Once again, center Polaris in the eyepiece and switch to a high-power eyepiece, magnifying the view enough so that when the star is in focus you can see the Airy disk
and the first diffraction ring.
Because the wavelength of visible light is about 0.5 [micro]m, the diameter of the Airy disk
is approximately equal to the f/stop number of the lens.
Phillip Kane replies: You are correct--as the obstruction increases, fewer photons end up in the Airy disk
's central peak and more end up in the outer rings.
Moreover, Dawes defined a double star as being resolved if he could discern a "notch" between the overlapping Airy disks
of the two components, as in the computed simulation at left.
Each of its two tight pairs was cleanly split, with tiny Airy disks
surrounded by a single diffraction ring and clear dark sky between the component stars.
The size of the Airy disk
is determined by the telescope's aperture and the light's wavelength.
And at these magnifications, stars appeared as textbook-perfect Airy disks
surrounded by a bull's-eye pattern of uniformly illuminated diffraction rings, which decreased in brightness outward from the Airy disk
The view of Epsilon Lyrae, the famous Double Double, was particularly impressive with both star pairs cleanly separated and each star's Airy disk
surrounded by a perfect set of diffraction rings.
The definition of CFZ assumes that the spot size of a defocused star (measured by its full-width at half-maximum intensity, or FWHM) becomes no larger than the Airy disk
. Based on geometric optics, the distance the focuser can move while maintaining this condition is found by multiplying D by f.
This measurement reflects how much of a star's light is concentrated in its Airy disk
. The larger the number, the more bloated and soft stars will appear.
But the level of astigmatism was very low and did not cause in-focus star images to noticeably deviate from the ideal Airy disk
diffraction pattern that a perfect refractor should have.
I noted occasional glimpses of the first diffraction ring around the Airy disk
when observing Albireo (Beta Cygni), particularly around the yellow star of this well-known colorful pair.