Terry now that you enlarge it like that i can see what you mean, it is a fairly heavy crop though, however the first image you posted appears fairly sharp overall, the stars appear nice and round.
On a closer inspection of your first image there is a little degrading at the edges.
I, have found with most lenses that the actual infinity make is not always the best setting, sometimes a little before or a little after that make can be your best focus.
It is only a fraction of a mm, but can make all the difference.
Leon
