Great first effort Mike!
May I suggest try tweaking focus a little at f1.8 and get the best possible stars in all 4 corners in the preview screen (i.e don't focus using the centre of the frame). Once the best point of focus is found note where this position in the distance scale. You will problably find it is not exactly at infinity but slightly to one side. Remember this point for future reference. Once the point of best focus is found, stop down to say f4. No matter what you do you will get coma at the corners at f2.2 and it doesn't go until around f3.5 or f4. This was a best effort at f1.8 with this same lens:
http://www.pbase.com/terrylovejoy/image/38609012/large
Phil I'll dig up a comparision between the 50f1.8 and the 17-70 at 50mm. The 50 1.8 definitely beats it at the same apertures.
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Terry