiple of half of the operating wavelength of the radio
telescope, the radio waves with the same phases received by the two antennas will
be added to each other and further strengthen. When d is an odd multiple of half of
the wavelength, the two waves with the opposite phases will cancel each other out
and further weaken to reach the minimum. Since the radio source generates diurnal
apparent motion with the Earth rotating, α is always varied so continuously that the
receiver outputs the periodic superposition signals with strong and weak intervals to
form interferometric fringe pattern. The angular resolution of the two-element radio
interferometer is still expressed as formula (2.2), but hereD no longer denotes the
aperture of the single radio antenna other than the distance between the two radio
antennas. Obviously, the resolution of the radio interferometer is equivalent to a radio
telescope whose aperture is length of the baseline. In other words, the observational
effects of the single-antenna radio telescope with large aperture can be achieved by
using two small aperture radio telescopes.
Modern radio interferometers consist of widely separated radio telescopes
observing the same objects that are connected together using coaxial cables, waveguides, optical fibers or other types of transmission lines. This not only increases the
total signal collected, but is also used in an aperture synthesis to vastly improve the
resolution. In order to produce a high-quality image, a lot of separations between
different telescopes are required. Although the resolution of radio interferometer can
be improved by increasing the length of baseline, there are some technical difficulties
in practice. For example, the transmission lines are so longer that the phase difference
between the signals from various paths will be brought up and the quality of signals
affected.
Beginning in the 1970s, with the stability of the radio telescope receivers
improving, a new technique of Very Long Baseline Interferometry (VLBI) was
introduced to combine the radio telescopes from all over the world, and even in
the Earth orbits. In the VLBI, the signals from a celestial radio source are collected
at multiple radio telescopes on the ground; the distances between the radio telescopes
are then calculated using the time differences between the radio signals arriving at
the different telescopes; and thus, this allows observations of a celestial object that
are made simultaneously by combining many radio telescopes. Instead of physically
connecting the antennas, the data received at each antenna in the array is paired with
timing information, usually from a local atomic clock, like a hydrogen maser, and
then stored for later analysis on magnetic tape or hard disk. At that later time, the
data is correlated with that from other antennas similarly recorded, to produce the
resulting images. Using this method, it is possible to synthesize an antenna with
the equivalent size of the Earth. The VLBI technique enables the distance between
telescopes to be much greater than that possible with conventional interferometry,
which requires antennas to be physically connected by various types of transmission