I've been getting worse-than-expected guiding performance from my Celestron CGEM mount with the C9.25 SCT. I use the "set marker" option in Stellarium to position my imaging camera so that a bright star is in the center of the off axis guider view, then perform a PHD2 calibration. I then compare guided and unguided pointing error using frequency domain analysis. PHD2 does a great job reducing gear train error by 10x using the Predictive Periodic Error Correction algorithm, but there's still enough error left over to cause problems. What next?

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