The stars we see in the night sky are so far away that they are essentially points. But the image that forms at the focal plane is not a point, because the light that passes through the circular aperture opening of the telescope suffers from the effects of diffraction.

Diffraction occurs because of the wave nature of light. In practical terms, a disk of light forms at the focal plane instead of a point. It is called the "diffraction" or "spurious" or "Airy" disk, after British Astronomer Royal Sir George Airy.

Airy Disk

This disk has a measurable diameter that can be calculated. It is surrounded by a series of faint rings which can grow brighter with larger secondary obstructions. So the actual image formed looks like a bright disk surrounded by a dark ring (the interspace) surrounded by the first bright ring, surrounded by another dark interspace, surrounded by another fainter bright ring, and so on till the outer rings become too faint to be seen. 85 percent of the light in a high quality optical system is in the disk.

The linear diameter of the Airy disk is the smallest when the image is at focus and is dependent on the wavelength of light being calculated.

For a given focal length, the linear spot size of the Airy Disk also represents an angular measurement on the sky, which can also be calculated.

FWHM and Linear Diameter of the first interspace of the Airy Disk.

There are two ways to calculate the linear size of the Airy Disk at the focal plane of the telescope or camera lens.

One is to calculate the diameter of the Airy disk to the first interspace.

The other is to calculate the Full Width Half Maximum (FWHM), which is the diameter of the star at half of its maximum brightness value. The FWHM value is often used in CCD literature.

According to Richard Berry and James Burnell in The Handbook of Astronomical Image Processing, half of a star's total light is contained in the bight core defined by the FWHM.