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Interferometers

Big telescopes are expensive, and doubling the size of a telescope increases the cost by a much bigger factor. In addition, bigger telescopes suffer from deforming under their own weight, so there is a limit to the useful size of a single telescope. There is however, a way to combine telescopes so that the effective size of a telescope is equal - in one sense - to the distance between the telescopes. Combining telescopes in this way is called interferometry.,BR /. The light gathering power of a telescope is proportional to its area. If we combine telescopes, the light gathering power of the set of telescopes considered as a single instruments is proportional to the sum of the areas.
The resolving power of a telescope with a primary lens/mirror of diameter  
\[D\]
  using electromagnetic radiation of wavelength  
\[\lambda\]
  is given by the Rayleigh criterion  
\[\theta \simeq \frac{1.22 \lambda}{D}\]
, so we can resolve objects an angular distance  
\[\theta\]
  apart using this interferometer and this wavelength
radio waves are radio waves of very long wavelength. Light emitted from distant objects is red shifted into the radio part of the spectrum in reaching us. This means that we can only really study these objects at radio wavelengths, and must use interferometers with a large baseline  
\[D\]
  to be able to resolve them and determine the source of the radiation.