To: atm{at}shore.net
From: "Stephen C. Koehler" 
Reply-To: "Stephen C. Koehler" 



John,

I'm not sure if Nils Olof's response answered your question.  Let me try again.

First, take the eyepiece out of the picture.  The linear vs. angular size
of the Airy disk does not require an eyepiece--it's a property of the
objective, only.

Take a telescope and point it at a star.  To get the linear size of the
Airy disk, take out a ruler and measure the size of the disk in the focal
plane. That one is easy.

Now, to get the angular size of the Airy disk, you could compare its size
to the size of a planet for which you know the angular size.  Say you point
your scope at Jupter and know through other means that the size of Jupiter
at the time is 30 arcseconds.  Take out your ruler and measure the size of
Jupiter in the focal plane.  Then compare the linear size of the Airy disk
to the linear size of Jupiter.  From this, you might conclude that the Airy
disk size is something like 1 arcsecond.  That's the angular size of the
Airy disk.

Now if you did the same comparison with a scope the same focal ratio as
yours but with twice the diameter you would find that the linear size of
the Airy disk remained the same (linear size is related to the focal
ratio), but that the angular size would be twice as small. (This is because
the linear size of Jupiter would be twice as big, but the linear size of
the Airy disk wouldn't change.)

> 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
--
Steve Koehler
koehler{at}securecomputing.com

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