thanks guys - comforting that there are many different opinions.
Rick, doubt we ever get close to dealing with Airy disks - wish the seeing was that good - even occasionally would do.
my basic problem I guess is that "really bright" on the screen implies that all pixels of all colours are flat out - surely that means that really right stars cannot have colour in an image that tries to use the whole dynamic range? A pure colour can only have about 1/3 max brightnesss?