Re: [ATM] Satisfying "Milles-Lacroix" is neither necessary norsufficient for "diffaction-limited" performance

[Jim Burrows <burrjaw@earthlink.net>]

At 2004-06-22 11:26 +0200, Nils Olof Carlin wrote:

>My simplified view is that the RMS says how much of the light is within the
>Airy disk and how much in the rings, but the RTA says something about how
>wide the light is scattered in the rings. A small amount that is widely
>scattered may be objectionable in much the same sense that the spikes from
>the spider vanes are. One other consequence is that turbulence seems to
>affect a high RTA mirror by increasing the "spikiness".

Interesting thought.  If you're looking at an isolated point source, light 
going out from the Airy disk to the third or fourth diffraction ring (as 
happens with rough mirrors with passing RMS) will be so dim that they are 
invisible (visually).  But suppose you're looking at a cluster (one o' them 
M fellers) - maybe all those diffraction rings would add up to a visible, 
but false, background cutting down the contrast.

         -- Jim Burrows
         -- mailto://burrjaw@earthlink.net
         -- http://home.earthlink.net/~burrjaw
         -- Seattle N47.4723 W122.3662 (WGS84)  

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