strehl ratio

From: Jan van Gastel (jhm.vangastel_at_wanadoo.nl)
Date: 06/11/04 

Date: Fri, 11 Jun 2004 16:10:09 +0200

In lots of articles about large professional telescopes, seeing and
adaptive optics, a Strehl ratio of 0.37 is said to be diffraction limited.
For photographic purposes of course, because these scopes are never used
visually. I don't understand why 0.37 is good enough. Does anybody know? 

-- 
Jan
http://home.wanadoo.nl/jhm.vangastel/Astronomy/