On Fri, 18 Jun 2004 02:16:00 +0000 (UTC), Brian Tung wrote:

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.

Except for spherical mirrors with the entrance pupil at the radius of
curvature. There're no such thing as off-axis rays in that situation,
which is why the Schmidt camera is so incredibly aberration free (ignoring
the presence and correction of spherical aberration).

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.

A rotatable 3D diagram would probably be required for an intuitive grasp.

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.

It's aperture width, not area, that makes the difference. It is, in fact,
a comparison of linear values.

A very complicated diagram would probably be required to get an intuitive
grasp of why the relationship is there.


--
- Mike

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