What is a radio image?

One of the great discoveries of the Renaissance was the theory of perspective. At the core of the theory is the realisation that a picture is a map of the directions from which light is coming as seen from a particular viewpoint. So every point on the canvas corresponds to a particular direction in space. The hue at each spot represents in colour and intensity the light arriving from the corresponding direction.

Now, colour is the eye's way of describing the spectrum of light; for instance, the colour blue tells us that the light coming from that direction contains a range of wavelengths in the visible band but is relatively strong at around 450 nm. Colour is actually a rather inaccurate measure of the spectrum; for instance, it is hard to tell a mixture of red and blue light (i.e. purple) from the very deep blue (i.e. violet). For technical work astronomers prefer to obtain a series of monochrome images through the use of coloured filters, much like the ones used in ordinary photography, so that each is a record of light with wavelengths within a specific narow band.

The astronomical B, V and R bands correspond roughly to the three basic colours, blue, green and red.
Combining the images in the different filters then allows astronomers to reconstruct a "false-colour" image of the observed object.Our images are then abstracted a futher step: the intensity of white light from our print (or computer monitor) is telling us about the insnsity of the red light on the sky.

There is no reason to restrict the wavelengths used to the tiny range that the human eye can detect. Visible light is just a tiny segment of the electromagnetic spectrum and with the appropriate technology we can make images, maps of "light" in a more general sense, at wavelengths far outside this familiar band; You are probably already familiar with such "invisible" colours like X-rays, ultraviolet, infrared and radio. In fact, the range of colours used by radio-astronomers would correspond to about twenty new colours (or bands) if we say that there are three basic ones in visible light! Fortunately, just as with monochrome images, we can use ordinary visible gray-scales to display these images of "invisible" light.

These composite images show M84, a massive elliptical galaxy in the Virgo Cluster, about 55 million light years from Earth. Radio data from the Very Large Array is shown in red. A background image from the Sloan Digital Sky Survey is shown in yellow and white.
(Credit: Radio (NSF/NRAO/VLA/ESO/R.A.Laing et al); Optical (SDSS))

The leftmost image is in radio wavelengths, the middle one in optical and the rightmost a combination of the two.