What is this obsession with tight stars? Like ray tracing they are the easiest things to record.
It is the Modulation Transfer Function of any optic that gives a complete assessment of the quality of the optic.
This takes into account all the variables and gives a mathematical measure of the performance of the optic.
In practice seeing, light pollution, weather, flexure and a multitude of other annoyances limit any system from its ideal.
Diffraction is the wall that all systems cannot pass through.
It would be quite informative if manufacturers actually published the MTF of the optic they are selling.
This is what Canon does.
http://www.usa.canon.com/cusa/consum...2_8l_is_ii_usm
Bert