Airy Disk

Because light behaves in some ways like a wave, it is bent or “diffracted” by a telescope’s structure (the edges of the optical tube, for example). This happens in the same way that ocean waves are partially bent or turned in a new direction as they pass a dock piling or the corner of a jetty. This scattered light prevents a star from being focused to a point. Telescopes always show stars as small disks of light (called Airy disks, after British Astronomer Royal Sir George Airy, 1835-1892). The disks are surrounded by faint rings of light called diffraction rings. The size of the Airy disk is determined by the aperture of the telescope – the larger the aperture, the smaller the Airy disk. The illustration shows the simulated Airy disk of a star seen through an unobstructed refractor. The brightness of the first and second diffraction rings has been emphasized for clarity.

. . . our 38th year