I think what Paul is talking about is field rotation. Field rotation is a result of imperfect polar alignment. It is exacerbated when there is an angular difference between the guide star and the centre of the imaging field (pretty much guaranteed if you are using separate guide and imaging scopes on a SBS system without tweaking them carefully to make them orthogonal).
The gory details are explained here for anybody who wants to dig further:
http://celestialwonders.com/articles...ntAccuracy.pdf
For those who just want an executive summary, the things that make field rotation worse are:
- less accurate polar alignment
- larger guide angle
- larger FOV
- smaller image scale (determined by focal length and pixel size)
- longer exposure times
- higher declination (field rotation is worst near the poles)
Peter: I reckon you probably have a mix of field rotation and flexure.
Cheers,
Rick.