Mark is a very sharp guy and as usual, he got this right! Inspired by his post on CN, I did my own quick analysis of this phenomenon. Here’s what the Air rings look like for an unobscured aperture showing the rings on a log scale (in db).
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You can clearly see that they monotonically decrease in amplitude into the quantization noise of the calculation in the extreme wings. If I then add a 47% obscuration (following Mark’s example), you get the following PSF.
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You can clearly see the periodic structure in the ring pattern that Mark identified and that makes sense. When you have an obscured aperture, the Airy pattern becomes the difference of the the diffraction pattern for the entire aperture minus the diffraction pattern for the obscuration. Because the two diffraction patterns have slightly different spatial frequencies, the difference produces a Moire pattern with a beat frequency that is lower than the fundamental frequency of the unobscured main aperture. That’s why the pattern becomes more easily visible—even when the seeing isn’t all that good.
The spatial frequency of the Moire pattern will of course depend on the diameter of the outer aperture and the size of the obscuration so it will be lower for small apertures and higher for larger apertures. This is probably why I’ve never observed this pattern with my 24” scope even when the seeing is pretty good. It will always be more visible for smaller scopes with a moderately large obscuration under pretty good seeing.
Finally I have to say, “Good catch Tony!” You pointed out something that’s actually an interesting and relevant optical property of obscured apertures. I’d never really thought much about this and it’s an interesting effect. It’s really just another “optical signature” for these types of optical systems.
John