I hope this helps Rob.
A formula for calculating the size of the Airy disk produced by a telescope is:
http://www.wilmslowastro.com/softwar.../formula17.png
and
http://www.wilmslowastro.com/softwar.../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/tut...hotography.htm