From: "John Sherman" 
To: 
Reply-To: "John Sherman" 




Hi there,

> Let me give a more qualitative answer. The angular size of the Airy disk
> (inversely related to D) is how much of the sky it takes up in the
>eyepiece

>>> Ooh, you're hurting my head.
>
>Hope your head feels better.

Nope, it still hurts.


So I point my scope at a star, and it does not make an image of a star. It
makes a diffraction pattern. Right there at the focus point of the scope is
a diffraction pattern containing rings and a disc. I can measure that
pattern, and determine that the size of the disc is dependent on the f/# of
the scope, and is independent of the aperture (assuming green light and a
scope without aberrations). So now I can put an eyepiece into the scope,
and look at that diffraction pattern. Now, all of a sudden, for no reason
that I have been able to determine, the size of the disc has nothing to do
with the f/# of the scope. Now the size of the disc is determined only by
the aperture diameter, and is independent of the f/#. How can this be?? 
This makes me think that what you call "angular resolution" is
nothing more than the "apparent size" of the disc. But I have to
suppose that you (or someone you are relying on) have actually measured the
apparent size in arcseconds of different scopes with the same aperture but
different focal lengths, and found them the same.

How can it be that the disc in one example be dependent on the f/#, and in
the next example be independent of the f/#? That is what I don't
understand.


Thanks,

John

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