If you're after sharpness and aperture and bang-for-buck, I'd go for a prime over zoom. The Canon 135mm f/2.0L is my fav with DSLR. Not sure how your tracker will go with lenses of focal length much longer than say 200mm.
The Samyangs are cheap, but my experience with the 14mm that the manual focus was a pain. I prefer to use APT for capture and focus. No point having a sharp lens if you can't absolutely nail focus, and using software for AF will always be sharper than hand/eye.
Some of my wide fields with 135L:
http://www.iceinspace.com.au/forum/s...d.php?t=123902
http://www.iceinspace.com.au/forum/s...d.php?t=123928
http://www.iceinspace.com.au/forum/s...d.php?t=123852
http://www.iceinspace.com.au/forum/s...d.php?t=123833