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Airy disks, diffraction theory, and out of focus stars

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I started reading this website to better understand airy disks and diffraction in a telescope without all the math.


This all makes sense and I can see how if you placed a CCD at the focal point, you'd end up with an image of the airy disk and rings. What I don't understand is what happens if things are out of focus and the CCD plane isn't at the focal point, but somewhere in front or behind.

From looking at out of focus star images, you tend to get sort of a volcano-shaped star profile where there is a brighter circle that expands the more you go out of focus and the middle where the airy disc used to be gets dimmer and dimmer as the light spreads out. But trying to explain this in terms of the wavefront diagrams of that site, I am confused.

If I move the camera further from the mirror, so that the light cone lines are wider apart, that would seem to fit with what you see when moving out of focus. However, why does moving the camera closer to the mirror also produce the same effect? It seems to me from the diagram that the light cone gets narrower and so the out of focus image should shrink, but I know that isn't what really happens. Clearly I'm not understanding something fundamental, or I need a different diagram.

So can someone end my confusion and explain what I am missing?


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  • 3 weeks later...

You need to read Suiter's "Star Testing Astronomical Telescopes" - he discusses this topic at length and uses the results to evaluate focus and optical performance...

That was my thought too when I saw the thread title, but I don't remember seeing anything in there about AP/CCD. Maybe in a later edition?

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Suiter's analysis doesn't depend on the type of sensor...he "generated" all the images mathematically using PostScript.

Yes, I understand Suiter's analysis and how he generated his images. What I'm suggesting, and what the OP is asking, is whether and how changing the placement of his CCD should change what he's seeing. This is his question, and I still fail to see how simply telling him to read Suiter's book will answer that IF there's anything about CCDs that ought to be known or taken into account, hence my qualification. Maybe just answer his question if you know?


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The larger/ smaller disks of light you would see as you change away from the focus point are geometric circles of light and nothing to do with the Airy disk. Their size will as you suggest change with the focal ratio and the OOF distance.

When smaller movements are made to and from the focus +/- couple of mm and the magnification of the system ( or the effective f ratio>f25 for imaging) then the extended Airy disk pattern will start to be seen. The changes to the Airy disk pattern in this region are the ones detailed by Suiter and would give some indication of the FWHM results obtained at various distances.


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I checked out the link you're referencing but what you need are images. They're everywhere on the internet--same images as you'll find in Suiter. Various aberrations show differently from extra-focus to intra-focus, but there are some that look the same. And yes, it may be that it doesn't matter whether you're talking eyeball or CCD--I don't know--but make sure you're familiar with what you're describing and then determine it's aspect intra-focal and extra-focal.

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Go here:


Note that coma, astigmatism and pinched optics will look almost identical whether you test intra-focally or extra-focally. And turbulence will look similar but with a discernable difference. The other aberrations are all markedly different depending on whether you're intra-focus or extra-focus.


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