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Your image scale is .57 pixel per arc sec not .54. I am not 100% sure with the Sky X if it automatically takes into account binning (I think it might) but if you are using 2x2 then that may be one source of error and your image scale would be 1.14. I know SB Precision PEC did not take into account binning.
The curve seems too large. Mine on both PME and PMX is very small and only a bit above and below the centre line. This could also be a result of a wrong image scale being entered ie. not taking into account binning.
I did use a 20 minute run on a night of good seeing (most nights the seeing is good at my dark site, sometimes its exceptional, rarely poor, occassionally its just OK). The manual does not say to do 20 minutes though so that would only help not make or break it.
Your image though looks fantastic. At 2 metres everything does get exaggerated. With my CDK17 guide errors are usually much more than AP140 on PMX gets. But I get round stars every time. As pointed out 2x focal length of autoguider (not scope). You are using an OAG right, I think you mentioned that? Another possibility is flexure. How stable is that OAG? I had a look one on the net and it had knobs and adjustments everywhere and seemed huge. Can you just put the guide cam in the eyepiece holder of the scope without all the OAG stuff?
Also worth asking, in the pictured graph you do not show the apply corrections box as ticked in the top right corner. That needs to be checked for PEC to activate on the mount otherwise it does nothing.
Greg.
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