Re[2]: ATM Airy disk frustrations

[Dwight Elvey <elvey@hal.com>]

Donald Taylor <dtaylor@unlinfo.unl.edu> wrote:
> 
> Reading what Dwight has said about diffraction and Airy discs, I would like
> to offer the following comment for what it is worth.
> 
> In regard to diffraction it sounds like there is too much emphasis on the
> effects of edges. If you look at the text book presentation of Huyghens
> principle applied to wave fronts you see that every point in the wave front
> acts as a point source for a new spherical wave and that it is the combined
> results of all of those point sources adding up that creates the next
> wavefront surface.   That being the case it is not just the effect of the
> point sources along the edges that creates the diffraction effects.  It is
> all of those missing point sources outside of the aperture as well as those
> missing because of obstructions that causes the diffraction effects. At
> least thats how I understand it. If it were only the point sources along the
> edge then it shouldn't matter for example how wide the obstruction of a
> secondary vane was only how long or what shape it has, but instead it is all
> of those point sources blocked by the width of the vanes as well that
> affects the diffraction pattern.

Yes, you are more correct. It is the lack of light that causes
the fringing and widenning of the disk. If all the light was there,
the light waves should cancel all of the light but the light
at the vary center. This is why wider area covered by secondary
has less contrast. Less of the outer rings are canceled and
less light sums at the center. If the light was a single frequency,
we would see the clasic sinc^2 function of light intensity, just
like what one sees in digital filtering. One can even do a three
dimentional FFT on the diffraction pattern to get the MFT curve just
as playing an impulse through an analog filter would create the
filter response. It is just harder for me to think about summing
all of the wave fronts from across the aperture. I didn't mean
to mislead and thank you for clarifying. I noticed that this point
isn't fully understood by others either that were trying to make a
distinction between area lost at the center of the mirror verses
some place else. Because the light lost is a parallel wave front, it
makes no difference at focus where the light is lost. The shape of the
area lost effects how it effects the diffraction pattern. Spiders,
make spikes and secondaries make complete rings. At least
this is true for on axis. Off axis, coma effects the diffraction
results, a little.
Dwight