Dr. Stuart Lowe: How Astronomers Make Pretty Pictures With Radio Telescopes
Making images of the sky with a single radio telescope is quite difficult. As well as having much lower resolution than a similarly sized optical telescope, radio telescopes usually only have a 1-pixel view of the sky. To make an image with a single radio telescope you have to do a raster-scan; slowly move left/right and up/down making many individual observations to build up an image. This technique is very time consuming particularly at shorter wavelengths because the resolution increases and you need more points to observe the same amount of the sky. As a result, there have been few all-sky images made with radio telescopes. One of the best known is a 408 MHz map of the sky created using observations from 3 radio telescopes in Germany, the UK, and Australia.
Over the past 30 years, radio astronomers have attempted to speed up imaging by putting arrays of receivers at the focus of radio telescopes. These radio "cameras" provide as many as 10s of pixels and are limited by the space available at the focus of the telescope and the smallest size of receiver that can detect a particular wavelength. These cameras speed up how quickly images can be made by roughly the same factor as the increase in pixels.
Interferometers can also create images of the sky but they do so in a very different way to single radio telescopes or optical cameras. Every pair of telescopes in an interferometer is called a baseline. The radio waves from a pair of telescopes are combined in a computer - a correlator - to create the virtual focus of a much larger radio telescope with the diameter equivalent to their separation.
Each baseline gives you information about the sky but only at the resolution determined by the telescope spacing*. A pair close together (a short baseline) can only see with a low resolution whereas a widely separated pair (a long baseline) only see high resolutions. If you only had long baselines you'd only be able to see the compact objects on the sky and large objects would be invisible to you! So, to produce a complete image you need a combination of different length baselines to get information about all the size scales. Using this technique an array of radio telescopes of 217 km in diameter can produce an image with a resolution equivalent to the Hubble Space Telescope.
* Unfortunately, this increased resolution only applies in the direction of the separation of the two telescopes. With two fixed telescopes you'd get an image that had high resolution in one direction but low resolution in the direction at right angles to that i.e. it would look a bit like a barcode. To get around this, radio astronomers cleverly make use of the daily rotation of the Earth. As the day goes on the direction in which you have high-resolution rotates with respect to the astronomical object and you can effectively combine all the highest resolution parts into a single image with the high resolution. This is called Earth-rotation aperture synthesis.
