Hi everyone,
Thank you for your comments and suggestions.
Greg, I tried the Tokina first at F2.8 but the coma was quite bad. On such a wide field of view its not too bad, especially if you crop some of the edges out. I will try F4 and see how it goes.
This image was a 120 seconds exposure. I have managed 4 minutes without a problem and still got round stars. I didnt use those images because the bright stars started to burn out on the 600Ds sensor. You can see some of that starting to happen on the 120 second image. I need some advice on how to avoid that from happening.
Matt, Ian, Andrew; All the details of my tracker is on another blog. See the detail at:
http://theo16dob.blogspot.com.au/
When I built the tracker I was less than impressed about its performance but I recently got myself a green laser and use it now to do very accurate polar alignment. Its incredibly easy and spot on every time. Bad polar alignment was my biggest problem all along.