Thanks guys. I can correct for the vignetting quite easily, that's not really an issue.
However longer exposures are quite difficult. My first night out the alignment wasn't perfect so I only got around 5 second before startrails started. The second night I thought I did pretty well but from the looks of things I need to setup Periodic Error Correction before I go any further. Every 400 seconds or so I would end up with 3-4 frames which showed streaking, so a 200 second exposure may prove quite difficult presently.
Time to read up on autoguiders
