Airy disk confusion?

[Robh]

Hi all,

I understand that the Airy disk is caused by diffraction and that its size is determined by the aperture diameter and aperture focal length. The Airy disk for all stars is the same size.
For bright stars, the Airy disk and some surrounding diffraction circles are visible. About 84% of the light is concentrated in the central Airy disk itself.
However, I note that the radius of detectable light within the Airy disk will shrink for dimmer stars as the light intensity falls below the threshold of detection. This is why dimmer stars look smaller than brighter stars.


Now, in another thread I noticed a comment here or there saying that the Airy disk gets larger with increasing magnification.
I hope someone can clarify a few things for me!

1. Is the calculated size of the Airy disk for a scope that which occurs in the image focal plane before magnification by an eyepiece?

2. For a star in focus, does the Airy disk enlarge with increasing magnification?

3. The resolution of a scope (say 0.5 arcseconds) depends on the size of the Airy disk. If question 2 is a "yes", how is it possible to separate close stars at higher magnification if the both the airy disks and apparent separation are increasing with magnification?

In need of enlightenment, Rob.

[DavidU]

I hope this helps Rob.

A formula for calculating the size of the Airy disk produced by a telescope is:
http://www.wilmslowastro.com/software/images/formula17.png
and
http://www.wilmslowastro.com/software/images/formula18.pngwhere:
D = Diameter of Airy disk in mm
λ = Wavelength of light in nm
FR = Focal Ratio of system
A = Angular diameter of Airy disk in arcsec
fl = Focal length of telescope in mm

I found this quite good as well
http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm

[Satchmo]

The 'apparent' angular size of the airy disc gets larger with increasing magnification. The physical size of the airy disc at the focal plane for a given f ratio is constant . The Airy disc has the same physical diameter at the focal plane in microns for example in a 0.1meter scope and a 10 meter, if the F ratio is the same.

[Robh]

Thanks, Mark and Dave.

I was familiar with the theory and formulae but was more interested in the practical interpretation. I think you've answered some of my questions.

In the case of a close binary, the apparent distance between the stars increases with magnification. If the apparent size of the Airy disk also increases, I'm guessing the radius of detectable light within the Airy disk shrinks as the magnification increases. Hence, the reason we can separate the stars more easily.

Regards, Rob