It is actually a small disk, known as the Airy disk
, surrounded by a series of diffraction rings, with the first ring being prominent and the rest dim.
Based on the expression of the Airy disk
width at the focal plane of the lens D = 1.22[lambda]f/w, w is the radius of the transmission beam.
At the focus the image of a star appears as a small bright disk surrounded by concentric rings of diminishing brightness, known as Airy disk
or diffraction spot.
As a result, constructive and destructive interference occurs, forming a so-called Airy disk
instead of an infinitely small focused point.
This pattern is called the Airy disk
after George B.
Let me add one minor point, however, regarding Phillip Kane's comment, "A scope with a central obstruction shows a slightly larger Airy disk
A scope with a central obstruction shows a slightly larger Airy disk
(the bright center of a star's diffraction pattern) than an unobstructed aperture.
The smallest focused spot is called the Airy disk
, which depends on the wavelength of light and the aperture of the optical system.
After slight tweaks to the telescope's collimation, the optics yielded classic Airy disk
diffraction patterns around stars at high powers, and double stars such as Epsilon Lyrae were well resolved.
But the Airy disk
's angular diameter is halved, so the light is now concentrated in one quarter the area.
Viewed at high power, stars looked "textbook perfect," with little evidence of spherical aberration disturbing their Airy disk
patterns, and there was no hint of astigmatism distorting them into ellipses or other odd shapes from malformed or pinched optics.
Under steady skies and using high magnification (25x or more per inch of telescope aperture), we will see a star's image as a tiny disk (called the Airy disk
) surrounded by a bull's-eye pattern of faint diffraction rings.