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Nik, a few thoughts from my astrophotography back in the film days.
Mounts like the EQ6 might seem solid but at the level of magnification used for planetary photography all sorts of things show up.
1. Asymmetric flexure in the mount and/or tripod that varies as the scope moves. Add to this changes due to contraction as the temperature drops.
2. Within the polar axis, if it has ball or roller bearings, these can flex slightly as the rollers or balls pass top-dead-centre (or bottom) where the load is maximum. The result is a slight periodic wiggle in dec and it will drive you nuts trying to correct it manually especially if there is backlash in the dec axis.
3. Variable forces between the RA worm and worm wheel as the worm rotates. Given where the worm is on the EQ6 this could easily result in a wiggle in dec correlating to the period of the RA worm. This one is the reason why old-school mounts did not put the RA worm wheel up between the dec axis and the first bearing - the best place for it is at the bottom end of the RA shaft where this can't influence the dec axis. But mounts like that are big, awkward and expensive.
4. Drive rate errors (affects RA only).
5. Atmospheric refraction - especially for objects less than 60 degrees altitude. Worse, atmospheric reflection can show slow semi-periodic movements of several arc secs. Affect its apparent position in RA as well as dec.
Solutions:
A. Instead of aspiring to perfect polar alignment I used to deliberately misalign the polar axis so that while guiding the scope would always require declination in one direction only. This avoids the backlash.
B. Use an autoguider.
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