Every few weeks we see another amazing discovery made using a radio telescope. Here is how high resolution images are made with radio telescopes compared with an optical telescope.

Visible light has a wave length that is thousands of times shorter than radio waves. In an optical telescope, the image is formed by focusing light in the telescope which is captured with a two dimensional photo sensor at the focal plane.

Radio waves have a wavelength thousands of times longer than visible light, even a large radio telescope disk can only capture a very low resolution image by scanning the sky. High resolution imagery in the radio spectrum requires an array of multiple telescopes which increases the effective aperture and resolution by thousands of times. The image is “focused” electronically by a process called interferometry. Precision delays perform the same focusing function as the curve of the mirror in an optical telescope.

The figure above shows the capture of the central pixel of an image with both kinds of telescope. In an optical telescope the adjacent pixels are captured by a sensor array at the focus point. Each sensor pixel receives light rays from a slightly different path with slightly different timing.

Slight adjustments in the electronic delays for each telescope in a radio telescope array, allows the array to look off the central axis. A complete two dimensional image is formed by combining the same data from each telescope with the appropriate delays for that region of the image.