Airy disk


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Airy disk

The best focused spot of light that can be created by a perfect lens system, assuming a circular aperture and limited by light diffraction. The disk is surrounded by concentric rings known as an Airy pattern. Since the disk is the smallest unit that makes up the image of a luminous or absorbing object—formed by a corrected microscope lens in focus—the radius of the disk determines the limit of resolution of the lens system or microscope.
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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.