diffraction limit
/ dif-RAK-shun /
You might think a perfect telescope could show a star as a perfect point. It cannot — and the reason is a fundamental property of light itself. Because light is a wave, whenever it passes through an opening it spreads out slightly, an effect called diffraction. This spreading sets a hard ceiling on sharpness that no amount of polishing or money can beat: the diffraction limit.
When a point source like a star passes through a round aperture, diffraction smears it into a tiny bright disc surrounded by faint rings — the Airy pattern. Two stars closer than the width of this disc blur together. The classic rule, the Rayleigh criterion, says the smallest resolvable angle (in radians) is about 1.22 times the wavelength divided by the aperture diameter. The two levers are clear: a larger aperture shrinks the disc and sharpens the image, while longer wavelengths spread more, so the same dish that is razor-sharp for visible light is blurry for radio waves.
The diffraction limit is the best a telescope can ever do, but on the ground the atmosphere usually keeps it from getting there. A telescope working at its diffraction limit is called diffraction-limited; one blurred by air is seeing-limited. The whole point of space telescopes and adaptive optics is to push a real instrument down to this theoretical floor, and it is why radio astronomers must build dishes hundreds of metres across — or link them — to match the sharpness an optical telescope gets from a single metre.
A single radio dish 100 m across, observing at 21 cm wavelength, has a diffraction limit of only about 9 arcminutes — coarser than the naked eye — which is exactly why radio astronomers link dishes into interferometers.
Long wavelengths need huge apertures for the same sharpness.
The diffraction limit is a ceiling on sharpness, not a guarantee of it. A perfectly built telescope can still fall far short if the atmosphere, tracking errors, or a flawed mirror blur the image first.