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As Colin says, The CFZ is directly related to the f ratio.
The Airy Disk diameter is:
2.44*lambda*F/D (micron) where F is focal length, D is aperture and lambda the wavelength of the light being studied.
A 200mm f8 telescope gives a diffraction disk of 11 micron, a 400mm f4 it would be 5.5 micron. To catch the "lucky seeing" the CCD pixel size needs to be 1/2 to 1/3 the diffraction disk.
To maintain the Raleigh limit (1/4 wave), the CFZ is = 4 *lambda*f ratio^2 micron. Based on green light 550nm examples:
f15 gives 495 micron
f10 gives 220 micron
f8 gives 141 micron
f5 gives 55 micron
(The tolerance at UV wavelengths is 1/2 that of Ha)
- the average human hair, for comparison is 70 micron!)
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