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Just to add to this one last time, you need to rule out the unknowns. With your RGB composites there is too much going on to be certain. Take 10, 20 ,30, etc minute exposures through clear (or H-a as has been suggested) in order to ascertain whether this is in fact field rotation. Longer trails should give you a better picture of what's going on. If you don't get longer trails, or if they don't appear as you would expect field rotation trails to appear , then the problem lays elsewhere.
At the very least, rule out any effects from field curvature or RGB compositing.
On another note, if you have a large and well modelled TPoint sample, use Protrack. It works extremely well for long exposure tracking by counteracting the effects of flexure over time. Tests we've performed over the years at D21 and D22 at long focal lengths have shown that (provided you have a good model) Protrack works very well indeed.
Good luck,
Paul
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