Craig Franck wrote:
It does not strike the mirror in a symmetrical fashion. It hits it
slanted, or "off axis." (I'm not sure this answer will help you, since
I'm not exactly sure what it is you're asking.)

Is it off-axis because the lens or mirror has a curved surface? That makes
sense. But I don't understand why it favors a tail on one side.

That's right; the lens or mirror has an axis of symmetry, and the off-axis
light rays are tilted with respect to that axis. That means that light
rays don't get refracted in a symmetric way by the lens, so that they don't
come together to a point.

To see why it favors a tail, I think you would have to do the math, or see
a ray-trace diagram, or something like that. I'm not sure there's a good,
simple, first-order explanation in words alone.

Yes. You only see the secondary mirror if the eye piece is out of
focus, so it seems the diffraction effect should be in that off-focus
focal plane.

That's not the way that diffraction works. You see diffraction effects
because the wave front has a hole in it. The wave front does come to a
focus at the focal point, but because some parts of the wave front are
missing, the focus is disturbed. This disturbance can be seen in the
eyepiece as diffraction effects.

It isn't, precisely speaking, an edge effect in the sense that it happens
*only* at the edge. And the edge of the telescope tube *does* diffract
the light. If it didn't, there would be no such thing as the Airy disc.
Light would focus down to an infinitesimal point, rather than the Airy
disc.

So with a lens or mirror it is the combination of waves of light being
gathered over the entire surface and combining that causes the airy disc.

Yes, that's right.

It is interesting that the airy disk gets smaller with larger aperture.
Is that because the wave length of light gets smaller in comparison to
the overall area of the objective?

Hmm, the wavelength of light does get smaller in comparison to the size
of the objective (you can't really compare a length with an area), but I
hesitate to say that that is the *cause* of the trend. The math does work
out that way, though.

It's sort of like being better able to triangulate a position when you
have a longer baseline. I don't know if I can come up with a hard physical
analogue, however.

Brian Tung
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