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Jase - huge thanks for you in-depth and knowledgable reply!
Basically I really want to perfect tracking so I can go on to target other challenges. I'm pouring through PHD logs to see the raw data (look like a get a consistent 1 pixel error in DEC every 12 seconds or five pixels a minute (about a star diameter) - so that's in line with being a small fraction of SCP alignment in DEC - Maxpoint says under 30 arc seconds.
That is larger than I suspected - looking at 6 minute unguided shots stars look round to me - at 10 minutes I see a 2-3 mm star trail on a 4" * 6" shot - with less than 1 mm in DEC error.
As to mirror shift and mirror flop - I have (maybe very ignorantly) ignored these because I only need to re-focus my SCT about once every 3 months. I have a Meade motor focus so I never adjust the mirror position with the knob on the back of the SCT. Focus on a carbon fibre tube seems so consistent I have read this as the mirror isn't moving at all so I have no worries... Does not having to re-focus indicate very little mirror shift or flop? Or asked the other way how do I best tell how much mirror shift / mirror flop I have and how to be compensate for it?
Yes I am getting into the bowels of PHD logs. My gut is maybe my MAK is going out of focus, maybe stars are still to large on the CCD for PHD to get the centeroid correct - its just my gut its the software and ghardware taking images from the guide CCD that is introducing errors.
MaximDL based guiding is my next to do, I will see if it gives me better results. I foresee that some time next year I will advance to on axis guiding. I've been spreading my purchases around - CCD is the next major purchase, then a larger APO refractor and then hopefully only more advanced software remains.
Autoguiding seems a bit like chess - not to hard to get the hang on the basic - very challenging to master!
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