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As I understand, the visible light from an optical telescope is focused on a sensor which correlates light exposure to an electrical voltage, which is then converted to an image.

A single antenna radio telescope's signal is focused on some sort a sensor but this signal represents only a single 'pixel.' To generate a composite image of an object, a time intensive scan pattern needs to be carried out.

Why does this difference exist? Why does a radio telescope see a single pixel, while optical telescopes see an entire image? Are radio telescope sensors only a single pixel, or is a radio telescope itself, regardless of the sensor, only capable of seeing a single pixel?

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It's all because of the wavelength of light.

In most bands the radio telescope is about the size of the wavelength it's observing - so it can only see a single point at once anyway. It would be like having an optical telescope that was a tiny microscopic pinhole - there wouldn't be much point in having a megapixel camera behind it. Radio telescopes that work with mm wavelength band now have multi-pixel detectors.

To get comparable resolution to an optical telescope with wavelengths that are a million times longer we would need a telescope a million times larger. So we link radio telescopes together - to make a single telescope. Each telescope measures a single part of the incoming signal and we re-create the detailed picture later in a computer.

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  • $\begingroup$ That makes perfect sense, I thought it might be related to the wavelength of the signal. Thank you very much for the answer! $\endgroup$ – monolith Nov 29 '13 at 17:13

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