Why are radio telescopes so big?

It’s great to hear that NZ is an integral part of the Australasian bid for a giant radio-telescope network. The Square Kilometre Array promises to produce some great images of southern skies in the radio frequency band. Radio waves are part of the electromagnetic spectrum, just like light waves, and can be used to provide images of what’s ‘out there’ that provide information that visible images don’t.

It will (if the bid is successful) comprise of many, small dishes, but scattered over vast distances,across Australasia. There’s a simple physics reason for having to do this. That’s diffraction. When you have an aperture through which your waves are captured, the waves diffract (bend) and that limits the resolution you can have. Very approximately, your angular resolution in radians is about the wavelength divided by the aperture. For visible light, wavelengths are really small (about 0.6 microns), so there’s not much diffraction – a rough estimate for diffraction caused by the pupil of your eye (say 6 mm across) would be one ten thousandth of a radian, or a bit less than one hundredth of a degree. That’s pretty small. More likely, your sight will be much worse than this – limited by your ability of the eye to focus. Apertures (the width of the objective lens or mirror) of visible telescopes don’t have to be particularly large to give really spectacular images of the planets and distant galaxies, etc. The main mirror of the Hubble Space Telescope is only 2.4 m across – that’s plenty to go on.

But radio waves are much, much, much longer in wavelength. At say 100 MHz frequency, radio waves are about 3 m long. Compare that to the 0.6 microns of visible light – a cool five million times bigger. That means to get the same resolution as for the visible light, you need an aperture five million times larger. So, to get the same resolution as the unaided human eye has for light, that would require an aperture about 30 km across. That’s why we need to go across continents to get really good images at the long wavelengths.

Effectively, two telescopes placed a distance apart (if they are suitably linked) provides a synthetic aperture of that size – and can produce an image of similar resolution to one with that aperture size. Scatter lots of networked telescopes across a continent and you’re talking a pretty decent radio telescope.



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