Re: ATM "Diffraction Limited" a definition

[Andy Hochstetler <andyhoc@mindspring.com>]

Alan Levin wrote:
<< if you are
going to pick one number as a measure of optical performance, then
the Strehl ratio, that is the % of the actual telescope's light that
falls in the theoretical central spot divided by 84% seems like the
right single number.
>>

I think that is the heart of the whole issue.  By quantifying a certain 
characteristic of a mirror that identifies quality, perhaps we can remove 
some of that false advertising which seems so prevalent in commercially 
produced optics.

Bob Pfaff wrote:
<<  I am not sure if I understood what you are saying but my understanding
is the the diameter of the diffraction spot decreases with decreasing
f ratio. That means a 2" f4 mirror and a 100" f4 mirror will
have the same size diffraction spot. The 100" mirror will have much
greater resolving power because of greater focal length.
More focal lenght gives more magnification at the same f ratio
enabling you to separate the spots.
>>

If the focal length stays constant and the diameter of the objective 
increases then the focal ratio decreases as well.  Just another way of 
looking at it.

Does increased focal length or aperture give greater image brightness at 
the same magnitude?  I believe it's aperture.  So that would mean that to 
gain resolving power increase focal length and to increase image brightness 
you would increase aperture.  Is this correct?

Andy H.