Re: ATM "Diffraction Limited" a definition

[Andy Hochstetler <andyhoc@mindspring.com>]

Jack Schmidling wrote:
<< Diffraction limits the resolution of a telescope (among other things)
to the size of the Airy (otherwise knows as diffraction) disk.  If the
optic is capable of producing an image that diameter or less then it
is diffraction limited.
>>

I wanted to better understand this topic, so...out of sheer curiosity, I 
looked up the term diffraction limited in the McGraw-Hill Dictionary Of 
Scientific and Technical Terms, 5th ed.

It gives the following definition:

"Capable of producing images whose separations are as small as the 
theoretical limit imposed by diffraction effects."

Please follow my inexperienced ramblings for a moment.  This example comes 
from the Texereau book - you're probably already familiar with it.

If the linear radius 'p' of the diffraction disk formed by an objective of 
diameter 'D' and focal length 'f' equals:

p=1.22 x l x f/D                (l=the wavelength of radiation: ~.56 micron)    

Then if an objective with a focal ratio of 6 were used as an example the 
linear radius of the diffraction disk would be 4.1 microns.  Tex defines 
this as the limit set by the diffraction effect.

Does this mean that as long as the objective is capable of producing 4.1 
micron images that the objective would be considered diffraction limited? 
 This doesn't seem to take into account the 1/4 wave Rayleigh criterion.

Thanks for shedding light on this on-going debate.

Andy H.