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John,
For a 12.5" f/5 and 5 mins subs, I think you are on the right track suspecting flex of some sort.
Next time you set up your rig, check everything is locked down tight and take a picture of the setup. If you post it here you can get a few suggestions how to reduce the flex (it can never be eliminated 100% with a separate guide scope)
In the mean time, double check your polar alignment and work out how long your subs can be before you get trailing stars.
If you want to know if it's flex or rotation, stack all your images without aligning them. Have a look at the attachment - it's 2 hours worth of field rotation on a self guided STL camera on a poorly polar aligned mount. Notice how the stars rotate around the top of the image? That's where the guide chip is located. Flex would be more like straight lines.
James
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