2.2 Principles of Radio Telescopes
51
NRAO, USA. The VLA project was started in 1972 and formally built in 1980. And
later, with the technology upgrading, the VLA has evolved into the Expanded Very
Large Array (EVLA). In 2012, the array was officially renamed the “Karl G. Jansky
Very Large Array”.
The VLA comprises 27 independent antennas deployed in a Y-shaped array, each
of which has a dish diameter of 25 m and weighs 209 metric tons. Each of the
massive telescopes is mounted on double parallel railroad tracks, and thus the radius
and deployment of the array can be transferred to adjust the balance between the
angular resolution and the surface brightness sensitivity. Three arms of the Y-shaped
array are 21 km long. Using the railroad tracks that follow each of these arms,
the antennas can be physically relocated to a number of prepared positions, which
allow aperture synthesis interferometry with up to 351 independent baselines. In
essence, the array acts as a single antenna with a variable diameter. There are four
commonly used configurations: the array A with a maximum antenna separation
of 36 km, the array B with 10 km, the array C with 3.6 km and the array D with
1 km. The telescopes are switched among these configurations every 4 months or so.
The available wavelength of the receiver at the VLA is from 0.7 to 400 cm, and the
angular resolution between 0.2 and 0.04 arc-seconds. Using the VLA, with only 8 h
of observation in the centimeter wave bands, a radio source image with a resolution
of order of arc-seconds can generally be obtained.
2.3 Modern Large Radio Telescopes
In the electromagnetic wave spectrum, the range of frequency that makes up the
radio spectrum is so wide that the types of antennas that are used as radio telescopes vary widely in design, size and configuration. According to the difference
in the structures of radio antennas, the radio telescopes are usually divided into
two major types, continuous and discontinuous aperture ones, where the former is
a kind of classic radio telescopes with single parabolic-dish antennas, and the latter
is a kind of combined antenna array based on the radio interferometry. According
to the difference in mechanic equipment and driven mode, the continuous aperture radio telescopes are subdivided into fully steerable, fixed and combined modes.
The fully steerable radio telescopes can be subdivided into the equatorial and horizontal, rotating in two coordinate directions. For the fixed radio telescopes, their
antenna reflecting surfaces are always fixed at the ground sites, and the sky surveys
are performed relying on the Earth’s rotation. The combined radio telescopes can
rotationally track along the declination direction, and sweep to observe along the
right-ascension direction relying on the Earth’s rotation. Moreover, according to
the difference in the shapes of radio antennas, the radio telescopes are divided into
parabolic, spherical and parabolic-cylindrical ones, as well as other antenna shapes,
such as horn, spiral, traveling wave, dipoles, rotational-parabolic cross section and so
on. In general, both the traceable parabolic antenna and the fixed spherical reflecting
antenna are used most commonly.
51
NRAO, USA. The VLA project was started in 1972 and formally built in 1980. And
later, with the technology upgrading, the VLA has evolved into the Expanded Very
Large Array (EVLA). In 2012, the array was officially renamed the “Karl G. Jansky
Very Large Array”.
The VLA comprises 27 independent antennas deployed in a Y-shaped array, each
of which has a dish diameter of 25 m and weighs 209 metric tons. Each of the
massive telescopes is mounted on double parallel railroad tracks, and thus the radius
and deployment of the array can be transferred to adjust the balance between the
angular resolution and the surface brightness sensitivity. Three arms of the Y-shaped
array are 21 km long. Using the railroad tracks that follow each of these arms,
the antennas can be physically relocated to a number of prepared positions, which
allow aperture synthesis interferometry with up to 351 independent baselines. In
essence, the array acts as a single antenna with a variable diameter. There are four
commonly used configurations: the array A with a maximum antenna separation
of 36 km, the array B with 10 km, the array C with 3.6 km and the array D with
1 km. The telescopes are switched among these configurations every 4 months or so.
The available wavelength of the receiver at the VLA is from 0.7 to 400 cm, and the
angular resolution between 0.2 and 0.04 arc-seconds. Using the VLA, with only 8 h
of observation in the centimeter wave bands, a radio source image with a resolution
of order of arc-seconds can generally be obtained.
2.3 Modern Large Radio Telescopes
In the electromagnetic wave spectrum, the range of frequency that makes up the
radio spectrum is so wide that the types of antennas that are used as radio telescopes vary widely in design, size and configuration. According to the difference
in the structures of radio antennas, the radio telescopes are usually divided into
two major types, continuous and discontinuous aperture ones, where the former is
a kind of classic radio telescopes with single parabolic-dish antennas, and the latter
is a kind of combined antenna array based on the radio interferometry. According
to the difference in mechanic equipment and driven mode, the continuous aperture radio telescopes are subdivided into fully steerable, fixed and combined modes.
The fully steerable radio telescopes can be subdivided into the equatorial and horizontal, rotating in two coordinate directions. For the fixed radio telescopes, their
antenna reflecting surfaces are always fixed at the ground sites, and the sky surveys
are performed relying on the Earth’s rotation. The combined radio telescopes can
rotationally track along the declination direction, and sweep to observe along the
right-ascension direction relying on the Earth’s rotation. Moreover, according to
the difference in the shapes of radio antennas, the radio telescopes are divided into
parabolic, spherical and parabolic-cylindrical ones, as well as other antenna shapes,
such as horn, spiral, traveling wave, dipoles, rotational-parabolic cross section and so
on. In general, both the traceable parabolic antenna and the fixed spherical reflecting
antenna are used most commonly.