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Thanks for this. So it sounds like typically 'hope for the best' unless one has this better software (I suspect my Celstron Nexstar software is not in that category, though I may be wrong).
The background to my question is that I use the scope primarily for long exposure video astro, and smearing or star trails sometimes start to become evident for exposures over say a minute.
In the absence of a polar scope on my CG5 mount I use the camera view to polar align. I can get the true SCP centred to a high degree of accuracy (a few arcminutes) but of course with this method the effect of any misalignment between the RA and optical axes remains. It would be really nice if there were an elegant way of reducing said misalignment, short of what I do once in a while whenever I change something significant, which is swinging the scope manually around the RA axis and carefully setting the dec -90 point and shimming the OTA height respectively to maintain the target centering, which is a pain because the two actions interact.
Some of those other factors you mention are beyond my control, short of buying a new mount. The scope is only a 6" newtonian (6kg all up), so I suspect flexure is not much of an issue in my case.
I just don't really know how good (or bad) my scope/ mount combination is. Sometimes after all this palaver the smearing is better, sometimes it is not. Periodic error may also be a factor.
Cheers,
Andrew
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