A night of planned star cataloguing can be ruined by unpredicted cloud-cover and large, expensive measures are taken to site Earth-based observatories in meteorologically stable places. One of the primary advantages with radio astronomy is that a lot of radio wavelengths can be collected with little interference - radio waves will pass through clouds. Astronomers can receive radio waves of wavelengths down to 2 centimetres at sea level, free from interference. However, because radio waves are used in day-to-day communications, certain wavelengths have to be protected by international agreements, solely for use in astronomy.
There are two types of radio astronomy:
RADAR Astronomy
Radio waves are transmitted from an emitter towards a known object. Upon encountering the object, some of the radio waves are then reflected back into a receiver and then the distance can then be extrapolated from the time taken for the wave to return. The most common implementation is RADAR altimetry in which an orbiting satellite beams down radio waves and uses the echoes to effectively ‘map' the contours of the surface.
Passive Radio Astronomy
Because interstellar distances are so vast, it is impractical to use RADAR astronomy to search for objects. Instead, these telescopes wait for the radio waves to reach them from certain stars, quasars or galaxies. This gives an excellent insight into some of the older bodies in the Universe as the radio waves have been travelling for many hundreds of thousands of years, so, to our detectors, the object appears as it did in the past. However, this technique will only work for objects that radiate their own radio waves.
Most radio telescopes are of the same basic design. A large dish (primary reflector) is used to reflect radio waves into another, suspended and inverted dish (secondary reflector), focusing the waves. The secondary reflector then returns these waves into a receiver. The main dish needs to be kept clear from debris - any protuberance on the surface will reduce the wavelengths the telescope can collect.
The resolution of the telescope is determined by the size of the dish. The larger the surface area of the dish the higher the wavelength that can be detected and so more objects can be spotted. To maximise the resolution, an array of dishes are linked to form a radio interferometer. All are aligned in the same direction, greatly increasing the resolution beyond any single telescope. The Very Large Array in New Mexico is a good example of this. Twenty seven dishes are linked together and are used to get high quality radio images of generalised radio emitting phenomena, and also used to receive communications from the Voyager 2 spacecraft.
Radio telescopes are far beyond the financial reach of a hobbyist astronomer - the dish maintenance alone would put most out of pocket. But still, radio telescopes are one of the most effective and far reaching devices we have at our disposal to study the Universe.
