= Radio astronomy
{wiki}
Radio astronomy studies celestial sources through their <radio waves>. The long <wavelengths> make <angular resolution> expensive: a filled telescope of diameter $D$ has a characteristic <diffraction> scale of order $\lambda/D$. <Aperture synthesis> instead combines measurements from separated <antennas>, using their separations to measure fine structure while their individual collecting areas determine how much radiation they receive. A kilometre-wide arrangement of small <antennas> does not collect the power intercepted by a filled kilometre-wide dish.
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