DSLR Lens - funky blue stars

[whzzz28]

Been playing around with my polarie and i believe i have it all down pat.
My only problem though is that i am getting some weird stars in my shots.

Attached is a backyard test shot in the m8 area. 30sec at ~9mm (APS-C sensor) using a Sigma 8-16mm wide zoom lens @ f4.5, 1600 ISO.
The bright stars on the left hand side (blue) looks rather terrible.
Is this simply a lens design constraint that it has issues with blue light?
Possibly bad chromatic abberations?
It is less of an issue with less exposure, say 15sec the stars don't distort too badly.

I tried another lens - this time a Sigma 18-200mm zoom. Ran it at 180mm and the issue is still there with blue stars. It also has the joy of reflections on bright objects, but i'm not too fussed with that lens as it was only $250~ and i don't plan on using it. I uploaded a copy of it as well.

Will be heading out west next week to some dark sites so would like to put the DSLR and polarie to full use.
If i can figure out why my stars look so terrible.

Cheers.

[jjjnettie]

Try knocking the F stop up a couple of notches. That should help a lot. :)
You can tidy them up in Photoshop or Gimp or Irfanview, using Astroplugins Chromatic Abberation filter. It will realign those colours for you.
Carboni Actions has an action for cleaning up the blue haloes too.

[jjjnettie]

http://www.grekalova.com/photo_Astroplugins.html
While you're there, download Star Rounder too. It's come in handy a few times in the past. :)

[AstroJunk]

I am way too inexperienced to help much, but here is an unprocessed photo I took in similar circumstances with the same lens:

37secs, f4.5. ISO1600 @ 8mm on the Sigma 8-16 on a 650D

A rather annoying streetlight caused the red reflection to the right of the LMC, but otherwise everything is reasonably in order. It is hard to get a tight focus across the whole field, and I think that this could be part (if not all) of your issue. Closer examination of this image shows that I need more practice with focus.

[whzzz28]

Thanks JJJ.
I'll have a play with some of those tools.