Nice shot for a first timer. You need one of these to flatten the curvature of the the telescope you have.
http://www.bintel.com.au/Astrophotog...oductview.aspx
Your guiding looks like it was good as the stars in the middle are nice and round, it's just you field isnt very flat. The item above will give you nice round stars across the whole field.
cheers.
Sandy