The "Northern Cross" Radio Telescope (owned by Bologna University) is one of the world's biggest transit radio telescopes. Its construction started in 1960 and was completed in 1967. Due to technical issues relative to the antennas, a partial revision and upgrading of the entire instrument was necessary. Since 1976, the Northern Cross Radio Telescope has started a new "activity era".
The Northern Cross was conceived in order to receive radio waves with a central frequency of 408 MHz (73.5 cm in wavelength) with a 2.5-MHz bandwidth. It is composed by two series of antennas: one is oriented from East to West (E-W), while the second one develops in the North to South (N-S) direction. Designed as a transit telescope, it is steerable only in Declination: this means it can observe celestial radio sources only when they cross the local meridian.
The E-W section is a huge single antenna with a cylindric-parabolic primary mirror. Its is 564 m long 35 m wide. The focal line is placed 20 m above the rotation axis of the antenna. 1536 dipoles are located (36 cm apart) and aligned on the focal line. Through the dipoles it is possible to detect the signal coming from the observed radio source, as they are the transducers between the incoming electromagnetic waves and the voltage signals – which are subsequently measured.
The N-S section is a linear array of 64 antennas. Each antenna has a cylindric-parabolic shape and its aperture is 23,5 m long and 8 m wide. The antennas are placed 10 m from each other. On each focal line 64 dipoles are present.