Radio Waves
The objects astronomers study such as stars, galaxies, quasars, pulsars, planets, supernovae and more, all emit visible light, as well as radiation that our eyes can't detect such as infrared and ultraviolet radiation. They also emit radio waves which are another part of the same electromagnetic spectrum. Radio waves have much longer wavelengths than the rest of the electromagnetic spectrum and range from several centimeters to several kilometers.
Radio Telescopes
Radio telescopes are used to study radio waves and microwaves between wavelengths of about 10 meters and 1 millimeter emitted by astronomical objects. Radio waves with wavelengths longer than about 10 meters are absorbed and reflected by the Earth's atmosphere and do not reach the ground. Many radio waves shorter than 1 centimeter are also absorbed by the Earth's atmosphere and only a few wavelength bands make it through. Wavelengths between 1 and 20 cm only experience minor distortions while traveling through the atmosphere and signal processing software can be used to correct for these effects.
Angular Resolution
Radio telescopes have to be much larger than optical telescopes because the wavelengths of radio waves are so much larger than the wavelengths of visible light. Radio wavelengths are between λ ≈ 3 km to λ ≈ 30 cm, while visible light wavelengths are between λ ≈ 4 x 10-7(violet) and λ ≈ 7 x 10-7(red). Angular resolution is a measure of how small details of an area in the sky can be seen. The larger the telescope, the more detail can be observed in a given wavelength.
Angular resolution (θ) of a telescope can be calculated using the wavelength of light or radio waves (λ) the telescope is being used to observe, and the diameter (D) of the telescope.
θ = 2.5 x 105 x λ/D, where θ is in arcseconds and λ and D are in meters
or
θ = 1.22 x λ/D, where θ is in radians and λ and D are in meters
So for example, one of LCOGT's 1.0 meter telescopes should have an angular resolution of approximately 0.1" when observing violet wavelengths. A 65 meter diameter radio telescope observing radio wavelengths of 5 cm would have an angular resolution of 192".
Interferometry
As you can see, the resolution achieved by a typical radio telescope at a typical radio wavelengths is not very detailed. To overcome this difficulty, radio astronomers use multiple radio telescopes at the same time, a technique called interferometry. This gives angular resolutions of 0.001" or better by effectively creating a single telescope as large as the distance between the two farthest telescopes. The light gathering power is not increased by this technique, but the angular resolution in greatly improved. The Very Large Array (VLA) in New Mexico consists of 27 radio telescopes each 25 meters in diameter, arranged in a Y shaped configuration. All 27 telescopes are used simultaneously to observe a target, then their observations are added together.
Image courtesy of NRAO/AUI
Very Long Baseline Interferometry
The longer the distance between two telescopes, the better the resolution when they are used together. Radio astronomers sometimes use telescopes that are thousands of kilometers apart to improve the resolution of their observations. This is called very long baseline interferometry or VLBI. At such great distances, it would take too long to send information from the observations back and forth, so each telescope has its own atomic clock and records the observation. Then later the observations from the various telescopes can be synchronized and combined.
