Airy Disc

Airy Disc – limiting what you can see

Larger telescopes can see smaller objects. They can also resolve closer binary stars and finer details. Why? Magnification? No, it is all to do with the Airy disc …

The apparent size of a star’s disc produced by diffraction effects in an optical telescope. No matter how well made a telescope is, it will never be perfect. The light passing through a telescope will be diffracted. The diffraction creates a sequence of rings of decreasing brightness. The central ring is the brightest and it is called the Airy disc.

The Airy disc has an inverse relationship to the aperture of the telescope. The larger the telescope, the smaller the Airy disc. The size of the Airy disc limits the resolution of a telescope.

In a refracting telescope, about 84% of the starlight reaching the telescope goes into the airy disc, the other 16% forms diffraction rings around the disc, degrading the image and limiting the resolution. Less light makes it into the Airy disc in a reflecting telescope because of the presence of the secondary mirror. It is for that reason that a reflecting telescope of a given aperture will always out perform a reflector of the same quality.

George Airy was the 7th Astronomer Royal of the UK and it is named for him. Why? Because he was the chap who worked all this out of course! But he wasn’t the first to observe the phenomenon, that honour falls to John Herschel. Or at least it was Herschel who first described it.

But how does the Airy disc arise. See the Wikipedia article for a thorough mathematical explanation.

For a simple non-mathematical treatment this will have to do. Most readers of this article will appreciate that diffraction occurs when light passes through holes of a comparable size to its wavelength. It’s not just holes that create diffraction effects. Edges can do that too. So because a lens or a mirror has a finite size, it also has an edge.  It is because of the presence of the edge that diffraction takes place.

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