Very nice.
I'm guessing this was taken from light polluted skies?
if so, try an LP filter. would help when taking long subs as LP tends to cast a brownish tint to most images.
here's a quick wipe from your jpg in startools.
your guiding looks perfect.
how long were the subs and which camera.
Hi jo and alistair, camera was an unmodded 450d and the subs were 6 at 10 mins each which is probably causing the lp,your image after being put through startools makes a huge difference will try shorter subs in future cheers.