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Thanks to everyone who has responded. I won't say I was tearing my hair out - just cursing quietly at the computer.
The rig is up at Coona and I'm doing this remotely - thankfully with some excellent support from the hosts at iTelescope.
Without wanting to jinx things, I am well ahead of where I was. Last night I was able to take a 120 second exposure in the "nasty" area of the sky with minimal elongation or drift. Tracking the main imaging camera with guiding switched off, I managed rms of about 1.5 in RA over 3 minutes. Much better than before.
More importantly, I've refined my guiding so I can take a decent image now.
So what did I do differently?
There were 3 elements to this:
1. I swapped the RA and Dec worm assemblies. There was nothing apparent when looking at the worm. Perhaps it was the way the worm was seated after switching them.
2. I dumped the old TPoint model and took a 179 point model. I had RMS across the sky of about 8 arc seconds with Supermodel turned on
3, I refined the guiding parameters - more on that in a moment.
With 1 and 2 I took a 10 cycle PEC with Pempro. Switching that and Protrack on - I could image comfortably to 60 seconds and even 120 seconds looked ok-ish. I did see some residual drift in RA - but its much better than it was. I tested this in both the West (previously bad) and East (previously better) at Dec 0 where the drift would be most noticeable.
As Peter R has warned me on the SB forum, its harder to pin down whether it was 1, 2 or both that changed things for me. But at the moment I don't mind. The test will be this evening when I will check pointing again - if it is out (as was last time I held a TPoint model over more than 1 evening), I will know that something funny is still going on.
The PEC is better but I will need to fiddle with it some more.
I have a suspicion that I had 2 overlapping issues. There was drift in RA - which reseating the worm addressed. Its not perfect but its better.
The shift in the TPoint model may just have been a corrupt model. No idea how that happens but Paul H had something similar recently.
I've posted the TPoint observations on the SB site - will be interesting to see what Mr Wallace has to say. He might tell me that it is something else entirely.
Guiding - this was interesting. I found that when I tried to guide out the RA drift, I was not able to bring it into a relatively steady state. The RA drift would resume after each guide pulse and if it was say 2 pixels out, the guide pulse would only bring it back by 0.5 - 1 pixel before it resumed its upward trajectory.
The answer was in the aggressiveness and the maximum guide pulses. I use CCDAP for automation. The author John Smith is quite particular about guide settings - he argues for smaller guide pulses to avoid chasing the seeing. There is a tool in CCDAP that recommends the minimum and maximum guide pulse. My maximum pulse was set to 0.22 seconds. When I upped this to 0.52, the guider became much more responsive. I was able to drop the aggressiveness from 10 down to about 5 or 6. This is with 5 second subs. Dropping them back to 2 seconds, I could drop the aggressiveness further to about 3.
Combining all 3, I could happily take a 300 second image with guiding on. Here is a screenshot of the centre of the image - not perfect but pretty good I think.
Now watch the clouds roll in!
Stay tuned - there may (doubtless will) be more grief ahead!
Pete
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