Radio telescopes are
primarily used to study naturally occurring radio emission from stars, galaxies, quasars, and other astronomical objects.
Due to atmospheric absorption (attenuation), the effective
frequency of observation is limited to between 30 MHz (10 meter
wavelength) and 300 GHz (1 millimeter wavelength).
Ground based radio observations of the cosmic sources are
limited to wavelengths that fall within the Atmospheric Windows. The
radio signals arriving on Earth from astronomical objects are
extremely weak -- millions (or billions) of times weaker than the
signals used by earth bound communication systems. Sophisticated
signal processing and multiple antennas can be used to improve
sensitivity and angular resolution. The signal quality is limited
only by the size of the instrument(s).
A radio interferometer consists of two or more radio telescopes, connected together using coaxial cable, waveguide, optical fiber, transmission line or memory devices.
The interferometer technique works by superimposing the radio waves from the different telescopes. Waves that coincide with the same phase will add to each other while waves that have opposite phases (noise) will cancel each other out, these signals can be combined electronically to
create an image
of exceptional resolution.
An interferometer gives much more detailed images of simultaneous observed radio sources than a single telescope of an antenna array. The resolution of a radio image
depends on the distance between two telescopes and the wavelength of the received signal (longer baselines + shorter wavelengths = higher resolution). The Very Long Baseline Array
(VLBA) is a huge interferometer that uses ten telescopes placed in sites from Hawaii to the Virgin Islands (see map above). This telescope is the 8,600 kilometers across and has a resolution as good as 0.0002 arc second.
A resolution about 50 times better than the Hubble Space Telescope!
A very good resource about Radio Astronomy and and what is available to
educators, students and amateur astronomers is the
Society of Amateur