Andy Resnick said:
The simple response is "do a better job polar aligning", but given my alignment procedure, I expect (assume!) polar alignment to be within a couple of arcmin.
Any ideas?
Okay, I see now that there may be bigger issues, possibly in addition to seeing. What mount are you using?
But I don't think we can completely rule out seeing just yet, since seeing alone
can cause the apparent position of an object to jump from one place to another as a larger blob of cold atmosphere passes into or leaves the direct line of site of the object. Think of atmospheric layers like ocean waves. Sometimes there are lots of small, turbulent regions, and sometimes, on top of that, there are larger, steady waves that have the effect of shifting things back and forth every few seconds or so.
But let's save this one for later. There may be bigger onions to peel.
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But the present, top-layer of the onion doesn't look to me like polar alignment either. I would instead look to tracking issues with your mount, or vibrational modes of your mount (see below).
The thing about polar alignment is, if you do the mathematics behind it, you'll find that polar alignment errors manifest exclusively as declination drift. In order for polar alignment to significantly affect right ascension, the polar alignment would have to be so far off as to have the mount's pole pointed to the wrong side of the sky.
And both altitude errors and azimuth errors both manifest as declination drift. It's even possible for one to cancel the other out at certain patches of sky, only to have them constructively attack you as huge declination drift at other patches. But never do they have much effect on right ascension.
So if your drift is primarily on the right ascension axis, I'd put polar alignment on the backburner for now. The big problems are probably from somewhere else.
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Tracking issues. This is a whole bag of onions here, but let's start with the lowest hanging onion. Balance.
Ensure your scope is balanced, complete with everything on your scope as it would be when imaging. Imbalance causes the tracking motor to work harder, and can cause inconsistencies in the tracking accuracy. Think of the starter motor of an old jalopy trying to "turn over" the engine: rather than rotating the engine at a uniform rate, the engine turns in surges and spurts.
[Edit: also, flexure can cause drift in both the declination and right ascension axes. This can happen even if the polar alignment is good. Having a scope that is properly balanced in both right ascension and declination axes can alleviate this somewhat. While precise balance might not eliminate it completely, it helps a lot. By that I mean drift due to mechanical flexure can be pretty bad if the scope is not balanced well.]
As an anecdote, I used to balance my scope meticulously on the night of imaging, before sunset was over. Then I would take off the objective cap and put on the dew shield. And blam: tracking issues that caused multiple images along the right ascension axis (the same as what you show in your first image of your last post). That's because I didn't balance my scope
with the dew shield on. Make sure you balance your setup in exactly the same configuration that it will be in when imaging. (E.g., don't adjust your balance with your lens cap on, since you won't have the lens cap on when imaging.)
Periodic error correction (PEC): if your mount supports PEC, consider using it.
Cable management: Don't have a bunch of cables dangling off the very end of the camera, down to the ground. This is where they are most susceptible to negative influence of angular moment on the system, if a breeze or wind is present. Try to tie any cables back such that they come back to the center of the system before running to the ground or elsewhere.
The same can be said for dew heater cables and whatnot. Don't leave them dangling. Try to run all cables to a central location on the system, such that if any cable does get perturbed, it doesn't exert much angular leverage.
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All systems have some wobble, and that wobble is more pronounced on some axes than others. Do you best to reduce the moment of inertia along all axes. [Edit: while maintaining balance, of course.]
Beyond that, there is the advice given in this forum in the advice to newcomers buying a first telescope (in a sticky post): Don't neglect the importance of the mount. If you have a set amount of money, don't spend most of it on the telescope (optical tube assembly), rather buy a quality mount.
