In the diffraction pattern for a circular aperture, instead of just having a circle, we have a central disk and several faint rings around the circle. Does the Airy disk consist of only the central disk or does it also cover the surrounding rings?

It's that I've seen written on several sites that it was just the central disk, but in others it says it covers the rings and now I'm confused.

And what is the difference between the Airy disk and the seeing disk?


1 Answer 1


1) The Airy disk refers to the central disk. When you consider both the central disk and the surrounding circles you should call it the Airy pattern.

But in my opinion it is just arbitraty taxonomy hence it is not so important as long as you understand what happens.

2) What is the difference between the Airy disk and the seeing disk?

In telescope optics, the Airy diffraction pattern is caused by the diffraction of light by the finite aperture of your instrument and it puts a fundamental limit on the spatial resolution of your telescope. This is called the diffraction limit.

But realize that when performing observations from the ground, the signal which is detected by your instrument has crossed the atmosphere. As random turbulence arise in the air, the electromagnetic wave carrying the information may be affected by the local variations of refractive index induced by turbulence. Hence you are not limited by diffraction only, but also by the following effect: as the wave front is distorted along the path between the source and the detector, the quality of what should be a diffraction pattern will turn into a blurred and twisted signal, that you are calling the seeing disk.

Have a look at these images that illustrate the effect of turbulence:

enter image description here

Seeing really is what alter the Airy pattern and reduce the spatial resolution of your telescope.

When seeing is perfect (when there is virtually no turbulence), your are diffraction limited: your resolution is optimal and constrained by the aperture of your instrument. As the quality of seeing decreases (when the atmosphere is windy for example), the resolution is diminished and the Airy pattern becomes less visible.

  • $\begingroup$ Thanks for the explanation. I just had a question. What is the seeing disk? $\endgroup$ Nov 22, 2018 at 23:26
  • 2
    $\begingroup$ The seeing disk can be defined as the sum of each individual Airy diffraction disks formed by each elements of the air column before the instrument. The seeing disk is what you visualize at the very right of the two figures. $\endgroup$
    – Gonstasp
    Nov 22, 2018 at 23:37
  • $\begingroup$ Possible source of the image (with explanation): telescope-optics.net/seeing_and_aperture.htm $\endgroup$
    – Cheng
    Jul 28, 2022 at 8:46

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